A comparative study of plastic deformation mechanisms in room-temperature and cryogenically deformed magnesium alloy AZ31

In this work, a commercial cast magnesium alloy AZ31 was compressed at room and cryogenic temperatures (RT and CT) to study how the twins and dislocations affect the flow stress, plastic strain and strain hardening. Cryogenic deformation suppresses the dislocation activities to enable the effects of dislocation slip and twinning on plasticity to be separately interrogated. A quasi-in-situ Electron Backscatter Diffraction (EBSD) method was used to trace the evolution and distribution of twins and geometrically necessary dislocation (GND) density manifested as kernel average misorientation (KAM) in the same group of grains before and after plastic deformation, at RT and CT respectively. Compared to the RT deformation, higher flow stress, higher strain hardening rate and moderately reduced strain to failure were observed in the CT deformation. These interesting findings were rationalised by the subsequent EBSD analysis. It is found that the increased fraction of finer twins and twin twin interactions rather than dislocation density leads to higher stress and higher strain hardening at CT. This suggests that the hardening effect induced by twin boundary and twin-twin interactions is considerably more effective than dislocation density hardening.

Automated summary

The compression of a commercial cast magnesium alloy at room and cryogenic temperatures showed that cryogenic deformation resulted in higher flow stress, higher strain hardening rate, and moderately reduced strain to failure compared to room temperature deformation. This was attributed to the increased fraction of finer twins and twin twin interactions at cryogenic temperatures, which was found to be more effective in hardening than dislocation density.