Effects of cooling rate on the microstructure and mechanical properties of Mg-5.0wt% Zn-2.5wt% Gd alloy

: The effects of cooling rate (0.32, 0.70, 7.7, and 10.08 °C/s) on the microstructure, fracture behavior, and mechanical properties of as-cast Mg-5.0wt% Zn-2.5wt% Gd alloy were systematically examined. The results indicate that increasing the cooling rate significantly reduces grain size, secondary dendrite arm spacing, and eutectic spacing. Additionally, the alloy's microstructure, including the type and volume fraction of secondary phases, changes with varying cooling rates. At lower cooling rates of 0.32 and 0.70 °C/s, the microstructure consists of α(Mg), W(Mg 3 Zn 3 Gd 2 ) and I(Mg 3 Zn 6 Gd) phases. In contrast, alloys cooled at higher rates of 7.7 and 10.08 °C/s mainly exhibit α(Mg) and I(Mg 3 Zn 6 Gd) phases, with a higher cooling rate promoting the formation of the I-phase. The volume fraction of eutectic compounds decreases from 17% to 9% as the cooling rate increases from 0.32 to 10.08 °C/s. Mechanical properties also improve with higher cooling rates. Specifically, the ultimate tensile strength, yield strength, and elongation increase by 58 MPa, 21 MPa, and 2.0%, respectively, representing an approximate 30% enhancement in each property when comparing alloys cooled at 0.32°C/s and 10.08°C/s. Strengthening analysis reveals that grain refinement contribute significantly to the improved mechanical performance. Additionally, the fracture behavior changes from predominantly intergranular fracture at low cooling rates to a mixed fracture mode (intergranular and transgranular) at high cooling rate. A mixed fracture mode improves the alloy’s ductility, toughness, and crack propagation resistance, making it more suitable for load-bearing structural applications.