Effect of cooling modes following solid solution treatment on microstructure and mechanical properties of Mg–6.5Li–5Y–2.7Zn Alloy

The effects of cooling modes, specifically the water-quenching, air-cooling, and furnace-cooling, after solid solution treatment on the microstructure and mechanical properties of Mg–6.5Li–5Y–2.7Zn alloy were investigated using XRD, SEM, TEM, and room-temperature tension tests. The as-cast alloy consists of an α-Mg/β-Li matrix and (Mg,Zn) 24 Y 5 eutectic with skeletal, snowflake-like and granular morphologies. After 500 °C × 24 h solid solution treatment, the (Mg,Zn) 24 Y 5 eutectic dissolves, while micrometer/nano (Mg,Zn) 24 Y 5 phases re-precipitate in the β-Li matrix, and 18R-LPSO precipitates at α-Mg/β-Li interfaces and within α-Mg in samples with the studied cooling modes. Volume fraction of Mg 3 Zn 3 Y 2 (W) phase rises from 0.8% (by air-cooling) to 2.4% (by furnace-cooling), but is not detected in the water-quenching sample. As the cooling mode changing from water-quenching to furnace-cooling, the volume fraction of 18R-LPSO increases from 6.19% to 9.3%, while the volume fraction of re-precipitated micrometer-/nano-sized (Mg,Zn) 24 Y 5 in the β-Li phase decreases from 3.9% and 5.7% to 1.6% and 0.2%, respectively. Stacking fault density in the α-Mg phase of the furnace-cooled sample is three times that of the air-cooled. Mechanical properties of the alloy decrease with decreasing cooling rate. The water-quenched alloy exhibits the best mechanical properties in ultimate tensile strength, and the largest volume fraction of fine re-precipitated (Mg,Zn) 24 Y 5 phases precipitates with an average particle size of 13.7 nm produces synergistic strengthening and enhanced ductility. Decreasing cooling rate leads to growth of re-precipitated (Mg,Zn) 24 Y 5 phase and deteriorates the mechanical properties.