Research On The Twinning Behavior Of AZ31 Magnesium Alloy Processed By Multi-directional Compression At Room Temperature

Twinning plays an important role in plastic deformation of magnesium alloys at low temperature.A commercial hot-rolled AZ31 Mg alloy plate with a strong basal texture was used in this thesis.Two kinds of cube samples were prepared for test.Multi-direction compression had been carried out at room temperature with different strain rates,strain paths and pass strains.The microstructure and texture evolution during deformation had been quantitatively characterized with electron backscattering diffraction(EBSD).Twinning behaviors of grains with various orientations under different deformation conditions had been systematically investigated.The effect of twinning structure induced by pre-deformation on subsequent deformation behavior and its underlying mechanism had been discussed in detail,demonstrating the influence of deformation condition on twinning variant selection mechanism.The main conclusions found are as follows:(1)The twinning behavior is closely related to strain paths and pass strains during quasi-static multi-direction compression.When compressed along TD-RD-ND path with a small pass strain,a minor amount of the volume fraction of twinning will be appeared.The twins are mainly generated in residual matrix when compressed along RD,leading to the increasing of the twin lamellae.However,twin lamellae produced by pre-deformation will hinder the expansion of new twins.When compressed along ND for next pass,the number of primary twin lamellae reduces and yield stress decreases because of the de-twinning.The volume fraction of twinning is enhanced with pass strain increasing.The second twins are generated within pre-deformation twins during second pass.The similar phenomenon takes place when given third pass which can decrease the occurrence of de-twinning and yield stress For ND-RD-TD path,basal slip becomes the main deformation mechanism and a few contraction twins and second contraction twins are generated when compressed along ND.But owing to the small quantities,these pre-compression twins have no obvious effects on subsequent deformation.Texture evolution is mainly controlled by tension twinning during multi-direction compression.Moreover,and the transformation is more obvious with the increasing of strain pass.The c-axis of the grains are rotated to nearly parallel to the compression direction by tension twins.And various twin variants are beneficial to weaken texture.Pre-deformation twins intersect with twins generated in subsequent deformation,which can separate grains and weaken texture.(2)Compared with quasi-static deformation,it is obvious that strain hardening rate and the twinning level increase during dynamic plasticity multi-direction compression.However,the texture weakening isn't so significant because of the large volume fraction of twinning.Through third pass,the grain refinement efficiency of dynamic plasticity samples is better than that of quasi-static,which can get more homogeneous microstructure.(3)The variant selection of {10(?)2} twinning basically corresponds to Schmid law during multi-direction compression.The twinning lamellae are linked to others in various ways,such as different variants connecting inside a grain,twin patterns between adjacent grains and twin chains crossing several grains.There generally exists strain compatibility among these connecting twins.At the same time,it can be observed that most connecting twinning lamellae both have high Schmid factor and strain compatibility factor.With the growth of pass strain,the amount of grains containing various second twin variants increases.However,the higher Schmid factor and strain compatibility factor decrease during dynamic plasticity deformation.