Research On Plastic Deformation Heterogeneity And Deformation Mechanisms Of AZ31 Magnesium Alloy

Magnesium alloys possess excellent properties and have a widespread range of applications.The poor plasticity at room temperature and significant heterogeneity during deformation limit,however,the production of magnesium alloys.As such studies on deformation heterogeneity and of the intrinsic plastic mechanisms play an important role in understanding the mechanical behaviors of magnesium alloys.In this thesis,a commercial rolled AZ31 magnesium alloy was chosen as the initial material,and a range of efficient methods to characterize the local strain and grain scale microstructural heterogeneity were explored,primarily utilizing digital image correlation(DIC)and electron backscatter diffraction(EBSD)techniques.Based on these investigations,a better understanding of the inherent relation between deformation heterogeneity and plastic mechanisms,and also of the plastic mechanism beh avior under local deformation conditions,was achieved.The main results found are summarized below:High resolution strain mapping at the surface of a deformed sample was realized with the help of nanoscale markers.The observed strain distribution at the grain scale,and even the twinning scale,demonstrate the strain heterogeneity of the deformed sample.Combined with an analysis of the microstructure and expected plastic deformation mechanisms,the results assist in understanding the generation of strain localizations.The results show that the strain distribution is not only influenced by the crystallographic orientation of one region,but may also be influenced by its neighbors.In particular,strain accumulates at some interfaces due to the lack of efficient plastic mechanisms for strain transfer across the interfaces.To understand further the local mechanisms of plastic deformation,a statistical study of twin transmission behavior at grain boundaries was carried out.The results demonstrate the propensity for twin transmission to take place at low misorientation angle boundaries,and reveal some influence of the Schmid factor on twin transmission.The non-Schmid twinning behaviors were explained from the perspective of strain accommodation by using a geometric compatibility factor,m'.More detailed analysis shows that whereas the m' parameter is useful for explaining the twin variant selection under local strain,it does not perform well for predicting where twin transmission events will take place.Based on the orientation data collected using EBSD,two visualization approaches to characterize the microstructural heterogeneity at the grain scale were proposed.It is demonstrated that the "in-grain orientation spread" method is efficient and superior to other methods in revealing the pattern of grain subdivision,and that the "in-grain orientation evolution" method can provide extra information about deformation band formation on a mesoscopic scale.The method shows that a clear pattern of grain subdivision occurs even at very small plastic strains.It is shown,moreover,that the different rotation behavior of different parts of a grain can be attributed to differences in the relative activity of a same set of slip systems according to a Sachs analysis and the low energy dislocation structures(LEDS)model.The proposed methods are also useful in understanding the inhomogeous microstructure evolution and the dynamic recrystallization nucleation mechanisms during warm deformation.It is found that grain coarsening at low strains can lower the system energy,thus weakening the intragranular deformation heterogeneity.The grain boundary migration during grain coarsening follows the law to reduce the total boundary energy.As the strain increases,in-grain heterogeneities become prominent,and grain boundary protrusions and a strain induced square-like grain boundary migration morphology are observed in the deformed structure.Using the “in-grain orientation evolution” method both discontinuous dynamic recrystallization(DDRX)and continuous dynamic recrystallization(CDRX)nucleation mechanisms are found in the magnesium alloy AZ31 during warm deformation at 200?.