Martensitic Phase Transformation in TRIP-Steel/Mg-PSZ Honeycomb Composite Materials on Mechanical Load

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DOI: 10.1002/adem.201100126
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Martensitic Phase Transformation in TRIP-Steel/Mg-PSZHoneycomb Composite Materials on Mechanical Load**

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By Christian Weigelt,* Christos G. Aneziris, Harry Berek, David Ehinger and Ulrich Martin
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Composite materials have been in focus of scientific studies since decades. Metal-matrix compositeshave received extensive attention in the last years. The combination of a metastable austeniticTRIP-steel with magnesia partially stabilized zirconia is presented in this study. The stress inducedmartensitic phase transformation in both components leads to advantageous mechanical behavior.Raised compression strength as well as increased specific energy absorption on plastic deformationoffers a range of structural and crash-absorption applications. Samples with zirconia additions arereinforced by volume increase during tetragonal-monoclinic phase transformation at compressivestrains below 35%. The microstructure and phase evolution of partially stabilized zirconia as well assteel has been investigated by EBSD with purpose to correlate mechanical properties with phaseevolution.

Composite materials have been in focus of scientific

researches since decades. The advantageous combination of

two or more different materials offers a range of micro- and

macrostructure designs according to their crashworthi-

ness.[1–3] Crashworthiness is concerned with the absorption

of energy through controlled failure mechanisms and modes

that enable a defined load profile during deformation.[4]

Cellular materials, such as metal foams and honeycomb

structures, are materials which stand out for the combination

of physical and mechanical properties, e.g., crash box

applications in automobile and train structures. Specific

energy absorption per unit mass and specific energy

absorption per unit volume are the key features for

crashworthiness, respectively.

[*] C. Weigelt, Prof. C. G. Aneziris, Dr. H. BerekInstitute of Ceramic, Glass and Construction Materials,Technische Universitat Bergakademie Freiberg, Agricolastraße17, 09596 Freiberg, (Germany)E-mail: [email protected]

D. Ehinger, Prof. U. MartinInstitute of Materials Science, Technische UniversitatBergakademie Freiberg, Agricolastraße 17, 09596 Freiberg,(Germany)

[**] This scientific work was supported by the German ResearchFoundation (DFG) in terms of the Collaborative ResearchCentre ‘‘TRIP-Matrix Composites’’ (CRC 799). The authorsalso acknowledge the support of Dr. B. Ullrich withSEM-analysis.

ADVANCED ENGINEERING MATERIALS 2012, 14, No. 1-2 � 2012 WILEY-VCH

Several groups have produced cellular structures by using

hollow spheres made of different steel compositions. These

materials exhibit a plateau stress of 5 and 23MPa with a

specific energy absorption of 2 and 10MJ �m�3, respectively,

up to 50% strain.[5,6] Rabiei and O’Neill[7] presented a new

composite material based on steel hollow spheres packed into

a dense arrangement by filling free space with casting

aluminum. The compressive strength of this material was

given at averaged 67MPa in the region of 10–50% strain with

an energy absorption of 30MJ �m�3 after 50% compressive

strain.

The combination of a ductile metal matrix with brittle

ceramic additions leads to failure tolerant metal-matrix

composites (MMC). TRansformation induced plasticity

(TRIP)-steels offer a matrix material with outstanding strain

associated with slight strain hardening. These properties

originate from a martensitic phase transformation from

austenite to martensite on mechanical load. Similar transfor-

mations can be observed for zirconia (ZrO2) ceramics. ZrO2

appears in three modifications: between melting and 2 370 8Cthe cubic phase (c-ZrO2) is stable. Further cooling leads to

transformation into the tetragonal phase (t-ZrO2) and below

1 170 8C highly distorted monoclinic ZrO2 (m-ZrO2) is

thermodynamically preferred. The phase composition can

be affected by stabilizer additions as well as heat treatment.

Particularly the t$m transformation is of technological

interest. In general this transformation appears with a volume

change of 3–5% exceeding the critical fracture length in

ceramics. Whilst this effect eliminates pure zirconia applica-

tions the transformation can be used for reinforcing a second

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C. Weigelt et al./Martensitic Phase Transformation in TRIP-Steel/Mg-PSZ . . .

Table 1. Chemical composition of TRIP-steel powder AISI 304 in wt%.

Fe Cr Ni Mn Si C

Bal. 18.40 9.30 1.13 0.67 0.01

Table 2. Chemical composition of Mg-PSZ zirconia powder in wt%.

ZrO2 MgO SiO2 HfO2 Al2O3 TiO2

Bal. 3.4 2.4 1.7 0.6 0.1

material. Guo et al.[8] achieved samples with strength of 1400

up to 2 100MPa for dynamic high-rate deformation. They

combined a low-alloyed steel with up to 35 vol% yttria

stabilized zirconia. Recent publications of Aneziris et al. [9,10]

presented cellular composite materials based on TRIP-steel

with magnesia partially stabilized zirconia (Mg-PSZ).

The standard route for the production of cellular

honeycomb specimens is the extrusion of plastic pastes. Since

decades filters and catalyst carriers are produced in close

related geometry. This method offers the ability of homo-

geneous mixtures of steel and ceramic particles in the range of

some mm. To achieve the properties for extrusion usually

organic additions, e.g., plasticizers, are necessary. After

drying and thermal debinding, a sintering step takes place

to achieve the final properties.

Within the present work the extrusion of austenitic

stainless steel matrix composites with Mg-PSZ particles at

room temperature was performed. After sintering the samples

were tested under uniaxial compressive load. The main topic

is the relation between mechanical properties and the

EBSD-observed phase evolution in both components. The

stress-induced martensitic transformation of single partially

stabilized zirconia particles and the identification of the

corresponding state of deformation are in focus. The key to

advantageous mechanical properties of the presented com-

posite material is the ability of both phases for the martensitic

phase transformation due to mechanical stresses. The

amounts of metastable austenite in steel as well as the

metastable tetragonal phase in partially stabilized zirconia

are important. Several interactions during sample production

and heat treatment may influence the phase proportion in

comparison to starting material. But the success of the

presented composite material is related to phase relations.

Experimental

The honeycomb square cell composites have been pro-

duced with the well-established ceramic extrusion technique.

Table 1 presents the chemical composition of the as-received

steel powder typeAISI 304 (German grade X5CrNi 18-10) with

an average particle size of 14mm. The percentage of 92%

Table 3. Composition of mixtures for paste preparation.

Raw materials

Material Type

Austenitic steel 1.4301, d50¼ 14mm Womet, Ger

ZrO2 PMG 3.4, d50¼ 1.3mm Saint-Gobain

Plasticizer flour Coarse grained flour Dresdener M

Plasticizer cellulose Methocel F4M DOW Wolff

Dispersant Castament FS60 Degussa, Ge

Surfactant Denk Mit Henkel, Ger

Sum

Plus additional water

Water Deionized

54 http://www.aem-journal.com � 2012 WILEY-VCH Verlag GmbH & Co.

metastable austenitic phase was calculated based on the

amount of ferrite measured by the aid of X-ray diffraction

(XRD). The ceramic phase consists of magnesia partially

stabilized zirconia (see Table 2) with 3.4wt% MgO (Saint

Gobain, USA) at an average particle size of 1.3mm. Referring

to XRD the starting powder consists of 10% monoclinic, 36%

tetragonal, and 53% cubic phase. The compositions of the

presented samples can be seen in Table 3.

Before extrusion the inorganic powder materials were

mixed within 30min in a tumble drum with yttria stabilized

zirconia balls. In a second step the plasticizers and binders

were added and the material was further mixed in a

conventional mixer (Toni Technik, Germany) for 5min.

Finally, water was added and the recipes were kneaded until

a homogeneous and plastic paste was achieved. Due to a

combined de-airing single screw extruder with vacuum

chamber and sigma kneader type LK III 2A (LINDEN,

Germany) honeycomb structures were extruded with a

square shape of 25� 25mm2 and a honeycomb structure

of 14� 14 channels (200 cpsi-channels per square inch). The

wall thickness differs between 350mm (inner walls) and

400mm (exterior wall). After extrusion the specimens were

dried stepwise at 40–80–110 8C in an air-circulated dryer

within 12 h at each temperature and decreasing humidity

from 80 to 20%. Afterward cubic samples of 25mm length

were cut and placed on alumina tiling. The debinding step

took place at 350 8C for 90min in air atmosphere with a

Recipe [wt%]

Supplier 0Z 5Z 10Z

many 96.72 93.11 89.43

, USA 0.00 3.60 7.29

uhlen, Germany 1.81 1.82 1.81

Cellulosics, Germany 0.94 0.93 0.91

rmany 0.14 0.14 0.16

many 0.39 0.40 0.40

100 100 100

5.91 6.02 5.88

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Table 4. Lattice parameter used in the EBSD phase analysis.

Phase ICDD no. Crystal system Lattice parameter

a [A] b [A] c [A] a [8] b [8] g [8]

ZrO2 0780047 Monoclinic 5.15 5.20 5.32 90 99.20 90

ZrO2 0140534 Tetragonal 5.09 5.09 5.18 90 90 90

ZrO2 0707304 Tetragonal 3.58 3.58 5.16 90 90 90

ZrO2 0270997 Cubic 5.09 5.09 5.09 90 90 90

Mg0.38 Mn0.62 SiO3 0832282 Trigonal 9.65 10.39 12.11 108.65 102.32 82.95

(Mg,Mn)Al2O4 0011154 Cubic 8.03 8.03 8.03 90 90 90

Austenite fcc 3.66 3.66 3.66 90 90 90

a0-Martensite bcc 2.87 2.87 2.87 90 90 90

Table 5. Density and shrinkage of the sintered honeycomb structures.

0Z 5Z 10Z

Density [g � cm�3] 6.77 6.62 6.61

Open porosity [%] 7.71 9.28 8.80

Linear shrinkage [%] 11.88 12.67 12.70

heating rate of 1 K �min�1. To prevent oxidation effects during

further heat treatment the honeycombswere placedwith stock

in graphite crucibles after debinding. Finally the samples were

sintered pressure less in a 99.999% argon atmosphere using an

electrical furnace typeHT 1600GTVac (LINN, Germany) with

oxidic furnace lining and MoSi2 heating elements. After

heating with 5 K �min�1 the temperature of 1 350 8C was kept

constant for 120min followed by cooling with 10 K �min�1 to

room temperature. Open porosity and bulk density were

determined based on Archimedes principle and DIN EN 1389

with toluol as immersion fluid.

The compressive tests were performed on bilateral

polished samples with a 500 kN servo hydraulic testing

machineMTS 880 (MTS, Germany)with a displacement rate of

0.022mmmin�1 at room temperature. In order to analyze the

Fig. 1. Honeycomb structure based on recipe 5Z as-sintered: (a) profile view, (b) top view.

Fig. 2. SEM image of a typical fracture surface region of 5Z before compression test, (a) magnification 1 000�,(b) magnification 3 000�.

crashworthiness stress–strain-curves were

recorded. To clarify the correlation between

mechanical properties and phase evolution

of metal and ceramic components deforma-

tion tests were stopped after 2, 5, 10, 20, and

30% compressive strain, respectively, on

different samples. Additionally, compression

tests were carried out up to deformation

degrees of 35 and 50% in order to identify

the material failure and structural damage of

the multi-cell columns. The specific energy

absorption was calculated according to

Equations (1a) and (1b). SEAV is defined as

specific energy absorption per volume (V)

unit, SEAm refers to mass (m) unit, respec-

tively.

SEA ¼ W

Vor SEA ¼ W

m(1a)

where

W ¼ZSb

0

PdS (1b)

W is the total energy absorbed during sample

deformation, P the load, S the displacement,

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and Sb is the strain at end of experiment according to Jacob

et al. [4].

Microstructure characterization was conducted using

scanning electron microscopy (SEM), energy dispersive

X-ray microanalysis (EDX) as well as electron backscatter

diffraction (EBSD). Mechanically tested as well as non-

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Fig. 3. (a) Stress–strain curves of the compositions 0Z, 5Z, and 10Z, (b) Specific energy absorption as a functionof engineering strain applying a quasi-static compressive load in OOP-direction.

Fig. 4. Collapse patterns of the honeycomb specimen reinforced by 10 vol% Mg-PSZ after interruptedcompression at 35% strain: (a) view of the outer skin, (b) view of the longitudinal section.

Table 6. Specific energy absorption after compressive deformation test, average of 5samples.

Recipe Strain SEA

[%] [kJ � kg�1] [MJ �m�3]

0Z 2 0.47� 0.03 1.47� 0.09

5Z 2 0.64� 0.08 2.04� 0.25

10Z 2 0.72� 0.07 2.25� 0.24

0Z 5 1.55� 0.04 4.89� 0.06

5Z 5 2.05� 0.12 6.48� 0.33

10Z 5 2.36� 0.15 7.40� 0.53

0Z 10 4.06� 0.06 12.79� 0.07

5Z 10 5.18� 0.13 16.37� 0.27

10Z 10 5.92� 0.31 18.55� 1.18

0Z 20 11.95� 0.24 37.65� 0.33

5Z 20 14.42� 0.05 45.62� 0.52

10Z 20 16.18� 0.83 50.76� 3.49

0Z 30 23.60� 0.65 74.34� 1.15

5Z 30 26.96� 0.64 85.26� 2.69

10Z 30 28.18� 1.22 87.80� 5.34

0Z 50 49.49� 2.06 157.63� 7.26

5Z 50 53.17� 4.77 169.10� 5.52

10Z 50 46.49� 1.14 145.84� 0.93

Fig. 5. XRD pattern of recipe 10Z after sintering at 1 350 8C for 2 h in argonatmosphere.

deformed specimens were prepared for analysis. Overviews

regarding the EBSD method are given in several contribu-

tions.[11,12] The incident electron beam is inelastically scattered

within the sample. A part of the backscattered electrons can

escape the sample. In their way they are reflected on lattice

planes. The electron backscatter patterns (EBSP) are a result of

interference following Bragg’s role. EBSP contain all informa-

56 http://www.aem-journal.com � 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tion about the lattice symmetry within the

excited sample volume and thus can be used

for phase analysis. They consist of parallel

lines for each reflecting lattice plane. The

angles between these lines in the so called

Kikuchi pattern or EBSP exactly correspond

to the angles in the lattice structure. The

analysis is realized by comparing the

measured angles with those of candidate

lattice structures. Some knowledge about

possible phases and their lattice structures is

necessary. The decision can be taken on the

base of medium angle differences. There is a

possibility to determine lattice spacing’s too.

But under normal conditions for ceramic

materials the accuracy is not acceptable. The

investigation was carried out in a conven-

tional scanning electron microscope Philips

XL30 equipped with an EBSD system TSL

(Edax/Ametek, Germany). In the case of

ceramic materials a conductive coating is

necessary. One also should keep in mind

that as a result of sample preparation a

disturbed region might occur. These effects

strongly reduce the quality of EBSP.

Phase determination can be carried out

for single spots or for defined scans. There

are two preconditions for a successful EBSD phase mapping.

First of all undisturbed and electrically conducting sample

surfaces are needed. Sample preparation thus is of great

importance especially in the case of multi-crystalline ceramic

materials which contain a high number of cracks. The sample

preparation was realized by vibration polishing in a

BUEHLER VibroMet2. All samples were coated with Pt-layers

in the nm range using a conventional EDWARDS sputter

coater. For each sample ten randomly spread regions with

zirconia agglomerate and surrounding metal phase were

analyzed. The second problem to be solved is a set of lattice

parameters of phases which really are involved in the given

sample. For this purpose an ICDD data base was used.[13] By

ADVANCED ENGINEERING MATERIALS 2012, 14, No. 1-2

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comparing the EBSD results with XRD a set of possible crystal

structures was defined. The results for the given PSZmaterials

and additional major phases for composite material are given

in Table 4. XRD measurements were done using a Philips

diffractometer with Cu Ka radiation scanning 2u angular

region from 5 to 808. Phase identification and quantification

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Fig. 6. Cross-section of zirconia agglomerate in the as sintered state, 5Z. (a) SEI, (b) EDX Zr map, (c) EDX Omap, (d) EDX Mg map, (e) EDX Mn map, (f) EDX Si map, (g) EBSD quality map, (h) EBSD phase mapshowing t/c-ZrO2 (black), and m-ZrO2 (gray).

have been done with aid of X’Pert Highscore

Plus analysis software (Panalytical, Nether-

lands).

Results and Discussion

The determined properties density, open

porosity and shrinkage are listed in Table 5.

The addition ofMg-PSZ powder has negligible

effects on the presented properties showing

differences only in the range of standard

deviation (average of five samples with a

deviation of less than 4%). Thus differences in

mechanical behavior at compressive strength

tests cannot be explained by differences in

porosity, Figure 1 shows a sintered sample of

recipe 5Z before deformation test.

During sintering and densification the

honeycomb sample structure changes from

200 cpsi after drying to �260–280 cpsi with a

cubic structure with length of 22mm in each

dimension. The thickness of inner walls can be

determined to 300mm whereas the exterior

walls Exhibit 350mm. Figure 2 presents the

microstructure of a typical sample fracture

surface of recipe 5Z after sintering. A dense

structure of TRIP-steel matrix with well

dispersed zirconia particles can be seen.

Usually ceramic additions appear as agglom-

erates. According to EDX-analysis three dif-

ferent regions can be identified. On the one

hand the composition of metal phases approx-

imate correlates with starting chemical com-

position of Table 1. On the other hand ceramic

regions with dominating zirconia content can

be observed. Beyond these particles with

elevated concentrations of e.g., manganese,

magnesium, and silicon are detectable.

Various interactions between TRIP-steel and

Mg-PSZ will be discussed later.

The results of the mechanical tests are

presented below. Figure 3 displays the

mechanical responses of the three material

conditions (0Z, 5Z, 10Z) under uniaxial

compressive loading in out-of-plane direction

(OOP, parallel to the extrusion axis denoted by

X3) with respect to the engineering stress

[Figure 3(a)] and the specific energy absorp-

tion per unit mass [Figure 3(b)] as a function of

strain. As has already been mentioned in


previous publications[14–16] the stress–strain curves of all

recipes start with a linear-elastic regime. Rising load is

associated with an interval of strain hardening with a

transition to a maximum compression stress peak of about

428� 22MPa. These phenomena are caused by a combined

mechanism of structural collapse and material damage. The


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mechanical behavior of the pure metal samples is different

from the zirconia toughened specimens. Increasing additions

of Mg-PSZ lead to enhanced compressive flow stresses related

with lower critical failure strains in contrast to the unrein-

Fig. 7. Cross-section of zirconia agglomerate after 20% compressive strain, 5Z. (a) SEI, (b) EDX Zrmap, (c) EDX O map, (d) EDX Mg map, (e) EDX Mn map, (f) EDX Si map, (g) EBSD quality map,(h) EBSD phase map showing t/c-ZrO2 (black), and m-ZrO2 (gray).

forced TRIP-steel condition. The reasons for these

early failures are the formation of cracks and cell

wall displacements as results of the global column

buckling mode as well as reduced ductility caused

by a lower amount of deformation-induced mar-

tensite in sample volume.[14,16] In Figure 4 the cell

wall buckling dominated by the formation and

cross-shaped arrangement of plastic hinges and the

crack initiation in the outer skin and the honey-

comb core are demonstrated for the specimen

reinforced by 10 vol% zirconia and deformed up to

35% compressive strain. As shown in the diagram

in Figure 3(b), the effect of particulate reinforce-

ment by 5 vol% of Mg-PSZ results in a higher

capacity of absorbed energy up to the applied

deformation degree of 50%. The distinctive failure

behavior of the recipe 10Z involves a decrease

of specific energy absorption above 35% strain.

The values for mass- and volume-specific energy

absorption calculated by Equations (1) and (2) are

presented in Table 6. In comparison to other

cellular crash-absorbing materials[1,17–19] the pre-

sented zirconia toughened TRIP-steel composite

honeycomb structures (recipes 5Z and 10Z) offer

excellent energy dissipation amounting to values

between 53.2 and 46.5 kJ � kg�1 (cf. 169.1 and

145.8MJ �m�3) after 50% compressive deformation.

Knowing the phase composition of the compo-

site material is of great importance for the under-

standing of the mechanical behavior and micro-

structure mechanisms during compressive

deformation tests. XRD are state-of-the-art for

qualitative and quantitative analysis in metal and

ceramic materials. Figure 5 presents a diffraction

pattern of recipe 10Z after sintering 2 h at 1 350 8Cin argon atmosphere. As already known from

literature the quantitative phase analysis in zirco-

nia demands XRD at high angles.[20] The amount of

ceramic phase in the composite material is limited

to 10 vol% regardingmechanical properties. Due to

this restriction of the volume ratio the intensities of

zirconia peaks and subsequently the validity of the

interesting peaks is poor. Hence, changes in phase

composition during compressive deformation tests

are hardly detectable by XRD and the demand of an

advanced phase analysis method becomes clear.

In contrast, EBSD measurements enable phase

identification of small particles and clusters,

respectively.[11] Lumpy sample surface even after

polishing, e.g., pores, as well as inhomogeneity in

crystal structure are restricting factors for analyses.

Grain boundaries and highly distorted crystal

58 http://www.aem-journal.com � 2012 WILEY-VCH Verlag GmbH & Co.

lattice or chemical reactions forming new compounds during

sintering, respectively, limit results. Not only microstructure,

but also mechanical properties are shifted by interactions. As

already known from[21] several interactions between zirconia


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Fig. 9. Volume fraction of fcc-austenite, bcc-martensite, and hdp-martensite afterdifferent compressive deformation rates measured by EBSD, 5Z.

and TRIP-steel occur during temperature treatment. Diffusion

of magnesium and alloying elements results in destabilization

of reinforcing ceramic particles during sintering.

Figure 6 presents the steps of EBSD-phase analysis of

non-deformed recipe 5Z starting with SEM image

[Figure 6(a)], EDX mapping [Figure 6(b–f)], EBSD quality

map [Figure 6(g)] and the resulting EBSD phase map

[Figure 6(h)]. The same procedure after 20% compressive

strain is presented in Figure 7. Both spots show zirconia

agglomerates embedded in TRIP-steel matrix. The EDX maps

Figure 6(b) and 7(b) clarify the edge between metal and

ceramic phase enabling separated phase analysis. In case of

the as-fired specimen the EBSD image quality for zirconia

particles is lower in comparison to steel. After plastic

deformation both phases appear with nearly the same validity

due to distorted crystal lattices. The dominating amount of

non-metastable monoclinic phase after sintering is obviously.

Nevertheless, the m-ZrO2 quantity is increased after com-

pressive deformation.

Figure 8 shows the calculated phase evolution of ceramic

particles in 5Z samples as a function of compressive

deformation. The first transformation in zirconia takes place

during sintering with a significant increase of non-

transformable monoclinic regions to 80.5� 4.5%. In reverse

the amount of metastable tetragonal (16.0� 4.0%) and cubic

(3.8� 1.6%) decreases. To keep in mind the as-delivered

powder possesses an m-ZrO2 quantity of 10%. The quantity

increases during deformation test to values of 90.3� 5.8% after

10% compressive strain. As expected the value of metastable

tetragonal phase decreases to 7.7� 4.7%. This is the result of

the stress induced phase transformation. Further compression

does not influence the phase composition significantly. The

scatter of the values is caused by lattice distortions and due to

the restrictions of a statistic method. The cubic phase is almost

completely destabilized during thermal treatment. Hence, in

the present paper only a negligible change from 3.8� 1.6%

after firing to 1.3� 0.7% after 30% deformation is detectable.

The possibility of phase transformation toughening in zirconia

ceramics is already known.[22] However, the influence of cubic

Fig. 8. Volume fraction of monoclinic, tetragonal and cubic phase in zirconia particlesafter different compressive strain measured by EBSD, 5Z.


zirconia for toughening effects during mechanical or thermal

treatment is not yet uniformly clarified.

The small amount of metastable tetragonal phase of the

as-fired specimens results from temperature treatment.

Sintering pure Mg-PSZ in the temperature field of

1 300–1 400 8C leads to a distinct increase of monoclinic phase

due to thermal destabilization.[21] Besides this, in the presence

of manganese, silica and alumina interdiffusion, and chemical

interactions occur. As can be seen in EDXmaps Figure 6 and 7

the concentration of these elements and magnesium, respec-

tively, is increased at grain boundaries. With the aid of EBSD

analysis these regions can be identified as Mg0.38Mn0.62SiO3

and (Mg,Mn)Al2O4. As a consequence themagnesia content in

zirconia lattice is reduced resulting in t!m transformation.

The most limitation factor for continuing advances in

TRIP-steel/Mg-PSZ composite materials can be seen as the

interactions during processing.

Figure 9 presents the corresponding volume fractions in

metal phases during compressive deformation of samples 5Z.

As expected the quantity of austenite (g) decreases with

increasing strain. At low strain particular the g ! e transfor-mation is triggered by deformation. Further compression

leads to e!a0 and g! e!a0 transformations, respectively.

The formation of martensite can also be observed by

deformation bands in Figure 7(g).

Conclusions

The ceramic plastic extrusion route at room temperature

has been applied in order to produce honeycomb macro-

structures based on austenitic stainless TRIP-steel and

TRIP-steel/magnesia partially stabilized zirconia composite

materials, respectively. It can be resumed that the reinforcing

mechanism of additions of ceramic particles enables compo-

site materials with superior mechanical properties. Advan-

tages have been achieved in terms of higher compressive

stresses as well as energy absorption during deformation. The

martensitic transformation in both phases was proven by

EBSD measurements resulting in a strain dependent phase

evolution.


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Three main conclusions after sample producing and

compressive deformation test can be drawn.
(i) H
60

eat treatment of the composite material leads to diffu-

sion and several chemical reactions between alloying

elements and zirconia stabilizing agent resulting in high

amount of non-transformable monoclinic zirconia even

before mechanical testing.
(ii) T he stress induced phase transformation in metastable
zirconia particles occurs at compressive deformations

below 10%. Furthermechanical load does not significantly

trigger the phase destabilization. Nevertheless, improve-

ments of strength at compressive strain below 35% are

significant.
(iii) T he deformation-induced austenite-to-martensite trans-
formation in TRIP-steel matrix is equivalent to strain,

even up to 30%.

In future work the phase composition of the partially

stabilized zirconia particles after sintering has to be changed

regarding higher amounts of metastable tetragonal and cubic

phase. A high potential of reinforcing effects of ceramic

particles can be expected when degreasing the amount of

non-transformable monoclinic zirconia before mechanical

load. Furthermore the mechanism of chemical interactions

between TRIP-steel matrix and zirconia during sintering has

to be investigated.

Received: April 27, 2011

Final Version: June 1, 2011

Published online: July 21, 2011

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