Two different types of deformation behaviors in ultrafine grained Mg alloys at high temperatures and development of the generalized constitutive equation for describing their deformation behavior

The deformation behaviors of very fine-grained Mg–Al- and Mg–Zn-based alloys prepared by severe plastic deformation were examined, compared and analyzed over a wide range of temperatures between 448 K and 653 K at strain rates greater than 10−4 s−1 using strain rate change tests and elongation-to-failure tests. The two types of alloys exhibited different deformation behaviors in the relation between strain rate and stress. At low strain rates, the Mg–Al-based alloy exhibited characteristics of Coble creep, which became important as the temperature increased, whereas the Mg–Zn-based alloy exhibited threshold stress-like behavior associated with grain boundary sliding (GBS), which became obvious as the temperature increased. A constitutive equation was developed that can describe these different deformation behaviors in a single equation. In the equation, the threshold stresses for Coble creep and GBS, which are correlated to each other, were considered. According to the proposed equation, if the microstructure of ultrafine-grained Mg alloys is sufficiently thermally stable to retain small grain sizes, the achievement of high-strain-rate superplasticity controlled by Coble creep is possible. The most important factor that makes Coble creep important in Mg alloys compared with other metals was analyzed to be its relatively high grain boundary diffusion coefficient.