Microstructure and thermal stability of nanocrystalline AZ31 Mg alloys reinforced by ultrafine vanadium particles

In this paper, the microstructure evolution and thermal stability of nanocrystalline (NC) AZ31-2.5 wt%VP magnesium matrix composites were studied, while the hardness was analyzed. The results showed that the NC AZ31-2.5 wt%VP still kept NC scale after annealing at 300 °C and 350 °C. Even at 400 °C and 450 °C, the grain size of the annealed composite only grew to 113 nm and 132 nm, which was still in ultrafine grain scale, illustrating excellent thermal stability of NC AZ31-2.5 wt%VP. Furthermore, the V particles, which was uniformly distributed in the Mg matrix, had no obvious change in dimension after annealing, while it also had no new phase formed. The stabilization mechanism was expounded and found that the pinning effect of V particles effectively inhibited the grain growth of Mg matrix at high temperature. In addition, the grain growth kinetics equation (D6−D06=kt) was utilized to explain the grain growth behavior of NC AZ31-2.5 wt%VP, and the activation energy (Ea) of grain growth was calculated to be 130.9 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Eventually, the hardness of NC AZ31-2.5 wt%VP was contrastive studied and the hardness evolution curves at different temperatures were obtained. After annealing treatment was performed at 450 °C for 180min, the hardness remained at 80 HV, which was higher than that of as-cast AZ31 (53 HV).