The Influence Of Interaction Between Dislocations And {10(1-)2} Extension Twins On Mechanical Behavior Of Magnesium Alloys

The ductility of magnesium alloys is usually limited at room temperature due to its lack of easily-activated slip systems, which makes twinning an important deformation mechanism during deformation. At certain loading conditions, such as compression along the extrusion direction of an extruded rod or the transverse direction of a hot-rolled plate, {101_<sub>2} extension twinning is the predominant deformation mechanism. In a {101_<sub>2} extension twinning predominant deformation, the stress-strain curve is featured by a low yield stress and rapid hardening. The process of twinning consists of nucleation and growth, which is closely related to the behavior of dislocations. Twin nucleation happens in an heterogeneous way through the emission of partial dislocations from grain boundaries or other facet defects. Twin growth or detwinning is realized by the gliding and climbing of twinning dislocations along twin boundaries（TBs）. In the present thesis, a compression-unloading-annealing-recompression experiment combined with statistical electron back-scattered diffraction（EBSD） analyses of pure Mg extruded rod was carried out to study the role of dislocations, as well as other mechanisms, in strain hardening during compression along the extrusion direction. Dislocations and TBs were introduced into the AZ31 hot-rolled plate by pre-deformations, and the influences of dislocations on twin nucleation and TB migration were analyzed via EBSD and transmission electron microscope（TEM）. Several conclusions can be reached as follows:（1） During compression along extrusion direction of the pure Mg rod, dislocation associated mechanisms and texture hardening play an important role in the work hardening while the Hall-Petch effect caused by TBs does not work effectively. At low strain, dislocation associated mechanisms are the main hardening mechanisms and their contribution to stress increase with strain. However, the contribution of texture hardening to work hardening is found to be related to the twin volume fraction. The reorientation by twinning hardly induces an obvious strain hardening with a twin fraction below 20%.（2） The pre-introduced dislocations can significantly change the twinning behavior. A decreasing stage appears in the work hardening curve and the peak value reduces due to pre-tension along rolling direction. Plenty of twin lamellae, of which the widths are no more than 1 μm, are observed, as well as secondary twins. Twin-twin intersection hinders the growth of twins and results in stress concentration to activate a new twin.（3） The accumulation and blockage of dislocations at TBs can pin the TBs and increase the activated stress for its migration. The dislocations improve the strain hardening rate during detwinning deformation and reduce the peak value of strain hardening rate. After completely detwinning, small angle boundaries ranging from 2o to 10 o are left behind where TBs previously presented. The interaction between dislocations and TBs damage the coherence of TBs, producing many BP/PB（basal//prismatic or prismatic//basal） facets. Besides, Frank partials blocked in the TBs result in an obvious deviation of the twin boundary from the twinning plane.