Grain Refinement Behaviors of Mg-Nd-Zn Alloy by Zirconium

As a fast developed magnesium alloy family, Mg-Nd-Zn alloys show great application prospect in varied fields. However, the grain refinement behaviors of this alloy family were still unclear. The present work investigated the process dependence of grain refinement effect by Zr refiner for the alloy. Gradually saturation of Zr dissolving as function of stirring time and melt temperature were revealed, giving out a saturate content of soluble Zr about 0.72 wt.%. The dissolve of Zr refiner undergoes three stages with increasing the added Zr refiner, respectively giving rise to grains microstructure without Zr-rich particles, single core Zr-rich particles and multi-shell particles. Most significant grain refinement effect occurred in the first stage without Zr-rich particles formation, due to refinement mechanism proved to be the grain growth restriction. The multi-shell Zr-rich particles were observed, for the first time, with soluble Zr beyond 0.68 wt.%. A limited Zr diffusion model was proposed to understand the formation of such multi-shell Zr-rich particles. Moreover, the Zr-rich particles evolution behaviors were revealed dependent on the melt holding time, building the relationship between Zr-rich particles and the grain refinement recession. The results shed new lights on the grain refinement process design of Mg-Nd-Zn alloys family.