GRAIN REFINEMENT IN ALUMINIUM ALLOY AlMgSi1 … · GRAIN REFINEMENT IN ALUMINIUM ALLOY AlMgSi1 DURING ECAP AT ROOM ... low temperature and high strain ... A Vicker hardness and tensile

METAL 2009 19. – 21. 5. 2009, Hradec nad Moravicí ___________________________________________________________________________

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GRAIN REFINEMENT IN ALUMINIUM ALLOY AlMgSi1 DURING ECAP AT ROOM TEMPERATURE

Tomáš Kovaříka, b

Jozef Zrnikb, c

Miroslav Cieslard

a University of West Bohemia, Plzen, Czech Republic

b Comtes FHT Ltd., Dobřany, Czech Republic c Technical University of Košice, Slovak Republic

d Charles University, Prague, Czech Republic

Abstract

The severe plastic deformation (SPD) was demonstrated as an effective approach to produce ultrafine grained materials. ECAP has emerged as effective deformation method to refine grain size in bulk metallic materials. Aluminium alloys AlMgSi1, prepared in three different states of structure, and was processed by this severe plastic deformation (SPD) technique at room temperature to a strain of 4. Development of deformation substructure and grain refinement due to straining is investigated after successive introduction of different passes corresponding to a different effective true strain by TEM of thin foils. Deformed substructure analysis showed no effect of different processed condition on deformation structure development. The microstructural changes are mainly characterized by evolution of high density dislocation substructures (subgrains) with relative low misorientations at strain below ε = 2 and deformation bands with moderate misorientation angle, frequently formed at higher strains. The fraction of high angle boundaries were observed at straint ε = 4. A new grains with random crystal orientation are them more frequently developed. Hardness measurement was used to evaluate the mechanical properties of deformed materials.

1. INTRODUCTION

Although the mechanical and physical properties of all crystalline materials are determined by several factors, the average grain size of the material generally plays a very important and often a dominant role. The strength of all polycrystalline materials is related to the grain size (d). According to the Hall–Petch equation which states that the yield stress σy is given by σy = σ0 + kyd

-1/2 (1) σ0 = friction stress, ky = constant of yielding. It follows from equation that the strength increases with a reduction in the grain size and this leads to an ever-rising interest in fabricating materials with extremely small grain sizes [1, 2]. The grain sizes of commercial alloys are generally modified for specific applications by making use of pre-determined thermomechanical treatments. The alloys are subjected to particular regimes of temperature. However, these procedures cannot be used to produce materials with submicrometer grain sizes


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because there is always a lower limit of the order of a few micrometers, which represents the minimum grain size easily achieved using these procedures. Therefore attention has been directed to the development of new and different techniques that may be used to fabricate ultrafine-grained materials with grain sizes in the submicrometer and the nanometer range. Ultrafine-grained (UFG) materials processed by severe plastic deformation (SPD) are defined as polycrystals having very small grains with average grain sizes less than ~ 1 µm. For bulk UFG materials, there are the additional requirements of fairly homogeneous and reasonably equiaxed microstructures and with a majority of grain boundaries having high angles of misorientation. The presence of a high fraction of high-angle grain boundaries is important in order to achieve advanced and unique properties. For example an increasing of strength, low temperature and high strain rate superplasticity [3]. Equal-channel angular pressing (ECAP) is an effective method for reducing the grain size to, typically, the submicrometer level by introducing large plastic strain into materials through repetitive processing. The ultrafine-grained (UFG) materials produced by ECAP have a very high strength due to their low grain size and high dislocation density [4]. As severe plastic deformation (SPD) procedures enable to produce UFG materials in bulk shape, there is an increasing interest for these materials with respect to their technological applications. This interest is especially large for SPD processing of those materials which are traditionally basic materials in the industry (e.g. precipitation-hardened Al alloys) [5]. In the present work, plastic deformation structures of an ultra fine grained (UFG) commercial Al–Mg–Si alloy (6082) manufactured by equal-channel angular pressing (ECAP) at room temperature have been characterized by transmission electron microscopy (TEM). The focus is on different deformation structures including deformation bands, which are claimed not to be present in coarse-grained alloys.

2. EXPERIMENTAL PROCEDURE The material used in this study is an AlMgSi1 alloy. The chemical composition (according to the norm DIN 1725 – 3.2315) is noted in table 1. Chemical composition of the experimental alloy was confirmed by the EDX analysis and is presented in table 2.

Table 1. The chemical composition of material according to the norm DIN 1725 - 3.2315.

Element Si Fe Cu Mn Mg Zn Ti Al

In mass [%] 0.70 až 1.40 0.50 0.10 0.40 až 1.20 1.20 0.20 0.05 rest

Table 2. The natural chemical composition of experimental material.

Element Si Fe Cu Mn Mg Zn Ti Al

In mass [%] 1.00 0.12 0.01 0.50 0.85 0.04 0.02 rest

Bars with dimension of 8 × 8 × 30 mm for ECAP were cut from the center part of continual cast rods with a diameter of 20 mm. The microstructure is presented by Fig. 2, 3. The heat treatments were applied to rods in two modes and the first mode


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(S1) was only in initial conditions with mechanical fabrication of forward pressing. In the second mode (S2) the rod was solution-annealed at 540 °C for 1.5 h followed by water quenching. In the third mode (S3) it was quenched to room temperature (with the same parameters) and followed by temper hardening at 160 °C for 12 h. A commercial 6082 aluminium alloy rods at pre-described conditions was subjected to ECAP at elevated temperature 150°C using route BC. The angle of intersection of the two channels (Φ) was equal to 120°C (Fig. 1). The numbers of passes for each mode were set to 6. The ECAP die used for the experiment was heated to pressing temperature and held for 30 minutes. The sample was heated for 300 s prior to pressing, which was done inside the pre-heated die until samples reached the pressing temperature. Each billet was pressed to four, five and six passes (N) through the die. The effective strain corresponding to one pass was ε ~ 0.67. Six passes correspond to the total strain of εef ~ 4. The billet was rotated between the consecutive passes about its longitudinal axis by 90° in the same direction. This procedure is generally referred to as the processing route BC and it was selected because it enables the formation of homogeneously deformed microstructure. It was not considered that the stress generated in sample after each pass, would be recovered (static recovery) due to repeated re-heating of the billet inside the die prior the next pass will be carried out. The microstructural examination of thermally treated and ECAP deformed samples was carried out by light microscopy (LM) and transmission electron microscopy (TEM). Thin foils for TEM observation were sliced normal to the longitudinal axis of ECAP pressed billets. The microstructures were obtained by usány JEOL JEM 200FX TEM operating at 200 kV. Selected area electron diffraction (SAED) was used to investigate the ultrafine grain transformation progress in dependence the strain introduced. A Vicker hardness and tensile test were carried out using MTS universal testing machine equipped with Multisens extensometer. Tensile specimens with gauge length of l0= 25 mm were tested at a constant cross-head speed of 0.016 mm/s until failure. The engineering stress-strain curves were constructed. 3. EXPERIMENTAL RESULTS AND DISCUSSION 3.1 Microstructure analysis Microstructural characteristics in ECAPed samples were analyzed using light microscopy in the first step. The initial state is represented by coarse grain structure from side and centre area. The specimen in the as-received conditions had a average grain size approximately 150 µm (Fig 2, 3). Influence of deformation after 6 passes and its character of homogeneity in capacity of forming material is displayed on figures 4 and 5. Analyses of microstructures by optical microscope demonstrate effect of ECAP process well-noticeable but for better understanding of detailed mechanisms in markedly extended structure is not sufficient.

Fig. 1. Schematic illustration of the die.


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TEM observation of the material after ECAP revealed a complex microstructure. It appears that the usual micromechanisms of fine substructure formation were operative, with an equiaxed dislocation cell structure forming initially (Fig. 6a). The cells showed different levels of dislocation density in their interior (Fig. 6c).

(a) (b) (c)

200 nm 200 nm 200 nm

100 µm

500 µm 500 µm

100 µm

Fig. 2. Image of the cast rod in initial condition (side area); Nomarski

effect.

Fig. 3. Image of the cast rod in initial conditions (center);

Polarized-light effect (PLE).

Fig. 4. Image of extended grains after 6 passes (side area); PLE.

Fig. 5. Image of extended grains after 6 passes (center); PLE.

Fig. 6. TEM images of typical grain substructure; Initial conditions after 6 passes: (a) dislocation cell structures inside a larger grain, (b) dislocation network,

(c) grains with high density of dislocations.


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This was followed by a process of subgrain formation in which cell walls were transformed into more regular dislocation network (Fig. 6b). It was also found that different regions of the material displayed different rates of recovery, i.e., subgrains in some regions were shaped concurrently with the formation of dislocation cells in others. The ECAP die construction with a channel angle of 120° showed that it is feasible to press hard and solid metals and alloys [6]. Our experiments complying with it but TEM provided the evidence that applied effective strain (with Φ = 120°, 6 passes, εEF ≈ 4) was not sufficient to deform structure uniformly. The effect of the elevated deformation temperature had main influence (in all states) to initiation and extent of polygonization segments and nucleation of new subgrains too.

3.2 Mechanical properties

The results of tensile testing performed at room temperature are shown in Fig. 7, which represents the three states after 6 passes. The engineering stress–strain curves are shown for comparison. Significant strengthening is evident after six passes in quenched and aged sample. YS of the state 3 (subsequent aged) ECAPed alloy is high as 370 MPa. This strength is higher round off by 430% than valuation of the unECAPed material (70 MPa). The deformation curves corresponding to mechanical properties listed in table 3 for all states even in the pre-deformed stage. The strength of the quenched and subsequent aged UFG 6082 alloy is significantly higher than in the other stages, indicating that it is possible to add the strengthening effect from ECAP processing to effect from the precipitation hardening.

Fig. 7. Engineering stress-strain curves:

1) Initial state; 2) Annealed followed by water quenched; 3) Quenched and aged.


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Table 3. Mechanical properties of commercial 6082 aluminium alloy after ECAP; IN (initial conditions); AQ (annealed followed by quenched);

QAG (quenched subsequent aged); UD (un-deformed).

spec.

AlMgSi1

D0

[mm]

S0

[mm]

l0

[mm]

E

[GPa]

Rp0.2

[MPa]

Rm

[MPa]

Ag

[%]

A4

[%]

Z

[%]

UD state 5,00 20,00 25,00 - 70 130 - - -

IN state 5,03 19,87 25,00 58,64 302,01 321,90 5,98 11,80 38,33

AQ state 4,95 19,24 25,00 73,33 178,93 208,06 5,06 14,96 46,81

QAG state 5,04 19,95 25,00 64,56 369,92 394,04 2,72 10,28 44,64

4. DISCUSSIONS and CONCLUSIONS The development of the dislocation density, the grain and subgrain in Al 6082

alloy processed by ECAP were investigated. The grain orientation distribution reached the random case of dislocation density and the subgrain size. 6 passes in elevated temperature 150° didn´t adeguate to compose high uniform UFG structure with the majority of high angle boundaries (60-70%). However a combination of SPD processing and precipitation hardening has the potential to render Al 6000 series alloys. The significantly high strength of the UFG 6082 Al alloy may be attributed to solid solution, grain refinement, dislocation and precipitation strengthening (the effect of hardening particles of β-Mg2Si phase precipitated during ageing process). Acknowledgement This paper contains results of investigation conducted as part of the MSM2631691901 project funded by the Ministry of Education of the Czech Republic. LITERATURE [1] HALL, E. O. Proc Roy Soc B, 1951, Vol. 64. [2] PETCH, N. J. J Iron Steel Inst, 1953, Vol. 174. [3] VALIEV, R. Z. Nature Mater, 2004, Vol. 3. [4] DALLA TORRE, F., et al., Pereloma, Acta Mater., 2004, VOL. 52, p. 4819. [5] CHOWDHURY, S., G., et al., Mater. Sci. Eng., 2008, A 490, p. 335. [6] ALEXANDROV, I., V., el al., Tungsten hard metals and refractory alloys, 2000,

Vol. 5, Metal Powder Industries Federation, p. 27.

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GRAIN REFINEMENT IN ALUMINIUM ALLOY AlMgSi1 … · GRAIN REFINEMENT IN ALUMINIUM ALLOY AlMgSi1 DURING ECAP AT ROOM ... low temperature and high strain ... A Vicker hardness and tensile