Phase Spheroidization and Solid-Solution Strengthening of Extruded Mg-Si-Zn-Ca-Y Alloy Induced by Low-Temperature Annealing

Microstructural evolution and mechanical properties of as-extruded Mg-2.5Si-4Zn-0.7Ca-1Y alloys were characterized under diverse heat treatment conditions. An optimal regime of 200 °C for 12 h emerged, delivering a peak Vickers hardness of ~100 HV0.2. After heat treatment, substantial dissolution of Zn into the α-Mg matrix occurred and Ca/Y-rich intermetallics underwent spheroidization. The microstructural changes facilitated the improvement of ultimate tensile strength (UTS) at 200 °C from ~160 MPa to ~200 MPa, while the elongation decreased from ~18% to ~6%. The enhanced high-temperature strength derives from Zn solid-solution strengthening and spheroidization of the Ca/Y-rich network phase, with a possible additional contribution from Y redistribution near the Mg2Si-containing regions.