Evolution of microstructure and texture during planar simple shear of magnesium alloy

Evolution of microstructure and texture during planar simple shear

of magnesium alloy

Jun-Yun Kang,a ,?Brigitte Bacroix b and Renald Brenner b

Korea Institute of Materials Science,797Changwon-daero,Changwon,Gyeongnam 641-831,Republic of Korea

LPMTM –CNRS,99Av.J.-B.Cle

′ment 93430Villetaneuse,France Received 23December 2011;revised 13January 2012;accepted 20January 2012

Available online 28January 2012

A rolled magnesium alloy was deformed using a planar simple shear apparatus,in which prismatic slip is expected to be a major deformation mode.Examination of the microstructure and texture indicated considerable tension twinning activity.Simulation of ?ow stress and texture using the viscoplastic self-consistent model was able to reproduce this twinning activity.Although the defor-mation twinning had a large in?uence on the microstructure and texture,the ?ow stress was mostly controlled by prismatic slip.ó2012Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.

Keywords:Magnesium alloy;Planar simple shear;Texture;Microstructure;Plastic deformation

Recently,magnesium (Mg)alloys have been

highlighted as lightweight structural materials for a range of devices and vehicles.However,their poor duc-tility at ambient temperature has been a limiting factor for the application of wrought Mg alloys,especially of sheet products [1–7].Their hexagonal close-packed (hcp)lattices have fewer primary slip systems (basal slip)compared with more symmetric cubic lattices.More-over,the critical resolved shear stresses (CRSSs)of other systems are large compared with basal ones.Inevitably,e?ective accommodation of deformation occurring in various geometries is di?cult [8,9].Tension twinning can additionally contribute to strain accommodation but its capacity is limited [10,11].At elevated tempera-ture,Mg alloys become more deformable as the non-ba-sal slip systems can be activated more readily.However,a strong anisotropy in system activation exists due to the still large di?erence in CRSS.Rolled Mg alloys have strong basal textures in which the majority of grains have their crystallographic c-axis parallel to the sheet normal [12,13];this strong texture also adversely a?ects the formability of rolled sheets [14–17].

Due to the relationship between plastic anisotropy and formability,the deformation mechanism of Mg al-loys is of great interest and has been extensively sur-veyed [17–33].While most research has dealt with

tension or compression of Mg alloys,only a small num-ber of works [29–33]concern shear deformation despite its relevance to sheet forming.

In this study,using a novel apparatus,the response of an AZ31alloy sheet under planar simple shear,i.e.the evolution of ?ow stress,microstructure and texture,is presented,then interpreted using a crystal plasticity model.

A rolled,5mm thick AZ31sheet was submitted to a simple planar shear test.Figure 1shows the sample dimension and the geometry of deformation.The veloc-ity gradient,L,in the gauge section is expressed as follows:

L ?0à_c

00000

0B @1

C A e1T

The test was performed using a universal mechanical test machine in which the sample was ?xed in a rigid frame that prevented the rotation of sample by non-coaxial loading.The strain measurement was performed with a video camera which measured the slope of a line marked on the sample surface.Ideally,there should be no reduction in gage section unlike conventional tensile testing,i.e.the measured strain and stress are true ones rather than engineering ones.As will be examined in this study,this type of mechanical test is especially useful to identify which hardening law is operating.The macro-scopic shear plane and direction can be directly adjusted

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3599;e-mail:?rice@kims.re.kr

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Scripta Materialia 66(2012)

654–657

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with respect to slip planes and directions when using tex-tured polycrystals or single crystals of sheet type.Fur-thermore,compared with conventional torsion testing, the sample geometry is much more convenient for prep-aration and analyses.Other details of the apparatus and the procedure can be found in Refs.[34,35].

In this study,from the geometry of deformation with respect to the initial basal texture,it can be presumed that the majority of grains would be situated for pris-matic slip,which is schematically described in Figure

1.The test was performed at room temperature up to

a strain of0.15.The microstructures and textures of the initial and the deformed specimens were then charac-terized by electron backscatter di?raction.

The microstructures with varying amounts of strain are shown in Figure2in which the salient feature is the evolution of deformation twins with strain.At the initial state,the microstructure consists of polygonal grains without twins.With a small strain of0.028,thin lenses of twins form in a few coarse grains.As strain in-creases,more grains are subject to twinning and the twinned volume expands.Within the sample area exam-ined in this study,it can be seen that there was only one variant of twin per grain,as shown in Figure2.Within a grain,multiple twins of the same orientation nucleate at grain boundaries as seen in Figure2b and c,expand in volume as they traverse the mother grain,and merge of the projection as shown in Figure3a and f,respec-tively.The most prominent change in texture is the development of two satellite peaks which are$90°apart from the central peak in the(0002)pole?gures.It known that these correspond to the formation and growth of tension twinning,which results in86 ==<1 210> rotation of lattice,and it can be inferred that the strong selectivity of twin variants produced the only two satellite peaks.At a strain of0.15,the distortion and weakening of basal?ber is evident due to the lattice rotation by slip as shown in Figure3e.

Using a crystal plasticity model,plastic?ow stress, deformation texture and contribution of active deforma-tion mode can be estimated.In this study,a viscoplastic self-consistent(VPSC)model[36,37]based on a?ne for-mulation[38]was used to analyze the strong plastic anisotropy of Mg alloys.In the model,basal,prismatic, pyramidalslip and tension twinning were input as the contributing deformation modes and a small rate-sensitivity index of m=1/19was assumed based on the deformation at ambient temperature.To incorporate deformation twinning in the model,the predominant twin reorientation(PTR)scheme of Tome′et al.[39] was adopted.In this procedure,a grain in which the vol-ume fraction of a speci?c twin system exceeds a thresh-old value is completely reoriented by the predominant twin system.The initial threshold value and the coe?-cient which controls the reorientation resistance[39] were set0.2and0.5,respectively,in this study.Although the assumption of complete reorientation is an oversim-pli?cation,PTR is more suitable than another conven-tional scheme called volume fraction transfer(VFT) [39]because the real microstructure in Figure2b–e shows deformation twinning caused by a single predom-inant system per grain.Moreover,VFT cannot incorpo-rate strain hardening in the simulation[39].

To simulate the evolution of?ow stress,a strain-hardening model was incorporated.Beausir et al.[32] have successfully simulated an experimental?ow curve of an Mg alloy without a hardening model at an ele-vated temperature(250°C).In their work,the hardening observed in the?ow curve can be attributed purely to textural hardening.At a high homologous temperature, microstructural hardening can be counterbalanced by softening due to recovery process,and therefore the evolution of?ow stress can be fully accounted for by

30 mm

18 mm

Prismatic Slip Basal Plane

Geometry and dimensions of the test specimen(SD,

SPN,shear plane normal;RD,rolling direction;

direction;ND,normal

accommodation of the

Microstructure evolution during the deformation(ND inverse pole?gure map overlaid on band contrast map):(a) shear strain.(Note that the acquisition surface and the magni?cation of(a)is di?erent from(b)–(e)).

J.-Y.Kang et al./Scripta Materialia66(2012)654–657655

equivalent of CRSS.H 0and a are constants and were determined to be 160and 0.1,respectively.All elements of the tensor a ij ,which accounts for the interactions be-tween slip or twin systems were set to 1,assuming isotro-pic interaction for brevity.Dc j is the incremental shear strain imposed on the system j .s 00j is the initial RSS while s 0j and s sat