Effect of cooling rate on the microstructure and mechanical properties of sand-casting Mg-10Gd-3Y-0.5Zr magnesium alloy

The present study investigated the effect of cooling rate on the microstructure and mechanical properties of sand casting Mg-10Gd-3Y-0.5Zr alloy with a cooling rate range of 0.7-3.6 °C/s. When the cooling rate was increased, the average grain size of α-Mg decreased from 59 μm to 39 μm, the volume fraction of the second phase increased from 17.6% to 24.5%, and the eutectic compound exhibited continuous network instead of coarsening discontinuous network of plate-shaped microstructure. Energy-dispersive X-ray test results suggested that solute content of RE elements (Gd and Y) was low within the area near the grain boundary of α-Mg with high cooling rate, and solid-solution reaction occurred in the sand casting process. In addition, Vickers hardness (HV) testing indicated that HV increased with the increasing cooling rate, and the volume fraction of second phase (V) played a main role in hardness of the alloys. There was a linear relationship between them, the fitting function was HV=1.25 V+63.05, R2=0.9989. Tensile strength test showed that both of ultimate tensile strength (UTS) and tensile yield strength (TYS) first increased and then decreased with the increasing cooling rate. Based on fracture observations, the types of fracture surface characteristics are correspondingly classified into three modes, transgranular, dimple-like fracture and intergranular fracture with the increasing cooling rate.