Twinning Behaviour And Twinning Mechanism In Magnesium Alloy AZ31

{1012} twinning plays an important role in magnesium and its alloys duringplastic deformation and it is always responsible for the low yield stress, high yieldasymmetry between tension and compression and anisotropy in these alloys. In order toresolve these problems, it is very important to understand the strengthening mechanismbased on twinning. However, nearly all the strengthening mechanisms in materialsscience are based on slip and the strengthening mechanism based on twinning is stillvery limited up to these days because of ambiguity in twinning behaviour and twinningmechanism. Therefore, it is still necessary to do more investigations on twinningbehaviour and twinning mechanism in magnesium alloys. In this paper, the essential ofmicrostructural evolution during compression in extruded magnesium alloy AZ31wasrevealed via texture and microstructure analysis. The possibility to decrese the yieldasymmetry via introducing lamellar {1012} twins by pre-compression was investigatedand discussed. It was found that the yield asymmetry can be decreased by pre-strain ifthe amount of pre-strain was appropriate. In order to further understand the twinningmechanism, a model was established to describe the atomic motion in {1012} twinningand the law of atomic motion in {1012} was revealed in magnesium. The majorconclusions can be summarized as follows.①Uinaxial compression along extrusion direction (ED) was carried out in anextruded magnesium alloy AZ31. The microstuctual analysis showed that the amount oftwins increased as a function of strain if the strain is relatively low. However, it began todecrease with continual increase in strain, or even disappeared at last. It seemed thatdetwinning occurred in this process. However, the texture analysis showed that nodetwinning occurred. The essential of microstructual evolution in this process is actuallytwin nucleation, twin growth and coalescence, leading to an analogous detwinningphenomenon observed in the optical micrographs.②Pre-compression along ED has an obvious effect on subsequent compressionperpendicular to ED in an extruded magnesium alloy AZ31. The results showed that theyield stress under compression perpendicular to ED increased obviously if there waspre-compression along ED. The reason for this change is mainly the reorientationproduced by twinning during pre-compression, leading to a low basal activity. Therefore,the yield stress of sample with pre-strain is lower than that without pre-strain. And also, samples under different loading modes exhibited different twin characteristics. Forextruded magnesium alloy AZ31, twins are parallel to each other in a grain whencompress along ED, but the twin morphologies are always multiple if the sample issubjected to8%pre-compression along ED and then3%compression perpendicular toED. For the sample subjected to8%pre-compression along ED and then3%compression perpendicular to ED, twins are parallel to each other in some grains butintersectant in most cases. The intersecting angle is always60°.③Lamellar {1012} twins produced by pre-strain have a significant effect indeformation behaviour during subsequent tension or compression.{1012}-typetwinning occurred when samples were subjected to compressive pre-deformation. Thecompressive yield strength of sample with compressive pre-deformation increasedgradually with the increase in compressive prestrain. The tensile yield strength of thesample decreased rapidly with the increase in compressive prestrain from zero to~1.7%,but it was nearly unaltered when compressive prestrain was higher than~1.7%.Detwinning occurred in the twinned regions played a role in the subsequent tensileprocess so that the yield stress was lower than sample without prestrain. Yieldasymmetry between tension and compression decreased gradually when compressiveprestrain was lower than~1.1%, but it began to increase after that. It suggests that yieldasymmetry can be effectively controlled by appropriate pre-deformation.④Detwinning is actually a twinning process occurs in twinned region whensample is subjected to a reversed stress. The maximum detwinning volume can bedetermined by controlling the pre-strain. It is meaningful to investigate the roles oftwinning in strain hardening via investigating the deformation behaviour of sampleswith different pre-compression. In extruded magnesium alloy AZ31, samples withdifferent twinned fraction were obtained by controlling the pre-strains. These samplesexhibited different strain hardening behaviour during subsequent tension. There was nothe stage that strain hardening rate increased with increase in true strain in samplewithout pre-compression, but for samples with pre-compression, that stage existed.Although samples with different pre-strain exihibited a similar strain hardening trend,the strain hardening behaviours were still different. For example, for samples with3%and9%pre-compression, there was the stage that strain hardening rate increased withincrease in true strain, however, for the sample with3%pre-strain, strain hardening ratebegan to decrease when the true strain was relatively low, but it lasted to a relativelyhigh true strain in the sample with9%pre-compression. Because detwinning completed earlier in sample with low pre-strain than that with high pre-strain, the results indicatedthat twinning played a very important role in strain hardening in magnesium alloys. Andalso, sample with higher pre-strain exhibited higher elongation because detwinningcontributed to plastic deformation.⑤Atomic displacement vectors during twinning were calculated based on thesymmetry principle. The law of atomic motion can be summaried as a movement ofatom group containing rotation and translation. The translation vector between twoadjacent layers of atom group is a twinning dislocation bT(bT=0.049nm). If many atomgroups complete rotation and translation, a twin with a certain thickness will beobtained.⑥In magnesium lattice, the rotation of atom group can be activated by movementof B-type atom which is closed to twinning plane. The movement vector is1/6〈1010〉.The rotation of atom group will promote the occurrence of twinning dislocation in〈1011〉direction, leading to the translation of atom group. Dissociation of grainboundary defects or dislocations pile-up may lead to1/6〈1010〉partial dislocation.