Effect of heat treatments on the microstructure and mechanical properties of an extruded Mg95.5Y3Zn1.5 alloy

The microstructure and mechanical properties of the extruded Mg95.5Y3Zn1.5 alloy under different heat treatment were systematically investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and an electronic universal testing machine. The results show that the as-extruded alloy is composed of 18R LPSO stripes and α-Mg matrix with stacking faults (SFs) in it. The fine 14H LPSO lamellas are formed in α-Mg matrix near the areas of SFs during solution treatment at 500°C for 8h. A great number of fine β′ phases are precipitated in the α-Mg matrix of T6-treated (aging of the T4-treated alloy at 225°C for 24h) and T5-treated (aging of the as-extruded alloy at 225°C for 32h) alloys. Moreover, the SFs which were first observed in extruded alloy are retained in T5-staged specimen, and exhibit a cross arrangement with β′ precipitates. The absence of 14H LPSO phase in T5-treated alloy indicates that the 14H structure cannot be formed during aging at 225°C. Tensile tests reveal that the presence of 14H lamellas improves the ductility of the alloy, but decreases the strength, suggesting that the 18R LPSO stripes are more effective in strengthening the alloy than 14H LPSO lamellas. The T6-staged alloy exhibits superior comprehensive mechanical properties with ultimate tensile strength of 358MPa, tensile yield strength of 226MPa and elongation of 6.1% at room temperature.