Superplasticity, dynamic grain growth and deformation mechanism in ultra-light two-phase magnesium–lithium alloys

The maximum superplasticity of 850% and 920% were demonstrated in Mg–7.83Li alloy and Mg–8.42Li alloy at 573 K and at an initial strain rate of 1.67 × 10−3 s−1 and at an initial strain rate of 5 × 10−4 s−1, respectively. Optical microstructural observations revealed that dynamic grain growth appears at 573 K. Different atomic mobilities of Mg, Li, alpha 5.7Li phase and beta(11Li) phase and different Gibbs free energies of alpha 5.7Li phase and beta(11Li) phase contribute to the obvious dynamic grain growth at 573 K. The dynamic grain sizes at 573 K were calculated by derived model. It is shown by comparing the stress exponent (n), grain size exponent (p), and deformation activation energy (Q) estimated experimentally with constructed deformation mechanism map incorporating dislocation quantities inside grains that the dominant deformation mechanisms in two alloys are grain boundary sliding accommodated by slip controlled by lattice diffusion.