On the role of twinning and stacking faults on the crystal plasticity and grain refinement in magnesium alloys

We have investigated the detailed microstructural mechanisms associated with the excellent crystal plasticity and ultra-fine grain refinement observed under high strain-rate deformation of Mg alloys, focusing on ZK60. Firstly, we have identified the clear formation of stacking faults in deformation-induced twinned crystal segments. Specifically, we have found that intrinsic I1 and I2 stacking faults bounded by 16<2¯ 023> and 13<101¯ 0> partial dislocations, respectively, were found to occur in very high number densities within the twins. This was due to the high Schmid factor for stacking fault shearing in twins and the critical role that twin boundaries played in emitting partial dislocations. Secondly, we have clarified the interplay between twinning and stacking faults on the enhanced crystal plasticity. Apart from the strain accommodated by the extensive twinning itself, we propose that the improved plasticity during high strain-rate deformation is mainly due to the nucleation of 13<11¯ 23>{112¯ 2} dislocation within twins, which provides enough independent slip systems to achieve a homogeneous deformation in the material. Finally, we have demonstrated the interplay between twinning and stacking fault formation on the nucleation of new grains via dynamic recrystallisation. The twin boundaries and stacking faults, especially those of the I1 type, facilitate the formation of low-angle grain boundaries that can subsequently transition into high-angle grain boundaries, and form ultra-fine dynamically recrystallised grains.