The effect of Ce on the modification of Mg2Si phases of as-cast eutectic Mg-Si alloys

Abstract

The eutectic Mg2Si phase in as-cast Mg-1.3Si alloys appears as a needle or Chinese-script-type morphology. However, when 0.5 wt% Ce is added to a Mg-1.3Si alloy, the morphology of the eutectic Mg2Si phase changes to very fine fibers. Previous studies found that explaining the effects of Ce addition on the modified Mg2Si phase in Mg alloys is challenging. In this study, a new mechanism for modifying the Mg2Si phase in Ce-containing Mg-Si alloys was considered based on microstructure-correlated crystallographic analyses. When Mg2Si phases are grown during solidification, Ce solutes pile up in front of the solid/liquid interface layer and react with Mg, Si, and Ce to form the CeMg2Si2 phase. Tetragonal CeMg2Si2 phases were investigated by high-resolution transmission electron microscopy (TEM) and they were found to be located on one side or on both of the edges of the modified Mg2Si phase. In addition, an analysis of the automated crystal orientation and phase mapping in TEM showed that thin nanorods such as those in the CeMg2Si2 and refined Mg2Si phases are oriented vertically in preferred growth directions <001> and <111¯> for CeMg2Si2 and Mg2Si, respectively. Thus, CeMg2Si2 phases are responsible for the restricted growth of Mg2Si phases.