A comparative study of indentation induced creep in pure magnesium and AZ61 alloy

Nanoindentation creep tests were performed on pure Mg and Mg-6 wt% Al-1 wt% Zn (AZ61) alloy at room temperature. Plastic flow and creep were more pronounced in Mg than in AZ61. Pure Mg was found to exhibit two distinct steady state stress regimes, with the values of stress exponent equal to 4.5 and 12. Twinning is most likely to dominate initially, in the high stress regime. Crystallographic re-orientation, caused by twinning renders the grains favorable for 〈c+a〉 slip in low stress regime. Grain boundary creep was also investigated, and appeared to be related to grain boundary sliding (n=2) in low stress regime. Creep response of AZ61 consisted of a single stress exponent regime (n~1) for both grain interior as well as grain boundary. A combination of extension and contraction twinning seems to be the dominant deformation mechanism. Chemical potential gradient induced diffusion is also likely to play an important role in creep deformation.