Effect of trace Gd on microstructure and tensile property of ZK60 cast magnesium alloy

This study systematically investigates the effects of trace Gd additions of 0.5% (mass fraction, the same hereinafter) and 1.0% on the microstructure and tensile property of ZK60 magnesium alloy. The as-cast and solution-treated microstructures of ZK60, ZVK600, and ZVK610 alloys are characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, differential scanning calorimetry, and X-ray diffraction. The tensile properties of the alloy specimens are measured and analyzed via room-temperature tensile tests. The results show that the as-cast ZK60 alloy has grain size of 95 μm, with coarse blocky MgZn phases and a small number of Zn₂Zr₃ particles present at grain boundaries. Trace Gd addition increases the fraction of secondary phases and transforms the MgZn phase into the Mg₃GdZn₆ phase, but does not refine the grain size. The room-temperature tensile properties of the three as-cast alloys are relatively close. The as-cast ZVK610 alloy exhibits lower yield strength and ductility, which is associated with its relatively large grain size and increased grain-boundary secondary phases. After T41 step solution treatment, the grain of ZK60 alloy become oarse, the secondary phases are nearly eliminated, and the ductility is significantly improved. However, the yield strength decreases slightly due to grain coarsening. In contrast, a small amount of grain-boundary secondary phases remains in ZVK600 and ZVK610 alloys. T42 and T43 processes, designed with prolonged high-temperature solution time or elevated solution temperature, further reduce the secondary phase fraction in the matrix but lead to additional grain coarsening, resulting in further reduced yield strength and no obvious improvement in ductility. Therefore, T41 solution heat treatment process is recommended.