Microstructure evolution and mechanical properties of an ultra-high strength casting Mg–15.6Gd–1.8Ag–0.4Zr alloy

Abstract

An ultra-high strength casting Mg–15.6Gd–1.8Ag–0.4Zr alloy has been developed by conventional ingot metallurgy and heat treatment. The alloy exhibits outstanding ultimate tensile strength of 423MPa and tensile yield strength of 328MPa in the peak-aged condition. The main factors contributing to the ultra-high strength are the high number density of the prismatic β′ plates and basal γ″ plates and their relative perpendicular distribution. Besides, a newly intermetallic compound is found in the as-cast microstructure other than Mg5(Gd,Ag) compound. Although, the retained intermetallic compound at the grain boundaries in the alloy solution-treated at 480°C for 18h and at 500°C for 8h has a similar composition to Mg5(Gd,Ag) compound, its crystal structure is demonstrated to be different to the two types of intermetallic compounds in the as-cast condition. Lastly, the effects of intermetallic compounds and precipitation phases on the fracture behavior of the alloy have been investigated, namely dominant inter-granular fracture in the as-cast and peak-aged conditions and mainly trans-granular fracture in the solution-treated condition.