Twinning Behavior And Twin-slip Interaction In AZ31 Magnesium Alloys Under Two Loading Methods

Basal slip and {10(?)2} extension twinning are most common room-temperature deformation mechanisms in Mg alloys due to their low critical resolved shear stress(CRSS).Their interaction in the grain interior and at grain boundary would significantly affect the microstructural evolution and mechanical properties of Mg alloys.In the present study,AZ31 alloys with typical extrusion texture and strong basal texture were deformed by uniaxial compression and Vickers indentation,respectively.Quasi-in-situ scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD)were combined to examine various slip patterns in twin interior and surrounding matrix.The analyses of Schmid factor(SF)and composite Schmid factor(CSF)were further employed to understand the slip-twin interaction in grain interior and at grain boundary.The orientation dependent twinning microstructure was characterized by serial polishing and EBSD.Specifically,the effects of twins on indentation morphologies were analyzed.Uniaxial compression along extrusion direction was applied on AZ31 rod to generate extensive basal slip traces and ｛10(?)2} extension twins in the grain interior.Their interactions would lead to several typical slip patterns near the twin boundary.As a result of the interaction between basal slip and {10(?)2} extension twins,the slip dislocations might be absorbed by the twin boundary or traverse across the twin boundary via the formation of basal slip traces in the twin.The initially basal slip traces would be deflected in the twin as a result of the twinning shear.The activations of[a60°]or[a120°]type basal dislocations are most likely due to the higher SFs.The statistical results indicate that the strain accommodation effect between slip and twinning was popular and could activate a certain amount of non-Schmid twins.According to theoretical derivation and crystal plasticity finite element modeling,a new composite Schmid factor(CSF)is acquired to describe the twin transfer across grain boundary by considering the applied stress and local shear stress produced by the predominated deformation mechanism in neighboring grain.The CSF for twinning is closely related to SF and grain boundary(GB)misorientation.The common grain boundary twins in a compressed AZ31 Mg alloy interrupted at two different strain levels are analyzed in a statistical manner.A CSF threshold for the activation of common-boundary twins is noticed and it decreases with the applied strain.By contrast,the SF threshold and its dependence with strain is not obvious.Compared with the traditional SF,the CSF is more effective in predicting the variants of common-boundary twins especially for small strain.Moreover,the statistical study also shows that high values of the CSFT/CSFBS and CSFT/CSFPS ratios favor twin transmission across GB s.The applicability of CSF on twin variant selection is discussed by analyzing two strain accommodation hypotheses for slip-twin interaction.The CSFST and CSF’ST parameters are obtained to describe the additional effect of local shear stress by neighboring slip on the twinning behavior.Their difference is associated with the SF difference and m’ value between the active twin and neighboring slip variant.The statistical study shows that both types of CSFs could effectively interpret the variant selection of slip-stimulated twin.Furthermore,most of the slip-twin interactions mainly occurs at the grain boundaries with high angle misorientations ranged from 30-70°.Compared to three other potential interactions at GB,relatively higher CSFST or CSF’ST values of slip-twin interactions are also statistically revealed.Therefore,CSF could be realized as an effective indicator to estimate the possible deformation mechanisms on the both sides of a given boundary.Several Vickers indentations were performed in fine-grained and textured Mg alloys with a wide range of crystallographic orientations.The indentation microstructures were characterized by EBSD combined with serial sectioning using electrolytic polishing.The spatial distribution of three-dimensional twinning microstructure is well explained by indented crystallography plane and diagonal orientation with respect to the crystallographic axes.Specifically,completely twinned grains and profuse twin chains were observed in the area around and underneath the indents.Conspicuous site-dependent twinning systems is responsible for the development of a sharp local basal texture.A geometric compatibility factor is used to illustrate the twinning strain crossing grain boundary,which is associated with a specific misorientation-angle distribution with a sharp peak near 30° in stir zone.The relationship between Vickers indentation morphologies and twinning microstructure is further interpreted.On the one hand,the symmetry of indentation morphology corresponds to that of twinning microstructure and diagonal orientation would not change the symmetry.On the other hand,predominate sink-in patterns are related to profuse {10(?)2} extension twinning.The sink-in patterns induced by extension twinning are mainly affected by twinning shear strain,texture hardening effect and barrier effect of twin boundary on matrix slip.In addition,pile-up was appeared in regions where just single slip traces were present.