A novel superplastic mechanism-based constitutive equation and its application in an ultralight two-phase hypereutectic Mg-8.42Li alloy

A superplasticity mechanism based constitutive equation incorporating deformation-induced grain growth and deformation-accelerated grain boundary diffusion was established. The constitutive equation established was applied to the superplastic two-phase hypereutectic Mg-8.42Li alloy. The contribution of net static grain growth to entire grain growth accounts for 51.65% while the contribution of net strain grain growth to entire grain growth accounts for 48.35%. It is shown by calculation that the grain boundary diffusion in the superplastic deformation of fine grained microduplex Mg-8.42Li alloy at low strain rates is not accelerated obviously and deformation-accelerated grain boundary diffusion takes place at high strain rates. It is indicated by normalizing the data of calculation and experiment that the flow mechanism of this alloy at 573 K at an initial strain rate of 5×10−4 s−1 is modified Mukherjee's grain boundary sliding controlled by the motion of the dislocations in the grain boundaries by a climb-glide process and Gifkins's grain boundary sliding occurring by the motion of grain-boundary dislocations that pile up at triple point.