Grain Boundary Precipitation Behavior of Nanostructured ... · grain-sized materials. Ultrafine-grained materials are especially interesting for light weight construction in automobile

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BNM-2007, Special Oral Session for Young Scientists (PhDs and PhD Students), 17 August 2007 Oral report

Grain Boundary Precipitation Behavior of Nanostructured Maraging Steel Syamak Hossein Nedjada,1 and Mahmoud Nili Ahmadabadib,2

a Faculty of Materials Engineering, Sahand University of Technology,

P. O. Box: 51335-1996, Tabriz, Iran. b School of Metallurgy and Materials Engineering, University of Tehran,

P. O. Box: 14395-731, Tehran, Iran. 1 [email protected] , 2 [email protected]

An Fe-Ni-Mn maraging alloy was cold rolled for 85% at room temperature and isothermally

aged at 753 K. Transmission electron microscopy was used to study precipitation behavior at

grain boundaries during isothermal aging. It was indicated that severe cold rolling and aging

treatment transforms initial lath martensite microstructure to a partially nanostructured steel.

In the areas containing equiaxed nano-scale grains, coarsening of grain boundary precipitates

was found to proceed in a rather homogeneous dissolution of fine precipitates at grain

interiors. The augmented homogeneous dissolution of precipitates at nano-scaled grain

interiors is attributed to high density structural defects facilitating lattice diffusion of alloying

elements. Arrays of precipitates were found at elongated grain boundaries in the initial stages

of aging. However, matrix precipitates at elongated grains were identified larger than

precipitates at equiaxed nano-scale grains.

200

300

400

500

600

700

0.1 1 10 100 1000Time, ks

Har

dnes

s, H

V

0

Figure 1. Changes in hardness of the cold rolled steel during isothermal aging at 753 K

222

a b

c d

Figure 2. Transmission electron micrographs of a specimen aged for 0.36 ks; (a) and (b) show grain boundary precipitates (arrows) at nano-scale grain boundaries; (c) and (d) show arrays of aligned precipitates (arrow) at

elongated grain boundaries

a b

Figure 3. Transmission electron micrographs of a specimen aged for 86.4 ks; (a) bright field image showing coarse precipitates at grain “A”; (b) dark field image lightening precipitates. Matrix precipitates in the nano-

scale grains (A) are smaller than precipitates at elongated grain “B”

223


Microstructural Stability of UFG IF Steel Under Cyclic and Thermal Loading Thomas Niendorfa,1, Hans Jürgen Maiera,2 and Ibrahim Karamanb,3

a Lehrstuhl für Werkstoffkunde (Materials Science), University of Paderborn,

33095 Paderborn, Germany b Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA

1 [email protected], 2 [email protected], 3 [email protected]

In the present work we investigated the cyclic stress-strain behavior of body-centered cubic

(bcc) ultrafine-grained (UFG) interstitial free (IF) steel under cyclic and thermal loading. The

fatigue behavior of UFG bcc materials has been only investigated in a small number of

studies, while no study systematically reported on the behavior under additional thermal

loading, yet.

In previous studies of our group it has been shown that the damage mechanisms that lead to

the low fatigue lives of UFG Copper, dynamic recrystallization / grain growth do not appear

in the UFG IF steel, when the fatigue experiments are conducted at room temperature. The

microstructural stability led to an increased fatigue performance in comparison to the coarse

grained material [1].

In the present study the fatigue behavior of UFG IF steel from route 8E at temperatures

ranging from room temperature up to 440 °C is investigated. All tests were conducted under

strain control with constant strain amplitude of 0.28% and a strain rate of

6 x 10-3 s-1.

The results of the current work can be summarized as follows:

The fatigue life of the UFG IF steel decreases with increasing test temperature. This behavior

is caused by strain localization due to local grain coarsening. These findings are supported by

results from electronoptical measurements, such as electron backscattering diffraction

(EBSD).

The UFG IF steel shows even at the high test temperatures cyclic hardening. This behavior

seems to be caused by a mechanism that is similar to strain aging.

[1] T. Niendorf, D. Canadinc, H.J. Maier, I. Karaman, S.G. Sutter, Int. J. Mater. Res., 97

(2006) 1328

224


Accumulative Roll-Bonding (ARB), Mechanical Properties and Deformation Behaviour of Aluminium AA1050 and Aluminium Alloy AA6016

Irena Topica,1, Heinz Werner Höppela,2 and Mathias Gökena,3 a Friedrich-Alexander University Erlangen-Nürnberg, Institute of General Materials Properties,

Martensstraße. 5, 91058 Erlangen, Germany 1 [email protected], 2 [email protected],

3 [email protected]

Over the past decade it was shown 224that the ultrafine-grained materials have superior

mechanical properties in terms of strength as well as ductility compared to the conventionally

grain-sized materials. Ultrafine-grained materials are especially interesting for light weight

construction in automobile industry, due to a high potential for cost reduction and energy

savings. A very promising method to obtain an ultrafine-grained microstructure is the so-

called accumulative roll-bonding. This is a relatively new and promising severe plastic

deformation process capable of producing ultrafine-grained materials with an average grain

size below 1 µm by applying large plastic strains. Tensile tests of the materials investigated,

i.e. the commercial purity aluminium AA1050 and the aluminium alloy AA6016, showed a

significant increase in strength and some increase in ductility by increasing the number of

rolling cycles. The enhancement of ductility is a crucial characteristic and provides an

immense advantage for the deformation processes of metal sheets. Surprisingly, no significant

difference in the mechanical properties was evident between the rolling and transverse

direction even though the microstructure revealed strongly elongated grains. During hydraulic

bulge testing both materials showed a tendency to higher achievable burst pressures and

strains with an increase in number of rolling cycles, indicating promising deformation

behaviour and good forming potential. Another demand placed upon the ultrafine-grained

metal sheets is the joining technique. The ultrafine-grained accumulative roll bonded

aluminium sheets were successfully joined by friction stir welding, but still require further

optimisation regarding process parameters.

225


Fatigue Behavior of Aluminium Alloys at Elevated Temperatures

Pawel Gabora,1, Hans Jürgen Maiera,2 and Johannes Mayb,3 a Lehrstuhl für Werkstoffkunde (Materials Science), University of Paderborn,

33095 Paderborn, Germany b Dept. of Materials Science and Engineering, Institute I: General Materials Properties,

University Erlangen-Nürnberg, 91058 Erlangen, Germany 1 [email protected], 2 [email protected] , 3 [email protected]

In the present study we report on the fatigue behavior of ultrafine grained (UFG) aluminium-

magnesium alloys with different amount of Mg processed via equal channel angular pressing

(ECAP). UFG materials produced by ECAP exhibit substantially increased tensile strength,

hardness, and fairly large ductility as compared to their coarse grained (CG) counterparts [1].

For many of the envisaged industrial applications, improved fatigue performance is a critical

issue that needs to be addressed [2, 3]. With this motivation we undertook the current study in

order to gain a deeper understanding of the effect of alloying elements on the cyclic stability

of the UFG microstructure at elevated temperatures. Specifically, we report on the fatigue

behavior of UFG Al-Mg alloys with 0.5 and 2.0 wt.% magnesium and compare the results to

their CG counterparts.

Fatigue tests were conducted in the low-cycle fatigue (LCF) regime at both room and elevated

temperatures using a MTS 810 servohydraulic test system. Strain controlled tests were

performed at a total strain amplitude (∆εtot/2) of 0.3% with a strain rate of dε/dt = 2.0 x 10-3 s-

1. Both materials have been investigated in the CG state and in an UFG state produced by

ECAP. In order to reduce small casting pores, all Al-Mg alloys were first subjected to one

ECAP-pass and a subsequent recrystallization treatment at 673 K for 0.5 hours. For obtaining

an UFG microstructure, eight ECAP passes using route BC

(rotation of the sample after every

pass by 90° in the same direction) have been applied.

The effects of different concentrations of magnesium and of severe plastic deformation

through ECAP on the cyclic response and stability at room and at elevated temperatures are

presented. Scanning electron microscopy and transmission electron microscopy were

employed to study the evolution of the microstructure. The results are discussed in terms of

microstructural processes to shed some light on the deformation mechanisms that determine

stability and fatigue performance of these materials.

226

[1] R.Z. Valiev, E.V Kozlov, Y.F. Ivanov, J. Lian, A.A. Nazarov, and B. Baudelet, Acta

Metall. Mater., 42 (1994) 2467

[2] P. Gabor, D. Canadinc, H.J. Maier, R.J. Hellmig, Z. Zuberova, J. Estrin, submitted to

Metall. Mater. Trans. A

[3] J. May, M. Dinkel, D. Amberger, H. W. Höppel und M. Göken, Metall. Mater. Trans A,

in print

227


On the Role of Grain Boundary Character in Creep Strength of FeCr Alloys Kwan-Gyu Tak1, U. Schulz and G. Eggeler

Institut für Werkstoffe, Ruhr-Universität Bochum, D-44801 Bochum, Germany 1 [email protected]

In a recent study the creep behaviour of two FeCr alloys was compared, a tempered

martensite ferritic steel (German grade: X20) and a binary iron chromium alloy (Fe10Cr) with

the same Cr content as X20 which was severely plastically deformed by equal channel

angular pressing (ECAP) [1]. After ECAP, tempering of Fe10Cr was performed in order to

establish the same micro grain size and room temperature strength as X20. The two materials

differ in one important aspect: the micro grain boundaries of X20 are stabilized by carbides

while there are no carbides on the micro grain boundaries of Fe10Cr. X20 was shown to

possess a much higher creep resistance, and this was attributed to the presence of carbides on

its micro grain boundaries. But in order to fully rationalize the difference in creep resistance

between X20 (high creep resistance) and Fe10Cr (low creep resistance) another factor must be

considered [1]. It must be clarified whether the nature of grain boundaries in the two materials

differs and whether this also affects creep resistance [1]. A detailed OIM/EBSD-study of X20

and Fe10Cr before and after creep was performed. The results are presented and discussed in

the light of what is known about the role of grain boundaries in creep. It is shown that the

nature of grain boundaries is less important than the presence of carbides on micro grain

boundaries in determining creep strength.

[1] A. Kostka, K.-G. Tak, R.J. Hellmig, Y. Estrin, G. Eggeler, On the contribution of

carbides and micrograin boundaries to the creep strength of tempered martensite ferritic

steels, Acta Mater., 55 (2007) 539

228


Characteristics of Vacancy Type Lattice Defects in SPD Nanometals Elena Korznikova1,a,b, Daria Setman2,a, Alexander Korznikov3,a,b, Michael Kerber4,a,

Erhard Schafler5,a and Michael J. Zehetbauer6,a a Physics of Nanostructured Materials, Faculty of Physics University of Vienna,

Boltzmanngasse 5 A-1090 Wien, Austria b Inst. for Metals Superplasticity Problems, 39 Khalturin St., Ufa 45000, Russia

1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected], 5 [email protected], 6 [email protected]

Recent research being focused on point defects in nanostructural materials obtained by severe

plastic deformation (SPD) revealed that a considerable amount of vacancy type defects is

produced. This fact can contribute directly and indirectly to outstanding mechanical properties

of SPD nanomaterials. Thus, any reason is given for detailed studies of point type defects in

severely deformed metals.

Among all methods for measurement of point type defects we have chosen two - residual

electrical resistometry (RER) and differential scanning calorimetry (DSC).

The dependence of vacancy concentration from shear strain and hydrostatic pressure was

investigated on pure Ni 99,998% and Cu 99,99%. These metals have different melting

temperatures and different stacking fault energies. In DSC curves from Cu only one peak,

caused by annealing of all defects is observed [1]. In case of Ni we found an additional peak

corresponding to annealing of single/double vacancies [2]. It is seen (Fig.1) that vacancy

concentration grows with increasing shear strain and pressure till some steady-state. The

temperature of annealing of single-double vacancies does not depend on the strain applied [1].

In case of vacancy agglomerates it is very close to dislocation annealing temperature and we

get an overlap of these two annealing effects in DSC and RER curves. For separation of this

effects dislocation densities, evaluated from X-ray Line Profile Analysis (XPA) were used.

Calculated concentrations of vacancy agglomerates are shown in Fig. 2 for Ni and in Fig.3 for

Cu.

In general (Fig. 1,2,3), the concentration of vacancy type defects increases with increasing

pressure and strain. After the release of pressure, vacancy annihilation takes place and its

intensity depends on the stacking fault energy of the material and the magnitude of pressure

and strain. This can explain the decrease of point defect concentration at high levels of these

parameters. In case of ECAP the decrease of vacancy type defect concentration was not

reached because of lower strains and pressures being present. [1].

229

Another important feature of DSC is the

possibility to determine the activation

enthalpy of defects migration. For this the

method of Kissinger evaluating the peak

temperatures shifts with changing the

heating rates [3] was used. The resulting

activation enthalpies are Qvac = 0.6 eV and

Qdisl = 0.9 eV.

In sum, the combination of the methods

DSC, RER and XPA reveals a powerful tool

in the investigation of vacancy type defects,

especially in materials produced by SPD.

[1] E. Schafler, G. Steiner, E. Korznikova,

M. Kerber, M.J. Zehetbauer, Mater.

Sci. Eng. A, 410 (2005) 169

[2] E. Korznikova, E. Schafler, G. Steiner,

M. Zehetbauer, Proc. 4th Int. Symp.

on ultrafine grained materials, ed. Y.T.

Zhu et al., TMS pub., Warrendale

(2006) 97

[3] H.E. Kissinger, Anal. Chem., 29

(1957) 1702

shear stra in γ0 5 10 15 20 25Va

canc

y co

ncen

tratio

n C

VAC

[10-5

]

3

4

5

6

7

8

9

10

8 G P a4 G P a

Figure 1.Concentration of single-double vacancies in Ni after HPT

1 1 0 100

vaca

ncy

aggl

. con

c. C

aggl

[10-4

]

0

1

2

3

4

5

6

D S C 8 G P a R E R 2 G P a

she a r s tra in γ Figure 2. Concentrations of vacancy agglomerates, obtained from DSC and RER data for Ni after HPT

re s o lv e d s h e a r s tra in γ1 1 0 1 0 0

vaca

ncy

aggl

. con

c. C

aggl

[10-4

]

2 ,5

3 ,5

4 ,5

8 G P a 4 G P a

Figure 3.Concentration of vacancy agglomerates in Cu after HPT

230


Improvement of Mechanical Properties of Al-Mg-Si Alloys Processed by Equal-Channel Angular Extrusion with Subsequent Annealing

Matthias Hockaufa,1, Lothar W. Meyera,2, Benjamin Zillmanna,3, Corinna Kuprina,4,

Michael Hietscholda,5 and Lutz Krügerb,6 a Technische Universität Chemnitz, Institut für Werkstoffwissenschaft und Werkstofftechnik,

Institut für Physik, Straße der Nationen 62, 09107 Chemnitz, Germany b Technische Universität Bergakademie Freiberg, Institut für Werkstofftechnik,

Gustav-Zeuner Straße 5, 09596 Freiberg, Germany 1 [email protected], 2 [email protected], 3 [email protected],


Very little information is available on the stability of ultrafine grained age-hardening

aluminium alloys at elevated temperatures and the resulting mechanical properties [1,2].

Therefore the objective of the present work is the investigation of the mechanical properties

and microstructure of the commercial aluminum alloys AlMg0.5Si0.4 and AlSi1Mg0.7Mn0.6

(AA6063 and AA6082) with ultrafine grain size after annealing. The materials were

processed by equal channel angular extrusion (ECAE) at room temperature up to total strains

of ~9.2 in the solution heat treated plus water quenched (W) and the peak aged condition

(T6). Hardness measurements were used initially to characterize the behaviour during

isothermal annealing at temperatures between 100°C and 400°C. Subsequently characteristic

states were selected for tensile and Charpy impact toughness testing. Generally, the aging and

recovery/recrystallisation kinetics are similar for both alloys, to the same alloying system. For

the AA6082 significantly smaller grain sizes and about 30 % higher strengths are achieved

compared to the AA6063 with lower alloying content in the as-processed condition. The

deformation of the AA6082 requires 100 MPa backpressure to assure homogeneous plastic

flow while the AA6063 can be processed without additional precautions. The post-ECAE

strengths of the peak aged alloys are slightly higher compared to the materials processed in

the solution heat treated condition. Depending on the annealing temperature the strength

decreased continuously in two stages (recovery and recrystallisation) for the material

processed in the T6 condition. Different behaviour was found for the material processed in the

W condition. An increase in both post-ECAE strength and ductility was found up to a total

strain of ~2.3 during annealing, both the ultimate tensile strength and uniform elongation can

be enhanced for ~40 % compared to the coarse grained commercial material when the

material is ECAE-processed before ageing (see Fig. 1).

231

At strains above ~4.6 this effect of additional

precipitation hardening during annealing is

overcompensated by the significant decrease of

micro-strains, as reported in the literature [3].

The ductility also decreases. The changes of

mechanical properties were found to be more

pronounced at lower annealing temperatures.

These findings are supported by comprehensive

micro-structural investigations. The study shows

that ECAE has the potential to significantly

improve the production efficiency and

mechanical properties of semifinished parts

made of commercial age-hardening Al alloys. The authors gratefully acknowledge the

Deutsche Forschungsgemeinschaft (DFG) for supporting this work carried out within the

framework of project SFB 692.

[1] J. Wang, Y. Iwahashi, Z. Horita, M. Furukawa, M. Nemoto, R.Z. Valiev, T.G. Langdon,

Acta Mater., 7 44 (1996) 2973

[2] N. Gao, M. Starink, M. Furukawa, Z. Horita, Ch. Xu, T.G. Langdon, Mat. Sci. Forum,

503-504 (2006) 275

[3] E. Cerri, P. Leo, Mat. Sci. and Eng. A, 410-411 (2005) 226

Figure 1. Engineering stress–strain curves from tensile tests for the UFG-AlMg0.5Si0.4 without and with annealing

232

Figure 1. Foils and Bars of Nanostructured Cu (top) and Inconel 718 (bottom)


Bulk Nanostructured Materials by Large-Strain Extrusion Machining and Micro/Meso Scale Components Thereof

Christopher Saldanaa,1, Wilfredo Moscosoa,2, James B. Manna, M. Ravi Shankara,

Srinivasan Chandrasekara, W. Dale Comptona, Alex H. Kinga, Kevin P. Trumblea and

Pin Yangb a School of Industrial Engineering, Purdue University, West Lafayette, IN, USA b School of Materials Engineering, Purdue University, West Lafayette, IN, USA

b Sandia National Laboratories, Albuquerque, NM, USA

1 [email protected], 2 [email protected]

Large-strain deformation processes, such as conventional machining, offer a route for

production of nanostructured and Ultra-Fine Grained (UFG) materials. The chip formation in

machining has demonstrated the ability to impose shear strains of 1-15 in a variety of

moderate to high-strength materials; however, the geometry of the deformation and resulting

chip is not determined a priori. A single-step deformation process – Large Strain Extrusion

Machining (LSEM) – is described, herein, that combines

microstructure refinement by large-strain machining with

simultaneous shape and dimensional control of the chip by

‘extrusion’. Bulk ultra-fine grained (UFG) and nanostructured

materials in the form of foils, plates and bars of controlled

dimensions are shown to result at small deformation rates that

suppress in-situ heating and microstructure coarsening (Figure 1).

The controllability of deformation strain and the microstructure is

demonstrated in foils and plates of copper, Al 6061T6, titanium

and Inconel 718. Micro- and meso-scale components can be

produced from these bulk nanostructured materials using

conventional manufacturing processes like micro-electrodischarge

machining and micro-milling.

233

Figure 2. Micro-gears produced from Nanostructured Inconel 718 (ball point pen tip at top left)

Figure 2 shows an example of micro-gears

created from high-strength, nanostructured

Inconel 718 for power-system device

applications. The LSEM process described here is

analogous to an earlier extrusion-cutting process;

however, these early studies were motivated by

the production of soft, metal strips by machining

at high speeds, a condition that results in high

strain rates, elevated temperatures and,

consequently, annealing of the metal strips

during the deformation. LSEM offers exciting

possibilities for the direct manufacture of foils,

plates, wires, and bars, with a fine-scale

microstructure in a variety of metal and alloy systems. Furthermore, because of the high

levels of superimposed hydrostatic compression prevailing in the deformation zone ahead of

the tool and the ability to vary the strain rate over 4 orders of magnitude, even materials with

a limited number of slip systems (or ductility) like titanium can be subjected to a high level of

deformation at ambient temperature.

234


Structure and Fatigue Properties of AM60 Magnesium Alloy As Processed by ECAP

Olga Kulyasovaa,1, Rinat Islamgalieva,2, Bernhard Minglerb,3,

Alexander Minkowc,4, Michael Zehetbauerb,5 a Institute of Physics of Advanced Materials, Ufa State Aviation Technical University,

12 K. Marx St., Ufa 450000, Russia b Physics of Nanostructured Materials, Faculty of Physics, University of Vienna,

Boltzmanngasse 5, AT 1090, Vienna, Austria c Ulm University, Institute of Micro and Nanomaterials,

Albert Einstein Allee 47, D-89081 Ulm, Germany 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected],

5 [email protected]

The structure and fatigue properties after equal channel angular pressing (ECAP) at different

temperatures have been studied. Special attention was paid to investigations of the structure

after fatigue tests at different stress amplitudes.

Cast billets of the magnesium alloy AM60 (Mg-6%Al-0.13%Mn) were subjected to an ECAP

processing using a die with two channels (circular in cross-section) arranged with an

intersection angle of 120o. The initial rods having a diameter of 20 mm and a length of 100

mm were pressed 10 times (route Bc). Three sets of samples were pressed at temperatures of

350°C, 210°C and 150°C, respectively, to evaluate the influence of the ECAP temperature on

the microstructure and on the mechanical properties.

Fatigue tests were carried out at a frequency of 20 Hz using asymmetrical loading cycles.

Standard flat fatigue specimens with a thickness of 1 mm were tested at constant amplitude

until failure or until at least 5×106 cycles were reached. The structural changes associated with

ECAP pressing were recorded using a transmission electron microscope Philips EM430.

All ECAPed samples exhibited an enhancement of the fatigue limit. This enhancement is well

correlated with grain refinement. For example, the maximum fatigue limit of 120 MPa has

been achieved in the samples having a mean grain size of 1 µm after ECAP at a temperature

of 150 oC, which is 70% higher in comparison with coarse-grained samples. It was

established that the samples ECAPed at 350оС with a mean grain size of about 18 µm do

reach the fatigue limit already at 90 MPa. Decreasing of the ECAP temperature till 210оС

leads to a decrease of the mean grain size to 2 µm and to a concomitant increase of the fatigue

limit up to 110 MPa. For comparison the fatigue limit of the coarse grained state of the

magnesium alloy AM60 was equal to about 80 MPa.

235

The mean grain size of the ECAP samples and the size of twins which appreciably emerge

after the fatigue tests depended strongly on both the structure before the test and the stress

amplitude applied. The twin density after fatigue tests was highest in case of high stress

amplitude applied and in areas where no precipitates were detected. After fatigue tests at low

stress amplitude no twins but a slight increase of the mean grain size occurred in the areas

with precipitates being present. The changes in structure of the ECAP processed samples after

fatigue tests have been examined by EBSD.

[1] H.K. Kim, W.J. Kim. Mater. Sci. Eng. A, 385 (2004) 300

[2] O.B. Kulyasova, R.K. Islamgaliev, Mater. Sci. Forum, 503-504 (2006) 609

[3] H. Mayer, M. Papakyriacou, B. Zettl, S.E. Stanzl-Tschegg, Int. J. Fatigue, 25 (2003)

245

[4] R.Z.Valiev, T.G. Langdon, Prog. Mater. Sci ., 51 (2006) 881

236


Consolidation of Nanocrystalline AlMg4.8 Powder via ECAP Back Pressure Marco Hüllera,1, Johannes Vlceka,2 , Heinz Werner Höppelb,3 and Mathias Gökenb,4

a EADS Deutschland GmbH, EADS Innovation Works, 81663 München, Germany b Department of Materials Science and Engineering, Institute I: General Materials Properties WWI,

University Erlangen-Nürnberg, Martensstr. 5, 91058 Erlangen, Germany 1 [email protected], 2 [email protected], 3 [email protected],

4 [email protected]

Ball milled nanocrystalline AlMg4.8 powder was subjected to ECAP in order to receive a

nanostructured bulk material. ECAP seems to be very beneficial for the compaction of

aluminium powders in the non-equilibrium state as processing temperatures are rather low

and severe shearing is beneficial for good particle bonding [1]. The quality of the

consolidation is crucial as according to Sanders et. al. [2] the mechanical properties of

nanostructured materials are very sensitive to defects.

ECAP route BC with applied back pressure was accomplished at a temperature of 200°C

without preliminary consolidation at Monash University (Australia). The ECAP processed

samples were investigated concerning density, grain structure by means of TEM, and

mechanical properties by means of hardness and compression tests after 1, 4 and 8 passes.

The as-milled powder exhibits a monomodal grain structure with an average grain size of

∼75 nm. After the first ECAP pass a bimodal grain structure is obtained. Larger grains of 300-

800 µm are embedded in the initial

nanosized grains. The average grain

size increased from 75 nm in the

powder state up to 240, 260 and

270 nm after 1, 4 and 8 passes

respectively. Concurrently the

hardness of the specimens decreased.

Characterisation of the microstructure

revealed full consolidation after the

first ECAP pass. During subsequent

passes the homogeneity of the microstructure was improved. These findings were confirmed

by density measurements (Fig. 1). Even after the first ECAP pass a density of 99.4% was

achieved.

93

94

95

96

97

98

99

100

HIP 1 Pass 4 Passes 8 Passes

Den

sity

[%]

Figure 1. Density after 1, 4 and 8 ECAP passes related to hot isostatically pressed sample (HIP-530°C/3h/1700bar)

237

The mechanical properties were

evaluated via compression tests

at a strain rate dε/dt of 10-4 s-1

and are given in Fig. 2. High

yield strength of ∼500 MPa and

no strain hardening was detected

for the 1 pass sample. 4 and 8

pass samples reveal both

pronounced strain hardening and

high strain values.

[1] K. Xia, X. Wu, Scripta Materialia, 53 (2005) 1225

[2] P.G. Sanders, C.J. Youngdahl, J.R. Weertmann, Mater. Sci. Eng., A234-236 (1997) 77

0

100

200

300

400

500

600

700

800

0 5 10 15 20 25 30

True strain [%]

Stre

ss [M

Pa]

1 Pass

4 Passes

8 Passes

Figure 2. Stress-strain curves from compression tests of AlMg4.8 after 1, 4 and 8 ECAP passes

238


Nanostructure and Properties of the Large Size Ti49,4Ni50,6 Alloy Sample Subjected to High Pressure Torsion Deformation

Alexander Lukyanova,1, Dmitry Gunderova,2, Egor Prokofieva,

Vladimir Pushinb and Aleksey Uksusnikovb a. Ufa Statement Aviation Technical University, Institute of Physics of Advanced Materials,

12 K. Marx St., Ufa 450000, Russia b. Ural Division of Russian Academy of Science, Institute of Metal Physics,

18 S. Kovalevskaya St., Ekaterinburg 620041, Russia [email protected] 2. [email protected]

It is well-known that TiNi-based alloys functional materials possessing shape memory effect

(SME) due to thermoelastic martensitic transformations. These alloys are widely used in

various engineering mechanisms and in medicine as implants and medical devices [1, 2]. A

perspective way of improving properties of metals and alloys is formation of NC structure

using methods of the severe plastic deformation (SPD) [3]. In earlier works [4, 5] amorphous

and nanocrystalline (NC) structures in TiNi have been processed by high pressure torsion

(HPT). Subsequent annealings lead to the formation of nanostructured state. However, these

works considered only small HPT-processed samples 5-10 mm in diameter and approximately

0.1-0.15 mm in thickness, which hampered the investigation of mechanical properties and

shape-memory effects and did not allow using high-strength NC TiNi for practical

application. A new unique equipment «SCROODG-200» allowing to produce HPT samples

with the diameter of 20 mm and thickness of up to 1 mm under pressure of up to 6 GPa was

created in IPAM USATU. Such HPT method enabled to fabrication of integral samples

without cracks and macrodefects as well as enhancement of their mechanical properties. In

this work the results of new HPT method of producing large size samples of Ti49,4Ni50,6 alloy

are first demonstrated for the first time. Fabrication of NC structure by subsequent annealings

of such HPT samples is first demonstrated for the first time as well. An initial Ti49,4Ni50,6

alloy from Intrinsic Devices Incorporation (San Francisco, California) has an austenite B2

structure with a grain size of about 60 mkm was used in the experiments. A sample in the

form of disc, cut from a quenched rod, was processed by HPT n=7 turns in a groove with the

diameter of 20 mm and depth of 0,5 mm under the pressure of 6 GPa. TEM studies of

microstructure showed that the samples processed by HPT, n=7 had amorphous-

nanocrystalline state with the size of nanocrystals of about 20 nm. Halo in electron diffraction

patterns pointer to a considerable amount of an amorphous phase. Annealing at 300°С (1

239

hour) did not result in a complete crystallization and amorphous phase was preserved.

Samples processed by HPT with n=7 had grains with an average size of 10 nm. After

annealing at 400ºС (1 hour) samples underwent a complete nanocrystallization and an average

grain size constituted 30 nm. The subsequent annealing at 500ºС leads to grain growth up to

45 nm. The grain boundaries become more clear and sharp, than after annealing at 400°C. The

results we obtained after annealings are close to results obtained for small HPT samples [5].

Mechanical tests showed that amorphous-nanocrystalline TiNi produced by HPT had much

higher strength in comparison with initial microcrystalline. Thus, microhardness

measurements showed values increasing from initial 1800 MPa to 5500 MPa after HPT

processing. Nanocrystalline state result s in the record value of strength for this material –

2600 MPa with the elongation of about 5%. Thus, HPT with n = 7 turns in heads with a

groove combined with subsequent annealings at varying temperatures allowed to obtain

integral nanostructured Ti49.4Ni50.6 specimens with various grain sizes. Now, it is possible to

carry out complex investigations of NC structure effect on mechanical and functional

properties of TiNi alloys.

[1] V.N. Zhuravlev, V.G. Pushin. Alloy with thermomechanical shape memory and their

application in medicine, Ekaterinburg, UrD of RAS, (2000) 151 p.

[2] V.G. Pushin, R.Z. Valiev, Sol. Stat. Phenomena, 94 (2003) 12

[3] V.G. Pushin, V.V. Stolyarov, R.Z. Valiev et al., Phys. Met. Metallogr., 1 94 (2002) S54

[4] S.D. Prokoshkin, I.Yu. Khmelevskaya, S.V. Dobatkin et al., Acta Mater., 53 (2005)

2703

[5] A.V. Sergueeva et al. Mater. Sci. Eng. A, 339 (2003) 1259

240

Poster session C

241

BNM-2007, 17 August, Poster Session C Invited poster report

The Effect of Non-Monotony and Deformation Center on Grain Refinement in Metals Imposed to Severe Plastic Deformation

Farid Z. Utyasheva,1, Georgy I. Raabb,2

a Institute for Metals Superplasticity Problems RAS, 39 St. Khalturin St.,

Ufa 450001, Russia b Institute of Physics of Advanced Materials, Ufa State Aviation Technical University,

12 K. Marx St., Ufa 450000, Russia 1 ufz [email protected], 2 [email protected]

Grain refinement occurring in metals imposed to severe plastic deformation is

considered as a process of fragment boundary and band formation due to the action of the

mechanisms of both crystallographic and non-crystallographic shears. It is shown that the

activity of the mechanisms identified and consequently the value of grain refinement depend

on the deformation center geometry and non-monotony of the process. A model of structure

refinement in metals subjected to equal channel angular pressing and torsion under pressure

has been proposed. In accordance with this model the sizes of forming fragments and bands in

pure metals reduce with increasing the curvature-twisting of a crystal lattice – a tensor density

of dislocations from the bend and turn of the sample in the center of deformation. The angular

misorientations of the mentioned structure elements rise with increasing the deformation non-

monotony to some definite limits. The average value of the accumulated curvature-twisting in

the center of deformation can be defined as a change in the ratio of the center surface area to

the volume. The dependence of this ratio on the accumulated deformation has been

determined which provides determination of a forming fragment size to the first

approximation. The paper evaluates a grain size of metals processed by different methods of

deformation. It has been shown that one can regulate the value of grain refinement by

changing the deformation center geometry.

242

BNM-2007, 17 August, Poster Session C Poster report

Evolution of Residual Stresses, Crystallographic Texture and Microhardness in Cu and Ti Subjected to Equal-Channel Angular Pressing

Igor V. Alexandrova,1, Jan Bonarskib,2, Alexander I. Korshunovc,3,

Vil D. Sitdikova,4 and Leszek Tarkowskib,5 a Institute of Physics of Advanced Materials, Ufa State Aviation Technical University,

12 K. Marx St., Ufa 450000, Russia b Institute of Metallurgy and Materials Science , Polish Academy of Sciences,

25 Reymonta St. , 30-059 Krakow, Poland c Russian Federal Nuclear center VNIIEF, 37 Mira Av., Sarov 607190, Russia

1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected], 5 [email protected]

Numerous studies of recent years testify to the fact that severe plastic deformation (SPD)

techniques provide formation of bulk nanostructured states and developed preferred

crystallographic orientations (crystallographic texture) in different metallic materials [1,2].

Bulk billets deformed by SPD techniques suffer very high plastic strain degrees. As a result a

high level of residual stresses, microstructure with the grain size of tens and hundreds

nanometers, developed crystallographic texture, strength properties magnified 2-5 times in

comparison with the coarse-grained states are typical of them. Besides, a definite

heterogeneity is specific for the mentioned parameters of microstructure and properties [3].

Analysis of the mentioned parameters of microstructure and properties is urgent from the

point of view of formation of bulk homogeneous nanostructured billets with high physical and

mechanical properties.

This report presents the result of detailed X-ray investigations of the evolution of residual

stresses and crystallographic texture. The analysis of microhardness in bulk Cu and Ti billets

subjected to equal-channel angular pressing (ECAP) is also delivered. The investigations were

carried out in different points of the central vertical section of billets, which partially had

passed through the die-set channels during the 1st and subsequent (up to the 4th inclusively)

passes of ECA-pressing (route ВС). The temperature for ECA-pressing was chosen room in

case of Cu billets and 450°С in case of Ti billets.

The XRD analysis was performed using the filtered CuKα radiation by means of “Burker D-8

Discover” diffractometer. Analysis of the diffraction effects in the chosen areas of the sample

surface enable to identify the topography of texture and residual stresses generated during the

ECAP. It was shown that planar distribution (longitudinal section) of the identified residual

stresses and crystallographic texture in the middle plane during ECAP process of Cu and Ti

243

ingots is strongly inhomogeneous. The main texture formation processes during the 1st ECAP

pass take place in the zone, adjacent to the plane of the die-set channels’ intersection. The

global texture is rebuild in the zone during subsequent ECAP passes which is resulted in its

softening.

The microhardness measurements were performed with computer-controlled Vickers

microhardness test unit “Duramin” using “Duramin 20” software. It was shown that

microhardness grew for Cu and Ti samples with the number of ECAP passes increasing. In all

the cases the microhardness value along the longitudinal axis is higher in comparison with the

microhardness along the cross section. The microhardness distribution in the vertical

longitudinal section becomes less homogeneous after the 1st ECAP pass in comparison with

the initial state but homogenizes during the following passes in both metals.

The correlation between the X-ray data and the microhardness measurement results is

revealed and discussed.

[1] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progress Mater. Sci., 45 (2000) 103

[2] M. Zehetbauer, R. Z. Valiev (Eds.), Nanomaterials by Severe Plastic Deformation,

Vienna (Austria), Wiley/VCH, Weinheim, Germany, (2004) 245

[3] A.I. Korshunov, I.I. Vedernikova, L.V. Polyakov, T.N. Kravchenko, A.A. Smolyakov

and V.P. Soloviev, Mater. Sci. Forum, 503-504 (2005) 693

244


Computer Simulation of Material Flow during Equal-Channel Angular Pressing Vladimir Zhernakova, Igor Budilova, Igor Alexandrovb,1

a Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000 Russia b Institute of Physics of Advanced Materials, Ufa State Aviation Technical University,

12 K. Marx St., Ufa 450000 Russia 1 [email protected]

Material flow has a complicated character in the conditions of severe plastic deformation

(SPD) realized by equal-channel angular (ECA) pressing. The latter is defined by the type of

crystalline lattice, character of strain hardening in material, its contained plasticity,

temperature and strain rate, geometry of the channels in the die-set, friction coefficient

between die-set walls and the surface of the deformed billet etc. On the other hand,

controlling the character of material flow, volume and shape of the deformation zone can

provide an optimum level and distribution of accumulated strains in the body of a deformed

billet. Computer simulation by means of finite element method is one of the most effective

tools for conducting such a research.

The report presents the results of 3D modeling of the processes of material flow during ECA

pressing of Ti billets. Influence of the character of strain hardening, number of passes, route

on the value of accumulated strains and their intensity was studied. Conclusion about

potential areas of crack nucleation during realization of different passes of a billet through the

die-set channels was made.

245


The Effects of Conventional direct Extrusion of the Twist Extruded Ti-6Al-4V by Means of Finite Element Method

Amir R. Shahab1, S.A.A. Akbari Mousavi2, M. Mastoori3 School of Metallurgy and Materials Engineering, School College of Engineering, University of Tehran,

P.O. Box: 11365-4563 Tehran, Iran 1 [email protected], 2 [email protected], 3 [email protected]

Recently some numerical investigations on Twist Extrusion (TE) process is reported as one of

the prospective method among SPD methods in order to produce nanostructured bulk

materials. Research on TE has been carried out till now imply on inhomogenities of structure

in cross-section and in longitudinal direction as well [1, 2]. The reason is existence of friction

among sample and die surfaces in turn cause more strain exertion at the corners of cross-

section specially on one of diagonals. Change of stress-strain mode is another reason of

creation of inhomogenities in mechanical properties of product.

In this investigation, the effects of conventional direct extrusion of TEed product as a

consequent cold work are studied and simulated using FEM. Appropriate constitutive

equation of Ti-6Al-4V is selected as the material properties and the temperature is chosen to

be 900°C. Effects of 17% reduction in direct extrusion

after one pass of twist extrusion (Fig. 1) with 90 degrees

turning on von-Misses stress, equivalent plastic strain and

shear strains distribution on cross-section.

Shear strains play important roles in grain refinement,

simple shear mechanism and characteristic anisotropy.

According to our results, consequent extrusion in addition

to calibrate sample for further deformation passes, acts as

a back pressure on the sample to complement hydrostatic

pressure. Also exerting extrusion cause more uniform distribution of shear strain and finally

moderate the intensity of equivalent plastic strain in different points of cross section and this

matter is in good agreement with experimental reported results.

[1] A.R. Shahab, S.A.A. Akbari Mousavi, M. Mastoori, 8th international conference of

nanostructured materials, NANO2006, August 20-25, Banglore, India

[2] S.A.A. Akbari Mousavi, A.R. Shahab, M. Mastoori, Journal of physics and technology

of high pressure, 1 7 (2007)

Figure 1. Schematic figure of die with twist and direct conventional extrusion

246


Molecular Dynamic Simulation of Interaction between Dislocations and Grain Boundaries in Thin Metal Films

Dmitriy Bachurina,1, Daniel Weyganda,2, Peter Gumbscha,b,3

a Institut für Zuverlässigkeit von Bauteilen und Systemen (IZBS), Universität Karlsruhe,

12 Kaiserstrasse, Karlsruhe 76131, Germany b Fraunhofer Institut für Werkstoffmechanik (IWM), 11-13 Wöhlerstrasse, Freiburg 79108, Germany 1 [email protected], 2 [email protected], 3 [email protected]

Thin polycrystalline films made from metallic materials are widely used in microelectronic

devices. Usually these metal films have thicknesses below 1 µm. Mechanical properties of

such films differ considerably from the properties of bulk materials.

Plasticity of polycrystalline metal films depends on the interaction between lattice

dislocations and grain boundaries (GBs). The type of GBs (tilt, twist, symmetrical,

asymmetrical) and its energetic are believed to determine the interaction mechanisms of

lattice dislocations with GBs: lattice dislocation can be absorbed or transmitted.

Experiments show a strong <111> texture of polycrystalline fcc metal thin films (with a

weaker <100> component). As the starting point for our investigations, symmetrical and

asymmetrical [111] tilt GBs have been chosen and their structure and energy have been

studied. An Embedded atom method potential for Nickel was used. Periodic boundary

conditions were employed in the direction parallel to the tilt axis. The structures were relaxed

at zero applied pressure.

For symmetrical GBs the calculations give structures well-comparable with structures known

from literature. However, it should be noted that the stable structures and the minimal GB

energies for some of studied boundaries can only be achieved by translation of one of the

grains parallel to the GB plane. Investigations show that there is no strong dependency of the

energy of asymmetrical GBs on the inclination angle. The energy of asymmetrical GB is

about 0.1÷0.2 J/m2 higher than the energy of corresponding symmetrical boundary.

Molecular dynamics simulation of dislocation – grain boundary interactions have been done

in the bicrystal. No periodic boundary conditions were used. The simulation cell sizes along

three dimensions were 26×35×18 nm3. An incoming dislocation glides under applied strain

tensor on the plane [ 1�1�1 ] and meets the GB. The simulation box was deformed up to 1÷2%.

An interaction with a number of different (high- and small-angle) symmetrical and

asymmetrical tilt GBs has been investigated. As it has been expected, calculations have shown

no transmission effects for high-angle GBs. The detailed analysis of the mechanisms of

247

interaction of lattice dislocations with 3D GBs is a challenging task and currently under

investigation.

248


Features of the Formation of the Nanostructured States in V-4Ti-4Cr Alloys under Severe Plastic Deformations

Ivan Ditenberga,1, Alexander Tyumentseva,2, Yury Pinzhina,3,

Alexander Korotaevb,4 and Vyacheslav Chernovc,5 a Institute of Strength Physics and Material Science, Siberian Division, Russian Academy of Sciences,

2/1 Akademicheskii Pr., Tomsk 634050, Russia b Tomsk State University, 36 Lenin Pr., Tomsk 634050, Russia

c A.A. Bochvar Research Institute of Inorganic Materials, 5 Rogov St., Moscow 123060, Russia 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected], 5 [email protected]

Transmission electron microscopy was used to examine the microstructure formed under

severe deformations (ε ≥ 93%) in V–4Ti–4Cr alloys rolled at room temperature. Microband

nanostructured states and high-energy defect substructures have been detected that feature a

high curvature (up to χij ≈ 200 µm–1) of the crystal lattice, a high density (∂θ/∂r ≤ 200 µm–1) of

partial disclinations at the microband boundaries, and local internal stresses reaching σлок ≈

Е/30 (Е being Young’s modulus). It has been shown that important features of the microband

structure are the prevailing reorientation of the microbands around type ⟨110⟩ directions and

the high density of large-angle boundaries with reorientation vectors θ = (50–60)0⟨110⟩. It

has been supposed that these features result from the plastic deformation and reorientation of

the crystal lattice through mechanisms of local martensitic type reversible transformations

(direct plus reverse transformations accompanied by a change of the reverse transformation

system) in fields of high local stresses. The most important factors involved in the new

deformation mechanism and the prerequisites to its realization are discussed, namely, the

degree of phase instability of the material, the intensity of local internal stresses, and the

possibility of the relaxation of these stresses by ordinary plastic flow mechanisms. Theoretical

analysis of the atomic mechanisms and distortions of the above transformations has shown

that the most important features of the carriers of this deformation mode are the absence of

any effective obstacles, under severe deformations included, and the possibility of the high-

defect structural states formed under these conditions to intensely relax. It is supposed that

the combined effect of these two factors underlies the phenomenon of ultrahigh technological

plasticity of the alloys under investigation: very high (practically unlimited) plastic strains can

be achieved by rolling them at room temperature without intermediate annealings.

The work was supported by Administration of Tomsk Region and Russian Foundation of

Basic Researches, grants RFBR № 05-03-98003 and 06-02-16312-a.

249


Methods and Tools for 3D Structure Prediction of Molecular Crystals and Nanoclusters

Alexander Dzyabchenko Karpov Institute of Physical Chemistry, 10 Vorontsovo pole, Moscow 105064, Russian Federation

[email protected]

The ab initio crystal structure prediction (CSP) of a molecular crystal assumes constructing by

theoretical computational methods from the only knowledge of the chemical diagram of the

molecule its full three-dimensional structure stable under the specified P,T conditions. As a

matter of fact, the global lattice-energy minimization results for a given molecule in a list of

structures ranked by energy that are theoretical crystal phases - both known and not yet

experimentally observed. The common methodology is to use the known crystal structures

and their observed physical and thermodynamic properties as experimental base to test and

refine the structure prediction methods in use, in particular to fine tune the semi-empirical

potential energy functions used to compute lattice energy. The quantum chemical methods

still hardly practical to use in global minimization of large molecular ensembles and solids

because of very high computational expense. Up to date they are normally applied to the

consideration of small model systems with the aim to calibrate less sophisticated methods

based on empirical atom-atom potentials. The global energy landscape in the multi-

dimensional parameter space contains all the information necessary to understand and predict

the solid state properties and phenomena that occur in response to a change in external

parameters (pressure, temperature, electric or magnetic field, light irradiation etc.). Along

with the high significance of the CSP problem in fundamental science, the interest to it is

promoted by prospects of application of the computational tools in crystal engineering of new

polymorphs of novel and known pharmaceuticals. In powder X-ray structure determinations,

the CSP tools are frequently used to find a structure solution suitable for its final refinement

by the Rietveld technique. Generally, the CSP methods are being developed towards powerful

computer instruments for molecular materials engineering which incorporate theoretical

energy-based structure simulation methods, screen graphics tools for structure design and

modification, structural databases and programs for structure characterization and comparison

with experiment. Over the several last years, the Cambridge Crystallographic Data Center

have organized several 'blind tests' on organic crystal structure prediction aimed at

independent assessment of the computational methods an programs presented by their authors

and research groups as structure-prediction instruments [1].

250

In this talk, we review the progress in the organic CSP studies made up to date. We report the

principal features of our programs used in the different steps of CSP computations. These

comprise the PMC program for global search of optimal packings in terms of rigid molecular

fragments of generally flexible organic molecules. Another program, CRYCOM, serves for

comparison of crystal structures by matching of crystal-lattice dimensions and rigid body

parameters of molecular fragments. Finally, we present the FitMEP program which serves to

adjust point charge magnitudes and positions to reproduce best the molecular electrostatic

potential from quantum chemical calculation.

Examples of successful structure predictions to be given involve a high-pressure phase of

benzene, C60-monomer structures, high-pressure polymerization products of C60 and C70

fullerenes, the biphenyl phases, the complex salt [Co(NH3)5NO2]Cl2, the Cambridge structure

prediction test structures, and a number of powder X-ray structure solutions based on lattice

energy minimization.

This work was supported by funding from RFBR (project 05-03-32808).

[1] G.M. Day, et al. Acta crystallogr., B61 (2005) 511

251


The Role of Processing and Sample Geometry for Ductility of SPD Nanocrystalline Metals

Arkadiusz K. Wieczoreka,1, Michael J. Zehetbauera,2 , Malgorzata Lewandowskab,3,

Kinga Wawerb,4 and Elias C. Aifantisc,5 a Research Group Physics of Nanostructured Materials, Faculty of Physics, University of Vienna,

Boltzmanngasse 5, A-1090 Wien, Austria b Warsaw University of Technology, Faculty of Materials Science and Engineering,

Woloska 141, 02-507 Warsaw, Poland c Laboratory of Mechanics and Materials Polytechnic School

Aristotle University of Thessaloniki, Greece 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected],

5 [email protected]

Thanks to their outstanding properties, nanocrystalline materials become increasingly

attractive for applications in the industry. Plastic deformation has proved to be very effective

in producing bulk nanostructured metals [1, 2], under conditions of enhanced hydrostatic

pressure and/or low deformation temperatures (“Severe Plastic Deformation, SPD”).

However, the efficiency of different SPD methods to achieve high mechanical properties is

different even when controllable parameters are the same [3]. Some of these mechanical

properties, namely the ductility, also vary if the dimensions of the tested samples are different.

The aim of this paper has been two-fold; (1) to compare the ductility of pure metals and of

alloys, after being processed by two different SPD techniques with the same experimental

parameters, and (2) find out the influence of tensile test sample geometry to the ductility. In

order to follow aim (2), various preparation procedures leading to different kinds of edges of

samples, as well as systematically changed sample geometries (Fig. 2) have been achieved.

For in-situ SEM and/or AFM observation of possible grain boundary sliding during

deformation, a miniaturized tensile device has been constructed in cooperation with the Erich

Schmid Institute in Leoben, Austria.

The lecture reports on ductility tests done at tensile test samples prepared by HPT and HE, i.e.

by two selected SPD techniques which have achieved under equivalent conditions. The

materials chosen were Cu, Al and Al 7475 alloy. Results of measurements will be presented

and analysed in terms of specific differences of the two SPD methods applied. Moreover, the

dependence of ductility on notch depth is demonstrated and attempts are reported to simulate

this dependence by means of the strain gradient theory [4].

252

Figure 1. Tensile test samples cut from the HPT disc

Figure 2. Tensile test samples cut from HPT disc with notches of different depth

[1] R.Z. Valiev, I.V. Islamgaliev, I.V. Alexandrov, Prog. Mater. Sci., 45 (2000) 103

[2] M.J. Zehetbauer (ed.), Adv. Eng. Mater., Special issue on Nanomaterials by Severe

Plastic Deformation 5 (2003)

[3] M. Richert, H.P. Stüwe, M.J. Zehetbauer, J. Richert, R. Pippan, Ch. Motz, A.E.

Schafler, Mater. Sci. Eng. A, 355 (2003) 180

[4] N.A. Fleck, G.M. Muller, M.F. Ashby, J.W. Hutchinson, Acta Metal. Mater., 2 (1994)

475

253


The Grain Boundaries Processes and Micromechanisms of Plastic Deformation of Nanostructured Materials

Alik K. Emaletdinov Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000, Russia

[email protected]

The special mechanical properties of nanostructured materials are defined by properties and

kinetics of lattice and grain boundaries dislocations. The change of lattice dislocations

properties is caused by very anharmonic lattice potential. In this paper the static dislocations

are investigated in anharmonic lattice and their fields of displacement and stress are

calculated. The fields of stress and power characteristics of dislocation pile-ups and

conditions of their interactions will be modified, for example, at formation microcracks, the

maximum density of dislocations and other.

In this work the physical model of grain boundary dislocations are proposed. It is shown that

grain boundary quasidislocations (Somigliana dislocations) are existed in usual boundaries.

The own elastic fields and energy of grain boundary quasidislocations, conditions of

dissociation of lattice dislocations entered into the boundary and time of spreeding are

investigated. The numerical curves of the fields of quasidislocation stress are shown. It is

shown that the using of method of long-acting stress dislocations cause the overstated

evaluations in geometrical models. The own elastic energy of quasidislocations is in two

orders less nearly than one of lattice dislocations. On the basis of kinetic Somigliana

dislocations describes the structural effects and mechanical properties of nanostructured

materials. The physical model of kinetics Somigliana dislocations explains the mechanical

properties of glass materials.

A new synergetic model is proposed for superplastic deformation of nanostructured materials,

based on the processes of self-organization and appearance of collective mode of movement

in kinetic of lattice and grain-boundary Somigliana dislocations. On the basis of these

conceptions a system of equations of kinetic defects is obtained which describes the moving

neck on specimen and principal causes of a number of structural effects which have not been

satisfactory explained before are given. The conditions for superplastic deformation are

obtained. Phase schemes of state of system are constructed. The minimal size of specimen for

the development of superplasticity is calculated.

254


Microstructure, Microtexture and GBs volution in FCC Materials During Ultra-HighSstrain Deformation

A. A. Gimazova,1, Alexander P. Zhilyaeva,b,2

a Institute for Metals Superplasticity Problems, Russian Academy of Science,

39 Khalturin St., Ufa 450001, Russia b Department of Physical Metallurgy, Centro National de Investigaciones Metallúrgicas (CENIM-CSIS),

8, Avda. Gregorio del Amo, 28040 Madrid, SPAIN 1 [email protected] 2. [email protected]

An investigation was conducted to compare the microstructure and microtexture of pure

nickel, aluminum and copper subjected to ECAP, HPT, machining and their combinations,

such as machining of ECAP specimens, HPT of ECAP copper and HPT of machining chips.

Microstructure, dislocation density and microhardness have been evaluated by x-ray,

transmission and scanning electron microscopy, OIM. Influence of different processing routes

is discussed in terms of accumulated strain and microstructure refinement. The

ECAP+machining+HPT sample shows the maximum microhardness value of 2 GPa that is

200% increase compare to initial sample. TEM investigation shows equiaxed grains with a

mean grain size of ~100 nm. In spite of small size grains’ interior still contains dislocations. It

has to be noted that the same deformation way but without prior ECAP gives noticeable

smaller value of microhardness almost equal to that after solely HPT deformation. On the

other hand the difference in microhardness between edge and center of sample is also smaller:

230 MPa for HPT against 150 MPa for machining+HPT. The grain size obtained by TEM is

around 400 nm for HPT sample and 200 nm for HPT processed from machining chips. Figure

1 shows microhardness and microstrain as a function of coherent domain size.

The greater values of microstrain corresponding

to high concentration level of lattice defects are

found for two samples which have machining

as final stage of deformation process:

machining and ECAP + machining. But ECAP

+ machining + HPT and Machining + HPT

processes did not lead to structure with high

microstrain value what means further HPT

deformation “uses” stored defects in boundary

formation. This may be the reason why HPT

50 100 150 200 250 300 350 400 450 500

1000

1200

1400

1600

1800

2000

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Microhardness

Hv,

MPa

Domain size, nm

Microstrain

Mic

rost

rain

, 10-3

Figure 1. Microhardness and micro strain as a function of coherent domain size

255

after machining gives more uniform distribution of microhardness than solely HPT.

Summary:

• The least grain size obtained in pure copper is around 100nm.

• The maximum microhardness value mounts 2GPa (200% of macrocristalline copper

microhardness value)

• High microstrain level appears for machining-ended deformation consequences. During

following HPT deformation microstrain level goes down.

256


Atomistic Simulations of Wedge Disclinations in a [123] Tilt Grain Boundary Albert M. Iskandarova,1, Ayrat A. Nazarova,2

a Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000, Russia 1 [email protected], 2 [email protected]

Atomic computer simulation is one of the most powerful methods for studying of the atomic

structure of defects and processes in materials. At present various properties of nanostructured

materials are studied by means of atomistic simulations. However, in these simulations

usually it is ignored that grain boundaries (GBs) and their junctions in real nanostructured

materials have a nonequilibrium structure and contain linear defects such as extrinsic

dislocations and disclinations. The nonequilibrium state of the GBs can result in an

enhancement of the GB diffusion coefficient in nanocrystals as compared to the diffusion

coefficient along GBs in conventional polycrystalline materials. Atomic computer simulations

can help to elucidate the underlying structural mechanisms resulting in this enhancement of

the diffusion. Another important issue concerning the nonequilibrium GB structure is its

effect on the mechanisms of interfacial crack formation and propagation. In particular, the

disclinations, being stress concentrators, can be sites for a preferential crack formation. And at

last, but not at least, disclinations can result in a local amorphization of crystals. This can

result in a transformation of the nanocrystalline structure into an amorphous one during severe

plastic deformation and ball milling.

Mechanisms of relaxation of wedge disclinations in bicrystalline nanosized cylinders of Ni

and Ti have recently been studied using molecular dynamics simulations [1-3]. These studies

have shown that above a certain critical strength, which depends on the temperature and

radius of the cylinder, the disclinations relax via a crack nucleation. In Ti competing

relaxation mechanisms, viz, new grain formation and disclination core amorphization have

been found to retard the crack formation. The cited studies have been done for disclinations in

special low-index tilt GBs, with axes [001] in Ni and [1100] in Ti.

The aim of the present work is a study of the possibility of a disclination core amorphization

in a more general [123] tilt GB in fcc metals. Simulations were carried out for Ni and Pd

bicrystalline nanowires of 20 nm radius containing a negative wedge disclination. Equilibrium

structures of the nanowires at different disclination strength values and temperatures are

studied. The results show that for both metals the disclinations relax via crack nucleation,

when the disclination strength is above the critical one. When the disclination strength

257

approaches the critical one from below, completely

ordered structures were observed in Ni, while in Pd a

trend for disordering of the disclination core was found

(see figure).

However, the disordered configurations have slightly

higher potential energy in comparison to the ordered

structure that suggests that they are not favored

structures in [123] tilt GBs in fcc metals. The results

imply that tilt GBs in fcc metals have a less trend for the

amorphization through disclinations than hcp metals.

This work was supported by a grant from the Science

and Education Foundation “Intels”, Magnitogorsk,

Russia.

[1] R.T. Murzaev, A.A. Nazarov. Phys. Metals Metallogr., 102 (2006) 198

[2] K. Zhou, A.A. Nazarov, M.S. Wu, Phys. Rev. B, 73 (2006)

[3] K Zhou, A. A. Nazarov, M. S. Wu, Phys. Rev. Letters, 98 035501 (2007)

Figure 1. Amorphized disclination core in[123] tilt GB in Pd

258


Technological Parameters Optimization of the Process for Producing Bulk Nanostructured Semi - Products Made of Ti

Irek V. Kandarova,1, Vladimir V. Latysha,2, Valery M. Polovnikovb,3 and

Gulnaz H. Salimgareevaa,4 a Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000, Russia

b Innovation Scientific Technical Center “Iskra”, 81 Pushkin St., Ufa 450000, Russia 1 [email protected] , 2 [email protected] , 3 [email protected], 4 [email protected]

There are important requirements for the development of new structural metals and alloys.

They are as follows: providing of high technological properties, i.e. high ductility at low

deformation stress and obtaining of limit values of service properties, i.e. structural strength,

high fatigue properties etc. Formation of nanostructures in commercial Ti is an advanced

approach for obtaining such properties. Production of Ti billets industrially requires not only

new structural solutions necessary for producing deforming equipment and die-set, but also

optimization of the technological parameters as there is a necessity to reduce a range of waste

material, an enhance in the process efficiency and enhance in properties stability in case of

mass production.

This work represents the results of investigations and optimization of the combined process

for producing nonostructured semi-products both at SPD stage and in the process of

subsequent strain – thermal treatment.

It was shown that the use of preliminary deformation before ECAP (at SPD stage) allowed

one to enhance a rate of work (process), to decrease the number of pressing passes from 4-8

up to 2. Besides, the quality of billets (a decrease in structured heterogeneity on the rings of

the billet and defective layer) increased.

On the basis of the plastometry investigations there were proposed and investigated several

variants of strain – hardening processing of UFG Ti billets after SPD. The data obtained were

used for creating the process model and the corresponding software (using Qform3D system),

that allows one to carry out a computer process modeling at the most critical stage of the billet

properties formation: SPD (ECAP) → strain hardening (forging broaching).

The results of these investigations made it possible to propose the technological process that

includes procedures of forging broaching, rolling and drawing. The technological process is

optimal from the point of view of productivity, material expenditure and obtainment of the

specified geometry and properties.

259


Atomistic Modeling of the Nanoparticle Agglomeration Ilya I. Karkina,1, Lidia E. Karkinaa,2 and Yury N. Gornostyreva,3

a Institute of Metal Physics, 18 S. Kovalevskaya St., Ekaterinburg 620041, GSP-170, Russia 1 [email protected], 2 [email protected], 3 [email protected]

In past decade, the increasing interest of researchers was turned to bulk nanocrystalline

materials and alloys and, in particular, one that is constructed from nanosize elements

(powders, films). Experimental results show the spontaneity agglomeration of nanoparticles

during sintering of the powder materials and this phenomenon seriously complicate the

fabrication of nanostructural materials with homogeneous space distribution of particles. To

date, the problem of atomic structural transformations during agglomeration two or more

number of nanoparticles is still unstudied both experimentally and theoretically. In recent

investigations the main attention was focus on problem of small clusters with hundred atoms;

while the particles with thousands atoms are especially interesting in context of nanostructural

materials. Currently, there is no sufficient understanding of the relation between the structural

state in the clusters of real materials, the aging regime on the one hand and specific features of

agglomeration on the other hand.

We present the results of the investigation of the mechanisms the metal clusters structural

transformations at agglomeration by molecular dynamic simulations. The modeling was done

for temperature 0,6-0,8 of melting point for nanoparticle Al, Ni, Au with size within the range

of 2-5 nm. We use of the many-body (EAM) potentials to realistic describe of the interatomic

interactions in considered metal particles. The atomic transformations during elementary

sintering step are studied by modeling of association by two disorientated interacting

nanoparticles. We investigated the stability of interparticle boundaries at long time aging

(with duration about few nanoseconds) for different disorientations of particles (low angle,

twin and some special type boundaries). The microscopic mechanism of the spontaneity

agglomeration was studied in details in dependence on temperature and disorientations of

particles. We show that size and size distribution of particles are important factors which

control the powder sintering.

260


Electron Microscopy Analysis of Disclination Defects in Severe Deformed Metals

Volker Klemma,1, Peter Klimaneka,2, Anna L. Kolesnikovab,3, Magsud Masimova,4,

Michael Motylenkoa,5, Alexei E. Romanovc,6 a Institut fϋr Metallkunde, TU Bergakademie, Zeuner St. 5, D-09596 Freiberg, Germany

b Institute of Problems Of Mechanical Engineering,

61 Bolshoi Pr., Vas. Ostrov, St. Petersburg 199178, Russia c Ioffe Physico-Technical Institute,

26 Polythechnicheskaia St., St. Petersburg 194021, Russia 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected],


Plastic deformation of metals up to large strains leads to the formation of fine grain

microstructure with high density of grain boundaries, characteristic lattice rotations and

disclination defects at grain boundary junctions [1-3]. At present, the methods for disclination

identification in deformed metals are not as well developed as those for dislocations or

stacking faults in weakly deformed materials. Transmission electron microscopy (TEM)

images of disclinations have specific features [4] which can be only determined in the

framework of a specially designed experiment. Long-range elastic fields of disclination origin

may also give rise to the pronounced characteristic picture of bending fringes (BFs) near non-

compensated (i.e. disclinated) grain boundary junctions (Fig.1). Bending fringes are global

and good visible objects and their modifications can be easily documented experimentally.

The modeling of the run of BFs next to disclinations is the subject of this report. We have

applied two-beam Howie-Whelan (H-W) approach for the calculation of a compound TEM

contrast caused by disclination defects and a BF near them. To solve the problem we have

introduced a non-zero deviation term in H-W equations. This deviation term accounts for both

the distortions of disclinations and thin foil bending.

261

Our results clearly demonstrate that the TEM contrast of BF is strongly modified near

disclinations (Fig.2). Therefore the analysis of the BF run in the vicinity of various defects in

strongly deformed metals can be proposed as an effective tool for the identification of

disclinations in such structures.

[1] V.V. Rybin, Large plastic deformations and destruction of metals, Мetallurgia,

Moscow, 1986

[2] A.E. Romanov, V.I. Vladimirov, Disclinations in crystalline solids, in: F.R.N. Nabarro

(Ed), Dislocations in Solids, North-Holland, Amsterdam, 9 (1992) 191

[3] R.Z. Valiev, R.K. Islamgaliev, I.V. Alexandrov, Progr. Mater. Sci., 45 (2000) 103

Figure 1. TEM observation of BF distortion near possible disclination location. (a)-(d) consecutive images of the BF run in deformed alloy Cu-Zn (23%). Image size is 1.5x1.5 µm

Figure 2. Modeled TEM image of BF near a disclination dipole. Disclination strength ω=1o; foil thickness 200nm, extinction length ≈gξ 40.9nm.

Coordinates are in units of gξ

262


A Study on Hard Cyclic Viscoplastic Deformation Behavior of Bulk UFG Metals Lembit Kommel

Tallinn University of Technology, Department of Materials Engineering

Ehitajate tee 5, 19086 Tallinn, Estonia [email protected]

The metallic materials with different initial microstructure states were tested by method of

hard cyclic viscoplastic (HCV) deformation [1, 2] with step-by-step strain amplitude

increasing by means of materials testing system INSTRON 8516 at room temperature. For all

test materials the strain-stress, time-strain and time-stress curves at strain amplitudes from

0.05% to 2.5% for 30 cycles were obtained, accordingly. As result of cyclic straining the

microstructure of metals was changed. These microstructural changes of specimens were

studied by means of field emission scanning electron microscope (FE SEM), atomic force

microscope (AFM), transmission electron microscope (TEM) and X-ray (XRD) investigation.

The before tested metals uniaxial stress behavior at high strain rates was studied at different

temperatures by Hopkinson techniques.

Our numerical HCV deformation test results suggest that the as received severe plastic

deformed (SPD) metals, after equal-channel angular pressing (ECAP) show cyclic softening

behavior at constant strain amplitude. By strain amplitude increase the stress amplitude was

increased too at very first cycles for each specimen. SPD-metals with UFG and NC

microstructures after heat treatment up to temperature, when their hardness took decrease,

shows at HCV deformation very stabile but lower strength properties. Their true stress at

break was increased up to 1600 MPa. Heat treated metals mainly shows at first cyclic

hardening behavior and after strain amplitude and cycles number significantly increase shows

cyclic softening behavior only. By this the preliminarily ECA pressed UFG metals strain

hardening behavior was higher. After numerical HCV deformation all metals show identical

values of stress amplitude, lower Young’s module etc. Therefore, this method can be used as

new SPD technique.

Our numerical results of microstructural investigation at different techniques show that ECAP

metals have mainly FG, UFG and NC microstructures. Structural behavior of metals subjected

to HCV deformation was studied and obtained results allow concluding that there is a

structural change on different stages of loading. The rates of grain refining or coalescence

depend on test conditions, first at all, on cycle’s number and strain amplitude. The

effectiveness of structural change is determined by grains boundaries orientation. Grains with

263

low angle boundary disorientation coalesce first at all. Before fracture at low cycle fatigue the

grain boundaries were merged and coalescenced slip bands were formed. In the necking

region at tension before fracture of HCV deformed specimens the grain size and true strength

were increased, proportionately. Such nanostructured materials exhibit the breakdown of

Hall-Petch behavior. Results, received by Hopkinson technique use at low temperatures (-

190°C) or higher strain rate (104s-1) shows higher strain rate sensitivity and resistance to

deformation. By X-ray investigation of tested materials before and after different testing

features was established, that microstructure, mechanical- and physical properties have good

correlations with level of interatomic interaction of metals.

[1] L. Kommel, in: Y.T. Zhu, T.G. Langdon, R.Z. Valiev, S.L. Semiatin, D.H. Shin, T.C.

Lowe (Eds.), TMS (2004) 571

[2] L. Kommel, R. Veinthal, Rev.Adv.Mater.Sci. 10 (2005) 442

264


Formability of Nanocrystalline VT6 Titanium Alloy Sheet Alexey A. Kruglov1, Ramil Ya. Lutfullin2, Oleg A. Rudenko3 and Rinat V. Safiullin4

Institute for Metals Superplasticity Problems, 39 Khalturin St., Ufa 450001 Russia 1 [email protected], 2 [email protected], 3,4 [email protected]

Superplastic forming (SPF) is one of advanced methods for producing thin sheet parts.

However, the use of rather high processing temperatures especially for titanium alloys retards

its wide application. The effect of “low temperature superplasticity” [1] provides decreasing

forming temperature. The advantages attained in creation of nanocrystalline titanium alloys

including the opportunity to produce sheets of a commercial size provide wide prospects for

developing efficient SPF base methods [2].

Formability is an important

processing characteristic for

commercial application of

nanocrystalline sheets. The present

paper considers the SPF of

hemispherical and cylindrical

samples.

A sheet of VT6 (Ti-6Al-4V)

titanium alloy with a mean grain

size of 200 nm, 0.8 mm in thick,

was taken for investigations.

Forming was realized in a

cylindrical die, 70 mm in diameter

and 35 mm in depth. The die was equipped with a device for control of dome [2]. For

evaluating the formability of the sheet the forming time of hemisphere with the radius R=35

mm was taken into account. The forming time was measured as the time required for the sheet

to touch the die bottom. The formability has been investigated within a wide temperature

range from 500 to 800°C under constant pressure (Fig.1). The dome with height H=16 mm

was formed at the temperature 500°C for 60 min. Fig.2 shows the hemispherical sample

formed at the temperature 650°C.

For VT6 titanium alloy sheets with a mean grain size of 200 nm the temperature range 600-

650°C is recommended, since SPF carried out at these temperatures requires minimum time

Figure 1. Various forming time of hemisphere (R=35 mm) on temperature

0

5

10

15

20

25

30

550 600 650 700 750 800 Temperature (°C)

Form

ing

time

(min

.)

P-const

265

and creates conditions providing the absence of alpha case. Fig.3 shows cylindrical samples

formed at the temperature 650°C (left) and 600°C (right).

The obtained results show that the nanocrystalline grain size provides decreasing SPF process

temperature of titanium alloy sheets.

Figure 2. Hemispherical sample formed at the temperature 650° C

Figure 3. Cylindrical samples formed at the temperature 650° C (left) and 600° C (right)

[1] R.Z. Valiev, O.A. Kaibyshev, R.I. Kuznetsov, R.Sh. Musalimov, N.K. Tsenev, Doklady

AN SSSR, 301 (1988) 864 (in Russian)

[2] G.A. Salishchev, R.M. Galeyev, O.R. Valiakhmetov, R.V. Safiullin, R.Ya. Lutfullin,

O.N. Senkov, F.H. Froes, O.A. Kaibyshev, Materials Technology & Advanced

Performance Materials, 15 (2000) 133

[3] F.U. Enikeev, A.A. Kruglov, Int. J. Mech. Sci., 37 (1995) 473

266


Mechanisms of Nanostructured States Formation in Austenitic Stainless Steels under Severe Plastic Deformation

Igor Yu. Litovchenkoa,1 , Natalia V. Shevchenkoa,2 , Alexander N. Tyumentseva,3 and

Alexander V. Korznikovb,4 a Institute of Strength Physics and Material Science SB RAS,

2/1 Akademicheskii Av., Tomsk 634021, Russia b Institute for metals superplasticity problems RAS, 39 Khalturin St., Ufa 450001, Russia

1 [email protected] , 2 [email protected] , 3 [email protected] , 4 [email protected]

A defective substructure of austenitic stainless steels after high pressure torsion in Bridgman's

anvils and cold rolling deformation was examined by transmission electron microscopy. It is

shown that the defective structure of steels was formed by the intersection of deformation

twins of one or more systems and strain localization bands with high-angle misorientation

boundaries. The particles of α and ε deformation martensite both inside the twinning structure

and strain localization bands were found. However, a small volume fraction of these phases

was not revealed by X-ray diffraction.

It is also shown that at the true logarithmic strain e> 3, a fragmented structure is observed

with the fragments being of submicrocrystalline and nanocrystalline scale. In this structure

high curvature-tensor components (up to χij ≈ 200 µm–1) of the crystal lattice and the

corresponding local internal stresses as high as σloc ≈ Е/30 (Е - Young’s modulus)

determined.

The presence of the magnetic α - phase at rolling deformation ε ≥ 60 % in the

02Cr17Ni14Mo2 stainless steel is verified by magnetic methods. The content of the α - phase

increases with deformation and amounts to ~ 0.5 % at the deformation ε = 99 %. The

formation of α- martensite correlates with that of strain localization bands with high-angle (θ

≈ 600<110>) reorientation. A concept of formation of strain localization bands and

deformation twins in austenitic steels due to the mechanism of local reversible

(fcc→bcc→fcc) transformations of martensitic type [1, 2] is discussed.

Based on the results obtained the mechanisms of formation of submicro- and nanocrystal

structural states at severe plastic deformation are discussed.

This work was partially supported by the Ministry of Education of the Russian Federation and

CRDF within the framework of program BRHE (project No. 016-02), grant of president of the

Russian Federation MK-7459.2006.8, grant of the RFBR No. 06-02-16312-a.

267

[1] A.N. Tyumentsev, I.Yu. Litovchenko, Yu.P. Pinzhin, etc. The Physics of Metals and

Metallography, 2 95 (2003) 86

[2] A.N. Tyumentsev, I.Yu. Litovchenko, Yu.P. Pinzhin, etc. The Physics of Metals and


268


Fracture of Bulk Ultrafine Grained Aluminum Alloys Processed by Severe Plastic Deformation

Michael V. Markusheva,1 and Maxim Yu. Murashkinb,2 a Institute for Metals Superplasticity Problems, Russian Academy of Sciences,

39 Khalturin St., Ufa 450001, Russia b Institute of Physics of Advanced Materials, Ufa State Aviation Technical University,

12 K.Marx St., Ufa 450000, Russia 1 [email protected], 2 [email protected]

The features of room temperature failure in 20mm plates of commercial aluminum alloys

1560 (Al-6.5Mg-0.6Mn) and 5083 (Al-4.4Mg-0.7Mn-0.15Cr) with fragmented and grained

submicrocrystalline (SMC) (d~0.4-0.5µm) and microcrystalline (MC) (d~5-8µm) structures

processed by severe plastic deformation (SPD) via complex angular extrusion and following

annealing are considered.

The analysis of pre-polished surfaces of tensile specimens has shown that irrespective the

alloys microstructure the fracture starts by microcracks opening in coarse primary particles of

excess phases and their penetration into the aluminum matrix. At strain increase the new

cracks are formed predominantly by particles brittle failure and the occurrence of these

processes is more intense in the SMC alloys (Fig.

1).

It has been revealed that less resistance to cracks

formation in submicrocrystalline structures is

attributed to earlier and stronger localization of

plastic deformation in aluminum matrix which is

accompanied by formation of coarse shear bands

passing through hundreds of grains. Concurrently

with the development of such a band structure

homogeneous initiation of cracks at grain

boundaries and in triple junctions occurs. As for

the MC alloys, the conditions for stress cracks

formation are realized at later stages of plastic deformation (at higher strains) in short

individual slip lines compatible with their grain size.

The data on qualitative analysis of the path of macrocrack growth and quantitative estimation

of energy expenditures spent on formation of free fracture surfaces (via surface area) and

plastic deformation at the crack tip (via size of plastic deformation zone (PDZ)) under

Figure 1. Microcrack density vs tensile strain of the 1560 alloy

Степень деформации, %0 2 4 6 8 10 12 14 16

Плотность

микротрещ

ин, м

м-2

0

200

400

600

800

1000

аб

Плотность трещин в сплаведо растяжения

SMCМC

Strain, %

Mic

rocr

ack

dens

ity, m

m-2

After SPD

269

bending tests are shown and discussed. In particular, it has been established that

transformation of the fragmented SMC structure into the MC grained one due to post-SPD

annealing leads to the change in the character of failure from brittle intercrystalline to ductile

transcrystalline one. This is accompanied by the deviation of the crack from the normal to the

axis of applied tensile stresses and the increase in crack surface area.

It has been concluded that the main factor determining the resistance to crack growth in SMC

and MC alloys is the size of PDZ at the crack tip. The dependences of PDZ size and crack

resistance parameters on structure of the alloys matrix are similar: the PDZ size and the

specific works of the alloys failure and crack growth are smallest in the SMC fragmented

materials, while in the MC alloys they

are largest (Fig. 2).

Besides, the fracture character essentially

depends on the observed distinction in

the changes of the alloys phase

composition upon annealing. Thus,

unlike 1560, the formation of SMC grain

structure in 5083 alloy upon low

temperature post-SPD annealing was

accompanied by the dissolution of

second β-phase (Al3Mg2) precipitates

formed at deformation processing and

the proportional increase in the PDZ size

and SMC alloy resistance to crack

growth which values become close to the ones apt to the alloy with MC structure.

Figure 2. Specific work for specimen failure and crack growth vs the size of plastic deformation zone at the crack tip in the 1560 alloy

050

100150200250300350400

2 3 4 5 6 7

PDZ size, mm

Spec

ific

wor

k, k

J/m

2

Роста трещины

РазрушенияММCC

SSMMCC

Crack growth Fracture

270


Pressure Welding of VT6 Titanium Alloy under Conditions of Low Temperature Superplasticity

Minnaul Kh. Mukhametrakhimov, Ramil Y. Lutfullin and Amir K. Galimov Institute for Metals Superplasticity Problems, RAS, 39 Khalturin St., Ufa 450000, Russia

[email protected]

The essential role of deformation processes occurring at pressure welding under conditions of

“low temperature superplasticity” arises urgent necessity in thorough analysis of the strain-

stress state in the zone of joining.

Conditions of low temperature

pressure welding have been modeled

using FE-code ANSYS 5.7.

The goal of investigations was to

optimize the neck geometry and

parameters of the strain-stress state of

contacting processed sheets for

analyzing processing and reducing

sound composition to components of

one-axial interaction. Such an

approach allows performing controlled

deformation in the zone of welded

joint and provides optimal

(appropriate) temperature-strain rate

conditions of deformation under

relatively low pressure.

Unlike the sample with the constant cross section area the sample with the alternating area is

distinguished by the at once occurrence of local deformation since the laws of its development

are different in these samples. Thus, varying the sample shape one can control the time of

neck occurrence and change the laws of its development. At that the basic deformation is

localized in the zone of joint providing the best weldability of the construction. Imparting a

cylindrical shape to the welded sample with a less cross section area promotes steady

localization of plastic flow.

Figure 1. Normal stress in the sample with the constant cross section area and marked near-contact zone

271

a b Figure 2. Micrographs of the solid state joint for the VT6 alloy after pressure welding at 600 (а) and 650оС (b)

under condition s of low temperature superplasticity

Using the results of modeling a full-scale experiment was carried out. The experiment on

pressure welding of bulk samples of nanocrystalline titanium alloy VT6 at 600oC was aimed

to produce a joint having a tensile strength above 1150 MPa at room temperature.

One of the most important processing aspects is application of nano- and submicrocrystalline

alloys as sheet inserts for solid state joining of large-scale bulk semi-products. The decreased

flow stress of the NC material of the insert under similar temperature conditions provides

localization of superplastic deformation in the zone of joining. Moreover, the processed

mechanical properties of the zone of joining are similar to the ones of the welded semi-

products.

0

200

400

600

800

1000

1200

0 2 4 6 8 10 12 14 16 18 20

Степень деформации, %

Напряжения

течения

, МПа

- а) - б)

а b c

Figure 3. Micrographs of solid state joint (а;b) and mechanical properties of VT6 alloy (c) after pressure welding at 650оС using sheet inserts of nanocrystalline (a) and submicrocrystalline VT6 alloy (b).

The application of nanocrystalline and submicrocrystalline sheet inserts provides sound solid

state joining at lower temperature (T) and welding pressure (P) as compared to traditional

superplasticity. The obtained result is of practical importance for machine building and open

premises for developing recourse saving processing methods applying nanocrystalline

materials.

272


Processing and Mechanical Properties of Bulk Nanostructured Nickel-Based Alloys

Shamil Mukhtarova,1, Nadezhda Dudovaa,2 and Vener Valitova,3 a Institute of Metals Superplasticity Problems RAS, 39 Khalturin St., Ufa 450001, Russia


The present paper deals with investigation of nanostructured nickel base alloys processed by

severe plastic deformation via multiple forging and high pressure torsion. The nanostructured

nickel base alloys - single-phase Ni-20%Cr and precipitation hardened Inconel 718 with a

mean grain size of 50-80 nm have been studied in terms of their thermal stability and

mechanical properties.

The nanocrystalline (NC) structure is characterized by high internal stress (relative

microdeformation about 0.33% of Ni-20%Cr) and microhardness being higher than the

microhardness of coarse-grained alloy (by a factor of 4 and 2, correspondingly, for Ni-20%Cr

and Inconel 718). The strongly nonequilibrium state of NC structure is responsible for

microstructure changes upon heating. It was established, that the NC structure of the Ni-

20%Cr alloy can be considered as thermally stable to 500oC (0.46Tm), which is lower than the

temperature threshold for the retention of this structure in the Inconel 718 (580oC = 0.5Tm)

[1]. Annealing of the alloys under study up to these temperatures results in occurrence of

recovery processes, which in turn results in a decrease in internal stresses, which is

accompanied by the decrease in the microhardness and the relative microdeformation. But in

the Ni-20%Cr alloy the anomalous increase in the microhardness (5.2-5.4 GPa) observed at

annealing temperatures of 400—500oC is probably connected with a short-range ordering in

the structure at these temperatures.

The increase in the annealing temperature of investigating alloys to 600oC causes static

recrystallization and transformation the NC structure to the submicrocrystalline (SMC) one.

It has been established that, alloy Inconel 718 with NC structure displays features of low

temperature superplasticity at 600°C and strain rate 1.5⋅10-4s-1. A value of relative elongation

is 350% and the strain rate sensitivity coefficient m is 0.37. The specific feature of NC

structure is more uniform strain distribution as compared a coarse-grained structure.

[1] V.A. Valitov, Sh.Kh. Mukhtarov, Yu.A. Raskulova, The Physics of Metals and


273


Vacancies in the Stress Fields of Disclinations: Relation to Bulk Nanomaterials Ramil T. Murzaev1 and Ayrat A. Nazarov 2

Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000, Russia 1 [email protected], 2 [email protected]

At present the mechanisms of grain boundary (GB) diffusion in nanostructured metals is one

of the least understood phenomena. Some experimental data indicate on a many orders of

magnitude enhancement of the GB diffusion coefficient in bulk nanomaterials as compared to

that in ordinary polycrystals, while others shown only an insignificant increase. Theoretical

analyses available in literature are mainly of a phenomenological character and do not provide

a sufficiently clear insight into the problem.

Atomistic computer simulation of grain boundaries and their junctions is an important tool for

elucidating the mechanisms and kinetics of diffusion in nanomaterials. In particular, an

information obtained on the energetics of point defects in GBs and their dependence on the

GB structure is very useful for an approximate comparison of the GB diffusion coefficients in

equilibrium and nonequilibrium GBs. Recently the authors have studied vacancy formation

and migration energies in special [001] tilt GBs in Ni containing extrinsic GB dislocations

and disclinations [1-3]. However, most of the GBs in real nanomaterials have a general

character and study of the properties of vacancies in such GBs seems very important.

The present paper is devoted to a computation and analysis of the vacancy formation energies

in a general twist GB in Ni containing a positive or negative wedge disclination with the

strengths equal to ω=-5.0° and ω=5.0°, respectively. The GB has a plane (310) and twist

misorientation angle 76.7°. The zero-temperature relaxation resulted in a fairly wide (7 Å) GB

region with an amorphous atomic structure. For this thickness, one period of the GB contains

269 atoms. By a successive removal of each of these atoms followed by relaxation, vacancy

formation energy on each site was calculated for this equilibrium GB. By a removal or

insertion of a wedge of material into one half of the GB positive and negative wedge

disclinations were created in the GB. The displacement fields of the disclinations were

calculated for a cylinder with radius R=100 nm in order to catch the influence of disclinations

on vacancy energies in bulk nanomaterials with the grain size of the same order. Then the

vacancy energies on all 269 sites were calculated for the GB periods lying on distances of 5.4,

9.0, and 12.6 nm from the disclination line.

274

The results show that the GB vacancy

energies have a bimodal distribution

(see, for example, the figure). In the

equilibrium GB the first peak is at 0.18

eV and the other at 1.33 eV (the lattice

vacancy energy is equal to 1.63 eV for

the embedded atom method used). The

stresses of a negative disclination result

in a 0.15 eV shift of the low-energy peak

to the left, while those of the positive disclination result in an opposite shift to the same value.

The vacancy formation energy averaged over all 269 positions is equal to 0.84 eV in the

equilibrium GB, 1.00 eV in the GB with negative disclination and 0.65 eV in the one with

positive disclination. Thus, the disclination stress fields can result in a significant, up to 0.2

eV change of the vacancy formation energies in general GBs.

[1] R. T. Murzaev and A. A. Nazarov, Phys.Metals Metallogr., 100 3 (2005) 228



Figure 1. The distribution of vacancy formation energies in an equilibrium (310) twist GB in Ni

275


Diffusion-Controlled True Grain-Boundary Sliding in Nanostructered Metals and Alloys

Evgeny F. Dudareva,1, Galina P. Pochivalovaa, Yury R. Kolobov b,

Evgeny V. Naidenkinc and Oleg A. Kashinc a Siberian physical technical institute TSU, 1 Novosobornaya Sq., Tomsk 634050, Russia b Center for NanoStructured Materials and Nanotechnologies, Belgorod State University,

85 Pobeda St., Belgorod 308015, Russia c Institute of strength physics and materials science SB RAS,

2/1 Akademicheskii Pr., Tomsk 634021, Russia 1 [email protected]

One of the physically independent initial mechanisms of unelastic deformation of

polycrystalline metals and alloys is true grain boundary (GB) sliding controlled by diffusion

along GB. As a result of such unelastic deformation the appearance of grain boundary internal

friction is observed. Taking into account the above mentioned the internal friction method is

used in the present work to investigate of true grain boundary sliding in nanostructured metals

and alloys produced by severe plastic deformation. On the example of pure Ti, Al and

different alloys based on the metals it has been established that grain structure refinement by

means of severe plastic deformation down to nanoscale sizes results in displacement of grain

boundary internal friction peak to the range of lower temperatures. The data show the

decrease of temperature of onset and development of true grain boundary sliding as a result of

change of GB state from equilibrium in coarse grained materials to non-equilibrium in

nanostructured materials.

The activation energy Q of true GB sliding determined by rising branch of grain boundary

peak of the internal friction is reduced with a transition from coarse-grained structure to

nanostructure. The decrease of Q depends on increase of energy of non-equilibrium grain

boundaries. The value of activation energy of true grain boundary sliding is higher then

corresponding values for activation energies of GB diffusion for coarse-grained structure as

well as for nanostructure. At the same time the Q value is lower then activation energies of

volume diffusion in the both states. The influence of structure of high angle random grain

boundaries on true grain boundary sliding and activation energy of the process is arise from

diffusion model where true GB sliding considered as thermo-activated process obeyed by the

micro-mechanism like GB diffusion. The external stress results only in transition from

random transference of vacancies to their directional movement. It will be accompanied by

mutual displacement of adjacent grains in the case of rebuilding of GB’s resulting in decrease

276

of boundary energy by diffusional mass transfer between boundary and grain volume. In

consequence of that the activation energy for true grain boundary sliding must exceed the

corresponding value of activation energy of GB diffusion but still less than activation energy

of volume diffusion.

277


The Model of Anomalous Grain Growth in Submicrocrystalline Metalls and Alloys Produced by Severe Plastic Deformation

Vladimir Perevezentseva,1, Alexander Pupynina,2 a Blagonravov Institute of mechanical engineering RAS, Nizhny Novgorod Branch, 85 Belinskiy St.,

Nizhny Novgorod 603025, Russia 1 [email protected] , 2 [email protected]

As it is known [1-2] that anomalous grain growth often observed in submicrocristalline

(SMC) materials produced by severe plastic deformation method. During short time annealing

grains whose size essentially exceeds this one of “matrix” grains appear in the structure of

material. Fraction of these grains depends on annealing temperature and time and it varis from

zero to 100%. Therefore, it is possible, using short time annealing, purposely create bimodal

structure in SMC materials with necessary ratio of fine and coarse grains and, thus, to

influence strength and plastic properties of material. The model to describe anomalous grain

growth in SMC materials is developed. It is based on the concept of grain growth boundaries

transfer into nonequilibrium state [3] according to generation of nonequilibrium vacancies

during sweeping of adjacent grain boundaries (GB) by migrating GB. Kinetics of

nonequilibrium GB migration and anomalous grain growth in SMC copper containing

disperse particles of second phase during annealing at the temperature T=0.4Tm, different

initial values of grain size deviations d∆ from grain size of “matrix” (d = 0.25 mkm),

dislocation densities in GB boρ and volume fractions of second phase fv are numerically

analyzed (Fig.1). It is shown that increase of d∆ , fv and decrease of boρ lead to decrease of

incubation period of anomalous grain growth (Fig.2).

[1] J. Lian, R.Z. Valiev, B. Baudelet, Acta Metal. Mater., 43 (1995) 4165

[2] N.M. Amirkhanov, R.K. Islamgaliev, R.Z. Valiev, Phys. Met. Metallogr., 86 (1998) 296

[3] V.N. Perevezentsev, Phys. Met. Metallogr., 93 (2002) 207

278

0 1 2 3 4

4

8

12

160d

d

1

2

3

4

210t ⋅ , s

Figure 1. The kinetics of anomalous grain growth at different volume fraction of disperse particles: (1) – fv = 0; (2) – fv = 0.005; (3) – fv = 0.01, (4) – fv = 0.025 at d∆ /do=0.5; boρ b=2.5⋅10-3; Rp=50nm, where

Rp – disperse particle size

a

b

Fig.2. The dependence of incubation period tL on the deviation value d∆ at tbo b∆ρ =2.56⋅10-3 (a) and initial

dislocation density boρ (b) at different values of volume fraction of disperse particles fv:

(1) – fv = 0; (2) – fv = 0.005; (3) – fv = 0.0125; (4) – fv = 0.025; d∆ /do=0.5

0 0.5 1 1.5 2 2.5

4

6

8 s,10t 2L ⋅

1 2

3

4

20b 10b −⋅ρ

0 0.2 0.4 0.6 0.8 1

0.4

0.8

1.2

1.6

0dd∆

s,10t 3L ⋅

1

2 3

4

279


Ultra Grain Refinement of High Purity and Particle Containing Aluminum Alloys by Accumulative Roll Bonding

Abbas Akbarzadeh1 and Hadi Pirgazi2 Sharif University of Technology, Department of Materials Science and Engineering,

Azadi Ave., P.O. Box 11365- 9466, Tehran, Iran 1 [email protected], 2 [email protected]

Abstract: In the present study, accumulative roll bonding (ARB) as a severe plastic

deformation (SPD) method was carried out on commercial pure aluminum (AA1100) and a

particle containing alloy (AA3003). Electron back-scattered diffraction (EBSD) technique

was utilized to investigate the effects of second phase particles on microstructural and

microtextural evolution in the ARBed sheets. The results indicate that the development of a

strong texture during the ARB process leads to

unrefined bands in AA1100 alloy and transition of

microstructure to a submicron grain structure occurs at

final stages of the process (Fig. 1(a)). It was also

found that large lattice rotation around the second

phase particles in AA3003 alloy leads to the increase

of local misorientation and production of new high

angle grain boundaries. The presence of these

particles prevents developing a strong texture and

improves the grain refinement during the process.

This results in a more homogenous microstructure of

ultra-fine grains in AA3003 alloy (Fig. 1(b)). By

reducing the grain size in both alloys, the hardness of

the sheets increased more than 2 times of the initial

values. The hardness data held Hall-Petch relationship and were in a good conformity with

the microstructural changes.

Keywords: accumulative roll bonding, ultrafine grains, aluminum sheets, second phase

particles, texture and microstructure evolution

Figure 1. The orientation scans of the samples processed by 6 ARB cycles. (a) AA1100 and (b) AA3003

280


Mechanical Behavior of Nanostructured Ti6Al4V ELI Alloy under Tension and Compression at 4.2, 77 and 300 K

Elena D. Tabachnikovaa,1, Aleksey V. Podolskiya,2, Vladimir Z. Bengusa,3,

Sergey N. Smirnova,4, Kornel Csachb,5, Jozef Miskufb,6, Lilia R. Saitovac,7 and

Irina P. Semenovac,8 a B. Verkin Institute for Low Temperature Physics & Engineering,

47 Lenin Av., Kharkov 61103, Ukraine b Institute of Experimental Physics, SAS, 47 Watsonova St. , Kosice, 04353, Slovakia

c Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000, Russia 1 [email protected], 2 [email protected], 3 [email protected],

4 [email protected], 5 [email protected], 6 [email protected], 7 [email protected], 8 [email protected]

Comparison of mechanical characteristics and failure peculiarities of Ti-6Al-4V ELI alloy at

300, 77 and 4.2 К under uniaxial compression and tension in the different structural sates [1]

was done. In the initial coarse grained state 1, average grain size (d) of α-phase is 10-25 µm.

Ultra-fine grained state 2 was produced from the state 1 through equal channel angular

pressing (ECAP) and has d value of ~ 0.5 – 1 µm. State 3 was obtained from the state 1 by the

thermal treatment, ECAP and an extrusion; average size d of α-graines is from 200 nm to

400 nm. Mechanical characteristics were investigated with a stiff testing machine, at strain

rate 5⋅10-4 s-1 , at 300, 77 and 4.2 K. Specimens for compression had a rectangular shape

(2×2×7 mm), and specimens for tension have dog-bone shape with the 5.5 mm gage length

and a square cross section of 0.75 × 2.4 mm2.

It is established that reducing of the grain size from the state 1 (d~ 10-25 µm) to the state 2 (d

~ 0.5 – 1 µm) leads to increasing of the yield stress σ0 and strength σf up to 50 % in the entire

temperature interval, moreover σ0 and σf values at the compression exceed corresponding

values under tension. Further decrease of the grains size (state 3) down to 200 nm - 400 nm

results in the additional increment of σ0 and σf for 25% in comparison with state 2.

In states 2 and 3 ductility of the material is about 3 – 4 % at 300 and 77 К. Fracture at 4.2 К

took place without macroscopic plastic deformation under σ~ 1.5-1.6 GPа, that corresponds to

the yield stress in the initial state at 4.2 К. Observed differences in the strength and ductility

characteristics of the Ti-6Al-4V ELI alloy, apparently, dependent on additional barriers for

dislocations motion in the 2 and 3 states: these are twins colonies (state 2) and additional

grain boundaries, which number increases essentially in the ultrafinegrained state 3 and (in a

lesser degree) in the state 2. Values of activation volume V for dislocations motion, obtained

from a stress relaxation data, are practically independent on the alloy structural state. Thus, at

281

the yield stress at 300 K and 77 К V ≈ 3.5x10-28 m3 and V ≈ 0.9x10-28 m3, correspondingly.

SEM fractographic analysis of the fracture surfaces of the ultrafinegrained Ti-6Al4V ELI

specimens shows that at 300, 77 and 4.2 К ductile failure takes place at the microscale.

[1] I. P. Semenova, L. R. Saitova, G. I. Raab, A. I. Korshunov, Y. T. Zhu, T. C. Lowe, R.Z.

Valiev, Material Science Forum, 503-504 (2006) 757

282


Severely Cold-Rolled and Annealed Ti-Ni Shape Memory Alloys: Structure, Transformations and Functional Properties

Sergey Prokoshkina,1, Vladimir Brailovskib,2, Karine Inaekyana, Vincent Demersb,

Irina Khmelevskayaa, Sergey Dobatkinc and Evgeniy Tatyanind

a Moscow State Institute of Steel and Alloys, 4 Leninskiy Pr., Moscow 119049 Russia b Ecole de Technologie Superieure, 1100 Notre-Dame Ouest, Montreal (Quebec) H3C 1K3, Canada

c Baikov Institute of Metallurgy and Material Science of RAS, 49 Leninskiy Pr., Moscow 119049 Russia d Institute for High Pressure Physics of RAS, Troitsk, Russia


Ti-Ni shape memory alloys (SMA) subjected to low-temperature thermomechanical treatment

(LTMT) by cold rolling with true strains encompassing moderate (e=0.30) and severe (e=1.5-

2.1) deformations were studied using TEM, DSC, microhardness tests at room temperature,

mechanical tests, and functional properties determination. Post-deformation annealing (PDA)

allows obtaining the following structures in a deformed material: recovered and polygonized

dislocation substructures (after LTMT with moderate strain) or nanocrystalline structure of

austenite (after LTMT with severe plastic deformation).

The best combination of mechanical properties (critical stresses for B2(R)→B19’

transformation or reorientation of B19’-martensite, σtr, yield stress of B19’-martensite or B2-

austenite, σy) and functional properties (maximum recovery stress, σrmax, and completely

recoverable strain, εr,1max) was obtained for the nanocrystalline structure (50-100 nm) as

compared to polygonized dislocation substructure with the same size of grains and subgrains

(Table 1). The maximum completely recoverable strain obtained after tensile deformation is about 6%

for Ti-50.0%Ni alloy and 7% for Ti-50.7%Ni alloy, both nanocrystalline. The maximum

recovery stress value obtained for both nanocrystalline Ti-50.0%Ni and Ti-50.6%Ni alloys is

about 1400 MPa. To all appearances, 1400 MPa is the higher limit or recovery stress value for

binary Ti-Ni alloys.

Knowledge of time-stability of as-deformed structure and also nanocrystalline structure

obtained after post-deformation annealing will be useful for practical application. Using DSC

method allows simple estimating the amorphous phase fraction and predicting the properties

of the alloys.

This work was carried out under partial financial support of the Federal Research and

Development Program “Development of Scientific Potential of the Higher School” from the

283

Ministry of Education and Science of Russian Federation and Natural Science and

Engineering Research Council of Canada.

1)Ti-50.6%Ni alloy subjected to e=1.5+773K; 2)Ti-50.6%Ni alloy subjected to e=1.9+723K; 3)Ti-50.6%Ni alloy

subjected to e=1.5+673K

Table 1 - σyM and σtr values (at -18 °C), σr

max and �r,1max obtained after various thermomechanical treatments

Alloy LTMT

strain, e

PDA

temperature, K

Structure σtr,

MPa

σyM,

MPa

σyM-

σtr,

MPa

σrmax ,

MPa

�r,1max,

%

973 Recrystallized 190 620 430 490 2.2

673 180 1100 920 890 5.4

0.3

623 Polygonized

180 1300 1120 910 4.4

973 Recrystallized 200 580 380 460 2

Ti-

50.0%Ni

1.9

673 Nanocrystalline 140 1600 1460 1420 6.2

973 Recrystallized 120 540 420 470 4 0.3

673 Polygonized 110 1100 990 820 5.4

973 Recrystallized 130 560 430 - -

773 Nanocrystalline 100 1260 1160 8501) 7.2

723 220 1570 1350 13802) 5.9

Ti-

50.7%Ni

1.7

673 Nanocrystalline

290 1880 1590 10203) 4.1

284


Effect of Severe Plastic Formation on Tensile Properties and Impact Toughness of a Zinc-Based (Zn-40wt.%Al) Alloy

Gencaga Purceka,1, Onur Saraya,2, Ibrahim Karamanb,3 and

Tevfik Kucukomeroglua,4

a Department of Mechanical Engineering, Karadeniz Technical University,

61080-Trabzon, Turkey b Department of Mechanical engineering, Texas A&M University,

College Station, TX 77843-3123, US 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected]

In recent years, Zn-Al alloys have emerged as a potential cost- and energy-effective, and

environmentally friendly system for substituting several ferrous and non-ferrous alloys in

various engineering applications. However, their low strength, very poor ductility and low

impact toughness limit the use of these alloys. Some traditional methods like alloying, heat

treatment and composite forming have been used; however, the resulting improvements in

these properties have not been satisfactory. In this study, the effect of equal-channel angular

extrusion (ECAE) processing at 130°C on tensile properties and impact toughness of two-

phase Zn-40Al alloy were investigated for up to four passes via route BC in order to improve

their inferior properties. Fracture behaviors of processed and un-processed alloy after tensile

and impact tests were also examined. As a result of multi-pass ECAE, elongation to failure

increased significantly with increasing the number of ECAE passes as shown in Fig.1(a). The

sample extruded 4 passes exhibited 88% elongation to failure at room temperature, which

were 13 times higher than that of the as-cast alloy. The ECAE also increased the strength of

the alloy after one pass, however, higher number of passes led to drop in the strength

(Fig.1(a)). Moreover, the impact toughness of the alloy was improved by multi-pass ECAE

due to the increased ductility as well as smaller fracture dimples as shown in Fig.1(b). By

means of multi-pass ECAE, Zn-40Al casting alloy having brittle fracture behavior were

transformed into the tougher alloy having typically ductile fracture behavior as shown in

Fig.2. These results indicate that the multi-pass ECAE is effective in improving the tensile

elongation and impact toughness of binary Zn-Al alloys.

285

Number of passes0 1 2 4

Stre

ngth

(MP

a)

100

150

200

250

300

350

Elo

ngat

ion

to fa

ilure

(%)

20

40

60

80

100

Tensile strengthYield strengthElongation to failure

Number of passes0 1 2 3 4

Impa

ct to

ughn

ess

(kg.

cm/c

m2 )

40

60

80

100

120

140

a b

Figure 1. The effect of multipass-ECAE on (a) strength and elongation to failure, and (b) impact toughness of Zn-40Al alloy

Figure 2. Appearance of the failed tensile specimens and their fractured surfaces. a) As-cast (0 P), b) one pass

(1 P), c) two passes (2 P) and d) four passes (4 P)

286


Dissipation and Deposition Discontinuous Harden Particles in Cu-1Cr-0.7Al-0.2Zr Alloy by ECAP

Svetlana N. Faizova1, Vladimir V. Latysh2, Valery N. Danilenko4, Elena A.

Sarkeeva3,a and Irek V. Kandarov2 1 Instititute of Mechanics, Ufa Science Center, Russian Academy of Sciences,

71 Oktyabr Pr., Ufa 450001 Russia 2 Designing Technological Bureau “Iskra”, 81 Pushkin St., Ufa 450000 Russia 3 Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000 Russia

4 Institute for Metals Superplasticity Problems, 39 Khalturin St., Ufa 450001 Russia a [email protected]

Chromium bronzes are a special class of high-copper alloys combining in their properties high

electrical conductivity with high strength what makes them attractive for many industrial

applications. Chromium bronzes of the Cu-Cr-Zr system are the precipitation hardening

alloys. The optimal composition of physical and mechanical properties for these materials

may be reached through the specific combination of mechanical and thermal treatments.

Using the severe plastic deformation (SPD) methods, particularly the multi-pass equal

channel angular pressing (ECAP), with the appropriate thermal treatment it is possible to

significantly increase the strength of the materials while preserving the high electrical

conductivity.

In the present work the influence of the solid solution treatment and the SPD parameters on

the dissolution and precipitation of alloying elements in Cu-1%Cr-0.7%Zr alloy has been

studied. The chemical composition, the morphology and the size distribution of the

precipitation particles have been studied using the extraction replicas technique.

The solid solution treatment of the commercial alloy samples was carried out at 1050°C for 5

and 10 hours. It has been found that the degree of dissolution and, correspondingly, the size

distribution of the remaining particles are different for these treatment durations. The size

distribution of the particles had the bi-modal form. The microhardness values were 510 MPa

and 670 MPa for 5 and 10 hrs, correspondingly, what well correlated with structural

differences.

The analysis of the structural changes demonstrated that the SPD (8 passes of the ECAP along

the Bc route) activates the diffusion what results in the stain-induced decay of the solid

solution prepared at the previous stage. The average size of the remaining particles

significantly decreased and became approximately equal for both treatment times. This result

287

may also be considered as the evidence for the strain-induced diffusion enhancement during

the SPD.

The ECAP resulted in the structure refining of the material to the sub-micrometer scale. The

microhardness values after the SPD were 1670 MPa and 1670MPa for 5 and 10 hrs,

correspondingly.

This work was supported by RFBR grant (projects No. 06-08-00971, 07-08-00567-а).

288


Bulk Nanostructured Materials: Pathways for Commercialization Andrey Shcherbakova,1

a Institute of Physics of Advanced Materials, Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000 Russia

1 [email protected]

Nanostructured materials are a new class of materials with unique consumer’s properties. As

far back as yesterday only products from organic chemistry and pharmaceutics were presented

in the market. Today the market is enriched with composites, textiles and powders, including

powder metallurgy and coatings. Tomorrow the market will see bulk nanostructured materials

(BNM) and ultrafine-grained materials (UFG) materials and engineering structures from

them. One of the steps into the tomorrow market can be made through processing BNM and

UFG materials by severe plastic deformation (SPD) techniques. A promising sector for these

materials in the industrial market is estimated at several billion dollars. Transport,

engineering, sports industry, industries of tourism and recreation, medicine constitute the

market.

Specialists from IPAM in close cooperation with Russian and foreign partners have been

working on processing BNM by SPD techniques over 10 years. Work on commercialization

of the technologies developed by them is performed. With the help of these technologies

semi-products and pilot samples for medicine, sports and engineering were manufactured.

Today work on launching the industrial production of semi-products and items from BNM is

conducted.

289


The Nanocrystalline Material LitAr and the Compositions Diagram of the System Collagen– CaO – P205 – H2O

Andrei I. Sechnoya,1, Tatyana V. Sudakovab,2 Sergey D. Litvinovb,2 a Samara State Aerospace University, 34 Moscow Highway, Samara 443086, Russia

b Samara State Technical University, 244 Molodogvardejskaya St., Samara 443100, Russia 1 [email protected] , 2 [email protected]

For the purpose of evaluating the strategy of creating the implant materials it is advantageous

to investigate the topological rules of

the compositions diagrams. The

investigation of this kind lies in the

fact that the composition of the

complex compounds is at the

intersection of the secants connecting

the binary phases tops in the

compositions polyhedron as it was

made for the multicomponent system

of Collagen– CaO – P205 – H2O

(Fig.1). This fact has made it

possible to reveal the identification

region of the nanocrystalline

compound material LitAr used for

filling the defects of the bone and parenchymatous tissues [1].

This compound material is a high integrated system of collagen fibers with nanocrystalline

hydroxyapatite. The problem of conformity of the present material with the native bone has

been successfully solved with the help of analyzing the diagram on which the composition

region (hydroxyapatite HA, enamel, LitAr, bone, blood cell, blood) (Fig.2) has been

distinguished.

The condition which determines the biological activity of the material is nanometricity (Fig.3)

of its salt component [2]. This fact ensures universality of the material stimulating the

patient`s stem cells [3].

CaO

Collagen

HA

P O2 5

H O2

H P O4 2 7

H PO3 4

HPO3

H[P O ]3 81:3

2:13:1

1:1

mol.%

HA region

5:1 10:31:1

Ca(OH)2

1:1

HA

Са5[P O ]2 10Ca(PO )3 2

Ca P O2 2 7

2:1

Са10 6 25[P O ]Ca (PO )3 4 2

3:1

Figure 1. The diagram of the compositions of the 4-component system Collagen – CaO – P205 – H2O

290

The application of the topological choosing

criterions will make it possible to determine the

promising region of synthesis of the new

nanometric compound materials of the medical

purpose.

[1] S.D. Litvinov, I.I. Markov, M.M.

Olennikova, Biomaterialien, 3 7 (2006) 186

[2] S.D. Litvinov, T.V. Sudakova, A.S.

Seryogin. Book of Abstracts of Topical

Meeting of the European Ceramic Society

"Structural Chemistry of Partially Ordered

Systems, Nanoparticles and

Nanocomposites". Saint-Petersburg, Russia

(2006) 162

[3] S.D. Litvinov, I.I. Markov, T.V. Sudakova.

Proceedings of International Conference

“Chemistry, Chemical Engineering and

Biotechnology”. Tomsk: TPU, 2 (2006)

395

HA

H O2

enamel

bone

plasma

bloodcell

LitAr

Figure 2. The compositions plane

Figure 3. The hydroxyapatite nanocrystals in the material LitAr

291


Modeling of Isothermal Forging of Compressor Blade Made of Nanostructured Ti-6Al-4V Alloy

Alexander V. Botkina,1, Azat F. Shayahmetova,2, Alexander A. Kuzminyha,3 a Institute of Physics of Advanced materials and Nanotechnology Department, Ufa State Aviation

Technical University, 12 K. Marx St., Ufa 450000, Russia 1 [email protected], 2 [email protected]

The samples made of nanostructured Ti-6Al-4V alloy have high mechanical properties after

isothermal straining [1]. It is supposed that the compressor blade of GTE made of the

nanostructured Ti-6Al-4V alloy produced by isothermal forging at temperature 650°C should

have elevated service properties.

Investigation of the blade stress and strain state in forging, determination of the process power

parameters are an essential problem the solution of which will make it possible to develop

forming technique.

The investigation of the stress and strain state was conducted by finite-element method at

isothermal setting of the problem. The program product “Deform-3D” was used for doing it.

An energy method was used for analytical dependences output of strain energy and specific

force of straining.

There was modeled a press forming of the blade with overall dimensions (taking into account

a technological lap on the pen width): 110×142 and a mean pen size of the blade t=3 mm, in

the closed press tool. The initial cylindrical billet was 23 mm in diameter and 135 mm in

length.

The investigation results:

The specific force of straining in press forming of the nanostructured billet exceeds the

specific force in press forming of the same blade made of BT-6 alloy in as-received condition

by 6 times and amounts to 760 MPa.

The specific force of straining 769 MPa corresponds to the strain rate 0.5 mm/s. At this rate in

all material particles of the billet at the final stage of the press forming the strain rate meets

the speed regime of low-temperature superplastic deformation of the nanostructured Ti-6Al-

4V alloy.

The pressure in the hydroprocess cylinder providing a required press forming regime was

determined by using the law of dynamics.

As it was shown by modeling, the deformed state of the billet at the final stage of filling the

press forming is close to the two-dimensional one.

292

As the deformed state is two-dimensional, by energy method there were obtained

dependences of the strain energy, the specific force, the hydroprocess pressure on the main

geometrical dimensions of the blade, the friction coefficient and flow stress of the metal.

The data of the dependence correspond well to the data obtained at the pilot press forming and

form the basis for performing scientific process designs of isothermal forging of the product

“blade”.

[1] I.P. Semenova, G.I. Raab et al, FMM, in press

293


Nanostructures and Grain Boundaries Migration at the Atomic Scale Dmitrii Titorova,1

a Physical-technical Institute, Ural Branch of the RAS, 132 Kirov St., Izhevsk 426000 Russia 1 [email protected]

The problem of the development of nanostructured materials is a problem of developing

materials with the high density of grain or phase boundaries. The occurrence of grain

boundaries is a consequence of the distinction of orientations of lattices of next volumes of

substance. The disorientation can be created by the very local and very non-uniform

deformation, and by providing heterogeneity of orientations of crystallization or

recrystallization centres.

It is known that boundaries of grains in sizes from tens even to hundreds of micrometers

significantly affect the level of mechanical, physical, and chemical properties of

polycrystalline materials. In nanostructured materials the share of atoms, which are in the

boundary one-atom layer, can be equal to about 1 % of all atoms, if the diameter of a grain is

about 100 nm, and almost 10 %, if the size of grains is reduced up to 10 nm. It is natural to

expect that the role of grain boundaries in nanomaterials significantly increases.

Experimental methods do not allow one yet to understand reliably the atomic structure of

grain boundaries, especially triple and fourfold joints of grains. The interpretation of the data

obtained by electronic microscopy and, especially, by other investigation methods of these

structural formations is too ambiguous and subjective. It is impossible to search for optimum

structures on minima at the surface of potential energy, because there is a great quantity of

these minima, and minimum of boundary structures can not be global.

Nevertheless, the properties, exhibited by complexes of the connected atoms, essentially

depend on the location of the atoms rather each other in these complexes. Therefore, there is a

need to try to understand and to predict somehow the atom structure, atom reorganizations on

boundaries and joints of grains to operate them.

To maintain the stability of the nanostructure it is necessary to prevent grain growth. But for

this to happen it is essential to know the grain growth mechanism. Growth of grains at the

atomic level is not the transition of atoms from a state with the raised energy in the state with

the lowered energy.

It is the transition of atoms from places where their centres are coincident with an

arrangement of points of the lattice of one grain, to places, where their centres coincide with

an arrangement of points of the lattice of another growing grain.

294

These atomic readjustments occur on the boundaries of grains, therefore the grain boundaries

drift giving the impression that the grain growth is caused by the movement of boundaries.

Only adequately understanding how these atom transitions are carried out, it is possible to try

to search ways making these transitions difficult, and, thus, ways for realizing the stability of

the nanostructure.

In the paper it is shown how the atom structure of boundaries, triple joints and the atomic

bonding between adjoining grains can be presented, and how the atomic relay-race

mechanism of grain growth in a solid phase is given using the model of interpenetration

atoms. This model has been found [1, 2] when solving the problem of modeling the formation

of well known crystal structures described by the Bravais lattice. It is the “structural atom

model” of its origin and thus more effective for the decision of structural problems in science

of materials, as opposed to an electronic-quantum model developed from the analysis of

results of spectral researches, that is this model of its origin is the “spectral atom model” and

consequently it is a more effective one in the solution of spectral problems.

[1] D.B. Titorov, Crystallography Reports, 1 (2001) 19

[2] D.B. Titorov, Povrhnost’(Surface), 6 (2003) 93 (in Russian)

295


Nanotechnology of Macroobjects Georgy M. Volkov

Moscow State Technical University MAMI, 38 B.Semenovskaya St.,

Moscow 105839, Russia [email protected]

On an example of carbon - carbon system the opportunity of realization the monostage

technology nanocomposits for engineering industry is shown.

Considering graphite as a limiting degree of condensation of aromatic hydrocarbons have

estimated a critical diameter it nanoparticles .For carbon in allotropic modification of graphite

that make up about 10 nm. The many years research of a representative samples of industrial

batches of nanocomposite has shown the satisfactory compliance of experimental value of

d cr.(9.2 nm) of graphite with the theoretical data.

Nanoparticles of carbon and carbon matrix, connecting them, are formed in uniform

technological process simultaneously. The industrial technology of the nanocomposite as

plates and tubular pieces, and also as specific articles with the size up to 200 mm and the wall

thickness up to 10 mm is developed during the practical production processes.

Structure and basic properties the nanocomposite of carbon–carbon systems compliance with

the theoretical data. The carbon nanocomposite has a unique combination of properties: it is

chemically and biologically inert, air and liquid impermeable, radiation resistant and

surpasses any carbon materials: in friction coefficient 5 times, in cathode sputtering

coefficient 15 times, in oxidizing resistance up to 300 times and surpasses tungsten in high-

temperature specific durability. The above properties ensure functioning capability of

advanced machines and devices.

The offered approach to solution of technological problems of nanomaterials can be used in

creation of one-stage technology nanosystems of a filler-matrix of other chemical

composition.

296


Brittle-Ductile Transition in Commercial Pure Tungsten with Ultrafine-Grained Microstructure

Yue Zhanga,1, Jing Tao Wanga,2 J.Q.Liua,3, Artur Ganeevb,4 and Igor V.Alexandrovb,4 a School of Materials Science and Engineering, Nanjing University of Science and Technology,

Nanjing, Jiangsu 210094, P. R. China b Institute of Physics of Advanced Materials, Ufa state Aviation Technical University,

12 K. Marx St., Ufa 450000 Russia 1 [email protected], 2 [email protected], 3 [email protected], 4 [email protected]

Tungsten is an important material with highest refractory property among all metals. Its

application is, however, restricted by its brittleness. The Brittle-Ductile Transition

Temperature (DBTT) of commercial pure bulk tungsten materials could be as high as 500oC.

Plastic deformation was well used to improve the ductility of tungsten, following a empirical

rule that DBTT of tungsten decrease monotonously with plastic deformation. The un-

recrystallized fibrous microstructure in the materials retards the propagation of crack and thus

improves the ductility. It is believed that equiaxed grains, even fine, will result in brittleness

in tungsten. Dopants of various kinds are thus well used to resist recrystallization in tungsten

products, which usually owns thin size to allow the necessary plastic deformation.

Equal channel angular pressing (ECAP) provides a useful processing to impose high plastic

deformation into bulk materials, which makes bulk ductile tungsten products possible.

Meanwhile, this process could also introduce sub-micro to nano-meter grain size into the

materials, which provide an opportunity to test the effect of quiaxed grains at a sub-

micrometer level. Commerial pure tungsten was used to approaching these aims in the present

work. During ECAP, the die with a channel angle of 135o was heated to 600 oC and tungsten

sample was heated to 1150 oC. ECAP was conducted up to 7 passes via Route C.

Figure 1. TEM microstructures of the tungsten samples

ECAPed As received

297

The present work indicates that: 1) DBTT of commercial tungsten decrease from > 483oC in

as received state with a grain size of ~3 µm, to <350oC in ECAP processed state with an

equiaxed grain size of ~0.9 µm, this shows that ductility of tungsten could also substantially

improved by equiaxed grain structured when ultra-refined; 2) ECAP substantially increased

the HV of the commercial tungsten. This provides an effect processing for obtaining bulk

tungsten with high strength and ductility, in contrast to the thin materials by conventional

working processes.

Figure 2. The dependence of Vickers-hardness on Testing temperature

Figure 3. HV test pit at 483oC and cracks around it in as received tungsten

60 µm

298


Influence of the Scale Factor on Strain Fields in the Materials Processed by Equal-Channel Angular Pressing

Vladimir Zhernakova,1, Anatoliy Ermolenkoa a Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000 Russia

1 [email protected]

Equal-channel angular pressing (ECAP) is a promising process which allows obtaining large

plastic shear strains. In this process a sample is deformed when passing through two equal-

sized channels situated at some angle to each other. Formation of large plastic strains in the

sample material is achieved by simple shear. Large plastic strains are provided by multiple

repetition of the process or at the expense of use of multichannel scheme of deformation. This

results in grain refinement and unique mechanical and physical properties of the material.

Finite element modeling is widely used to study inhomogeneity of deformation during ECAP.

Inhomogeneity of deformation across the width of a billet, a multichannel deformation

scheme require a more thorough and deep description of the process by numerical procedures.

Study of ECAP samples with square and round cross sections is promising in this field.

The report presents the research data on behavior of the material subjected to equal-channel

angular pressing (ECAP). Influence of the scale factor on formation of nanostructures in Cu

and Ti samples after the 1st pass is studied. The samples after several passes according to the

multichannel scheme of deformation were also investigated. Influence of sample sizes on the

degree of deformation inhomogeneity was studied. The process was modeled by the finite

element method with the help of the software SFTC DEFORM-3D v.5.1. The calculation data

were compared with the experimental ones obtained at the Institute of Physics of Advanced

Materials (IPAM).

299


Mechanics of Plastic Deformation of Cu and Ti Ingots during Multipass Combined ECAP

Igor Budilova,1, Yuri Lukaschuka and Vladimir Zhernakova,3 a Ufa State Aviation Technical University, 12 K. Marx St., Ufa 450000 Russia


The report analyzes the plastic flow of material during equal-channel angular (ECA) pressing

of Cu and Ti ingots. The analysis of accumulated plastic strains at different stages of ECA

pressing was carried out within the frames of the isotropic model of the material.

Numerical techniques play an important role during development of the most optimal

geometry of the die-set and modes of ECA pressing. In particular, this approach allowed

investigating the influence of such ECAP parameters as a friction coefficient, a channels’

intersection angle, a pressing route and number of passes, geometric sizes of a channel, a fillet

radius of channels, a strain rate and a diagram of material deformation of a billet, back

pressure. Combination of these factors results in contradictory conclusions about regularities

of the plastic flow, evenness of channels filling and a value of accumulated total strain.

Influence of the friction coefficient and outer radius curvature radius on the peculiarities of

the plastic flow and distribution of accumulated over the billet body is studied on 3D models

in detail. Special attention is paid to evaluation of evenness of distribution of plastic strains in

the billet body and analysis of presence of zones with maximal values of plastic strains. Grain

refinement was assessed in the process of ECAP. Homogeneity of plastic strain fields was

analyzed.

300

18 August

Innovation Trends and

Applications of Nanomaterials

(ISTC session)

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