Microstructure and Mechanical Properties at Room and Elevated Temperatures in AM50-0.3 wt%CaO Alloy

Abstract

The present study is intended to comparatively investigate the changes in microstructure and tensile properties at room and elevated temperatures in commercial AM50(Mg-5%Al-0.3%Mn) and 0.3 wt%CaO added ECO-AM50 alloys produced by permanent mould casting. The typical microstructure of AM50 alloy was distinctively characterized using two intermetallic compounds, <TEX>${\beta}(Mg_{17}Al_{12})$</TEX> and <TEX>$Al_8Mn_5$</TEX>, along with <TEX>${\alpha}$</TEX>-(Mg) matrix in an as-cast state. The addition of a small amount of CaO played a role in reducing dendrite cell size and quantity of the <TEX>${\beta}$</TEX> phase in the AM50 alloy. It is interesting to note that the added CaO introduced a small amount of <TEX>$Al_2Ca$</TEX> adjacent to the <TEX>${\beta}$</TEX> compounds, and that inhomogeneous enrichment of elemental Ca was observed within the <TEX>${\beta}$</TEX> phase. The ECO-AM50 alloy showed higher hardness and better YS and UTS at room temperature than did the AM50 alloy, which characteristics can be mainly ascribed to the finer-grained microstructure that originated from the CaO addition. At <TEX>$175^{\circ}C$</TEX>, higher levels of YS and UTS and higher elongation were obtained for the ECO-AM50 alloy, demonstrating that even 0.3 wt%CaO addition can be beneficial in promoting the heat resistance of the AM50 alloy. The combinational contributions of enhanced thermal stability of the Ca-containing <TEX>${\beta}$</TEX> phase and the introduction of a stable <TEX>$Al_2Ca$</TEX> phase with high melting point are thought to be responsible for the improvement of the high temperature tensile properties in the ECO-AM50 alloy.