The contribution of grain boundary sliding to the deformation in an ultrafine-grained Mg-Al-Zn alloy

Severe plastic deformation through high-pressure torsion is used to refine the grain structure of the Mg-Al-Zn alloy down to similar to 140 nm and low temperature annealing is used to produce samples with different grain sizes, within the ultrafine range. The mechanical behavior is investigated using different testing techniques including microhardness, indentation creep, plane-strain compression, creep, and miniaturized tensile testing. The results allow a comprehensive analysis of the deformation mechanism. It is shown that the relationship between the flow stress and the inverse of the square root of the grain size is not linear in the ultrafine grained range and depends on temperature and strain rate. Grain refinement hardening and grain refinement softening can be observed at different temperatures and strain rates. There is an increase in strain-rate sensitivity and a decrease in apparent activation volume with decreasing the grain size. These experimental observations agree with the deformation mechanism of grain boundary sliding provided the thermal contribution for the threshold stress is taken into account.