AbstractThe alloys Mg–6Si and Mg–4Si–4Zn (wt-%) were extruded at 573 K and the mechanical properties investigated by tensile testing at strain rates from 2×10−5to 2×10−2s−1and at temperatures in the range 673–773 K. The Mg–Si alloy behaved as M type at any given strain rate. The Mg–Si–Zn alloy behaved as A type at strain rates above 2×10−4s−1. The activation energy for the deformation process was higher than those for lattice self diffusion and for the chemical interdiffusion of Zn in Mg. This is probably related to the cross-slip of dislocations. For the Mg–Si–Zn alloy, superplastic behaviour was found at strain rates below 2×10−4s−1. The dominant superplastic deformation process is likely to be grain boundary diffusion controlled grain boundary sliding.MST/3192