Development of high mechanical properties and moderate thermal conductivity cast Mg alloy with multiple RE via heat treatment

A new cast Mg–2Gd–2Nd–2Y–1Ho–1Er–0.5Zn–0.4Zr (wt%) alloy was prepared by direct-chill semi-continuous casting technology. The microstructure, mechanical properties and thermal conductivity of the alloy in as-cast, solid-solution treated and especially peak-aged conditions were investigated. The as-cast alloy mainly consists of α-Mg matrix, (Mg, Zn)₃ RE phase and basal plane stacking faults. After proper solid-solution treatment, the microstructure becomes almost Mg-based single phase solid solution except just very few RE-riched particles. The as-cast and solid-solution treated alloys exhibit moderate tensile properties and thermal conductivity. It is noteworthy that the Mg alloy with 8 wt% multiple RE exhibits remarkable age-hardening response (ΔHV = 35.7), which demonstrates that the multiple RE (RE = Gd, Nd, Y, Ho, Er) alloying instead of single Gd can effectively improve the age-hardening response. The peak-aged alloy has a relatively good combination of high strength/hardness (UTS (ultimate tensile strength) > 300 MPa; TYS (tensile yield strength) > 210 MPa; 115.3 HV), proper ductility (ε ≈ 6%) and moderate thermal conductivity (52.5 W/(m K)). The relative mechanisms mainly involving aging precipitation of β¢ and β′′ phases were discussed. The results provide a basis for development of high performance cast Mg alloys.