Enhanced Grain Refining Effect of Mg–Zr Master Alloy on Magnesium Alloys via a Synergistic Strategy Involving Heterogeneous Nucleation and Solute-Driven Growth Restriction

Zirconium (Zr) emerges as the most effective grain refiner for magnesium (Mg) alloys incorporating Zr. Typically, Zr is introduced in the form of an Mg–Zr master alloy. However, within Mg–Zr master alloys, Zr predominantly exists in a particle form, which tends to aggregate due to attractive van der Waals forces. The clustered Zr is prone to settling, thereby reducing its refining impact on Mg alloys. In this work, a combined pretreatment process for Mg–Zr master alloys was proposed, encompassing the introduction of a physical field to intervene the agglomeration of particle Zr and the employ of high-temperature dissolution and peritectic reactions to promote the solid solution of Zr. The results demonstrate that the particle Zr within the pretreated Mg–Zr master alloy is effectively dispersed and refined, and greater solute Zr levels can be achieved. The subsequent grain refinement ability was studied on a typical Mg–6Zn–0.6Zr (wt%) alloy. The outcome highlights that an improvement in the grain refinement efficacy (32.4%) of Mg–Zr master alloys was obtained with a holding time of 60 min. The pretreated Mg–Zr master alloy significantly augments the efficiency of grain refinement for Mg alloys through a synergistic strategy involving heterogeneous nucleation and solute-driven growth restriction. The crucial factor in achieving effective grain refinement of Zr in Mg alloys lies in regulating the presence and morphology of Zr in the Mg–Zr master alloy, distinguishing between particle Zr and solute Zr. This study introduces a novel method for developing more efficient Mg–Zr refiners.