Precipitate Characteristics and Mechanical Performance of Cast Mg–6RE–1Zn–xCa–0.3Zr (x = 0 and 0.4 wt%) Alloys

Abstract

In this study, the Mg–4Y–1Gd–1Nd–xCa–1Zn–0.3Zr (x = 0 and 0.4 wt%) cast alloys with low rare earth concentration were prepared in different routes of heat treatments, and their microstructures and mechanical properties were investigated. The Mg–4Y–1Gd–1Nd–1Zn–0.4Ca–0.3Zr cast alloy with ultimate tensile strength (UTS) of 264 ± 7.8 MPa, tensile yield strength (TYS) of 153 ± 1.2 MPa and elongation to failure (EL) of 17.2 ± 1.2% was successfully developed by appropriate heat treatment. The improved mechanical performance was attributed to the combined strengthening effects of fine grains, Mg₂₄RE₅, $\beta ^{\prime}$, $\beta _{1}$, $\gamma ^{\prime}$ and LPSO phases. In the heat treatment process, cooling method of T4 treatment affected the microstructure, which consequently determined the mechanical properties air cooling, rather than water cooling, gave rise to the formation of $\gamma ^{\prime}$ phase in the alloy without Ca addition. However, Ca addition facilitated the formation of $\gamma ^{\prime}$ phase, and the $\gamma ^{\prime}$ phase precipitated in the alloy after T4 treatment either by water cooling or by air cooling, but the air cooling increased the number density of $\gamma ^{\prime}$ phase in comparison to the water cooling. Although the $\gamma ^{\prime}$ phase strengthened the studied alloys, the formation of $\gamma ^{\prime}$ phase inhibited the precipitatition of $\beta ^{\prime}$ and $\beta _{1}$ phases in the following T6 treatment, and consequently reduced the strengthening effect of $\beta ^{\prime}$ and $\beta _{1}$ phases. The results showed that the mechanical performance of the studied alloys was largely determined by the precipitation of $\gamma ^{\prime}$ phase, which was regulated by the Ca addition and the cooling method of T4 treatment.