G–D Map Method for Analyzing Effects of Elements on Size Control of In Situ Formed Mg2Si in Mg Melts

Abstract

An approach for selecting alloying elements to control the size of an in situ formed phase is proposed based on a G–D map, which is derived from basic thermodynamic and kinetic principles. The G–D map is divided into four zones. Elements in Zone III are potential refiners whereas those in Zone II should be avoided in processing. When applied to the Mg2Si/Mg system, we find that Sr, P, Sb and Sn refine Mg2Si by increasing the nucleation rate and inhibiting the growth rate. Conversely, Ti, Mn, Zr and V contribute to coarse-shaped Mg2Si by postponing nucleation. The content of alloying elements should be matched with the Si content, and the required amount can be pre-estimated from the G–D map. Mg2Si formed in Mg-Al and Mg-Zn alloy is smaller than that in Mg-Si and Mg-Mn alloy, because Al and Zn suppress the diffusion of Si, particularly at high Si contents. The predictions of this approach are consistent with experimental observations. Hence, the G–D map can provide useful guidance for quantitative phase refinement design.