A Mg-6Y-3Zn-1Al Mg HPDC alloy having high thermal stability: Study of grain growth kinetics

Abstract

The thermal stability and grain growth kinetics of a high-pressure die casting (HPDC) Mg-6Y–3Zn-1Al (WZA631, wt%) alloy with long-period stacking ordered (LPSO) phase is studied by subsequent annealing at different temperatures and time. The grain size of the as-cast and annealed samples is determined using the linear intercept method. It is found that grain size of WZA631 alloy hardly changes up to 450 °C, indicating the microstructure is thermally stable. The grain growth kinetics of WZA631 alloy between 450 °C and 550 °C is well described by the kinetic equation, Dn−D0n=kt. The grain growth exponent varies with temperature, ranging from n = 4.98 at 450 °C to n = 1.45 at 550 °C. The values of activation energy Eg are approximately 122.5 kJ/mol and 139.1 kJ/mol, respectively, for annealing temperature below and above 500 °C, which identifies the bulk diffusion to be the principal grain growth mechanism in the studied temperature range. The growth rate constant k at 450–500 °C is around 2–4 orders lower than other typical Mg alloys, suggesting the excellent thermal stability below 500 °C. The grain coarsening temperature TGC and softening temperature TS are all around 500 °C, implying the critical temperature related to the thermal stability of WZA631 alloy is ~500 °C. The microstructure evolution shows that LPSO network structure is a predominant factor inhibiting the grain growth of HPDC WZA631 alloy, and the dissolution of LPSO results in the variation of grain growth exponent.