Development of heat resistant Mg-Zn-Al-based magnesium alloys by addition of La and Ca: Microstructure and tensile properties

•Microstructural evolution and tensile properties of Mg-Zn-Al-La-Ca alloys were studied.•The Al2LaZn2, Al4Ca and Ca2Mg6Zn3 isomorphous phases were formed in the as-cast alloys.•The combination of La and Ca has an obvious effect on the strength and ductility.•Mg-4Zn-4Al-2La-2Ca alloy shows optimal mechanical properties at room temperature and high temperature.

In order to clarify the potential of developing Mg-Zn-Al-RE-Ca-based heat resistant magnesium alloy, the influences of different La and Ca combinations on the microstructure and mechanical properties of Mg-4Zn-4Al-xLa-yCa (x + y = 4) alloys have been investigated. The results show that the high content of La and Ca additions promotes Al2LaZn2, Al4Ca and Ca2Mg6Zn3 isomorphous phases forming in the as-cast Mg-Zn-Al-La-Ca alloys, instead of the formation of low melting point Mg-Al and Mg-Zn-Al eutectics. Among all the secondary phases, the Al2LaZn2 phase is prior to form by consuming up La atoms, and then the Al reacts with Ca to form Al4Ca, and finally the surplus Zn reacts with Ca and Mg to form Ca2Mg6Zn3 phase. The Ca2Mg6Zn3 isomorphous phase contains a certain amount of Al, and its volume fraction is increased with increasing Ca content. The tensile strengths at both room temperature and 175 °C are increased with increasing Ca addition, while the ductility is decreased due to the increased formation of networked Ca2Mg6Zn3 phase. The Mg-4Zn-4Al-2La-2Ca alloy shows the optimal tensile properties at both room temperature and high temperature.