{10-12} Twinning Behavior At Room Temperature In AZ31 Magnesium Alloys

This work aims to investigate evolution process of deformation twins at room temperature in magnesium alloys subjected to various plastic processes.A series of compression experiments were carried out on several commercial AZ31 hot-rolled plates.In situ electron back-scatter diffraction(EBSD)technique was used to study the evolution microstructure during deformation twinning process of AZ31 magnesium alloy under quasi-in-situ compression.The microstructure evolution was examined on the ND-RD plane by electron backscatter diffraction(EBSD)analysis using an HKL Channel 5 System equipped with a scanning electron microscope.First of all,the twinning behaviors are investigated through in situ EBSD observation in an attempt to explore the deformation mechanism and strength mechanism in the process of plastic strain from 0% to 5.4% along the rolling direction(RD).After compression to a true strain amount of 5.4% along the RD,the initial texture with the c-axis aligned parallel to the ND becomes weak.Meanwhile,the twin texture with the c-axis aligned perpendicular to the ND is formed and part of the(0002)poles rotate to RD.Four typical twin-twin structures are investigated:(1)Parallel twins with single variant;(2)Interacting twins with single variant;(3)Parallel twins with double variants;(4)Interacting twins with double variants.Twinning is impeded by the presence of another twin,which is considered as a barrier for intersecting twins to further propagate and growth.Combining experimental characterization and theoretical dislocation analysis,we drew some conclusions that the interaction between twins and dislocation can contribute to strain hardening.(1)Parallel geometry between identical twins would facilitate the twin growth because they have similar stress state and crystallographic orientation.(2)Intersecting twins restricted the twin growth because the incident twin intersected the obstacle twin and then underwent a slow thickening rate at the intersection especially at twin tips.(3)Different twin variants restricted the twin growth by making high misorientation intersection boundaries,causing a reduction in the contribution of twinning to the deformation.(4)Different twin variants have more obvious effect on restricting the twin growth than non-parallel geometry between twin bands.Twin-twin boundary plays an important role in twin-twin interaction for it act as a barrier to retard twin growth.To accommodate a further imposed strain through twinning process,the nucleation of new twins is required.Migration of twin-twin boundary is also an important deformation mechanism to coordinate deformation for further propagation and growth when twins are intersected.Based on the above,one can draw a conclusion that twin-twin interaction induced hardening.The previous study has demonstrated that {10-12} twinning mechanism following three-step sequence: nucleation,propagation and growth.But localised de-twining caused by shear banding induced by uniaxial compression deformation was first found through in situ EBSD observation in this paper.More interestingly,twinning and de-twinning deformation operate simultaneously in a single twinning system,which was not in good agreement with the prediction of previous reports.Uncertain local stress state in individual grains was considered as the cause of this kind of deformation mechanism.The back stress induced in a twin by the surrounding matrix plays an important role in localised de-twining during uniaxial compression.The twinning deformation process consists of the following three steps: twinning,de-twinning and re-twinning.This twinning,de-twinning and re-twinning process needs more energy and larger load than the process of twin growth and coalescence and complete consumption.So we believed that localised de-twining deformation mechanism is one of the strengthening mechanisms in magnesium alloy.The process of twins impacting grain boundaries and stimulating the nucleation of new twins in neighbouring grains is traced by in situ electron backscatter diffraction(EBSD)technique.The formation of paired twin is considered as the mode to relax the stress in the neighbouring grains and four paralleled twin pairs develop in two neighbouring grains indicates that paired twins play an important role in strain compatibility between connected grains.Besides,twin pair and twin chain are the result of local strain compatibility.Twin variants with the second and third highest SF in paired twins have been found in EBSD map,which indicates that the strain compatibility between connected twins affects the variant selection.Furthermore,multiple variants twins inside one grain was also the result of paired twin,which was considered as the cause leading to strain hardening for it significantly retard the rate of propagation and growth of twinning during subsequent deformation.With the strain increase from 1.5% to 3.5%,the width of twin pair increase simultaneously.It suggests that grain boundary sliding or movement move parallel to the boundary and the movement of twin dislocation is along the grain boundaries.These findings are believed to extend the study of nucleation and propagation of {10-12} twins in AZ31 magnesium alloy.