Effect of heat treatment on microstructure and mechanical properties of Mg97.5Gd1.9Zn0.6 alloy

A ternary alloy strengthened by the phase with long period stacking ordered (LPSO) structure with composition of Mg97.5Gd1.9Zn0.6 (atom fraction/%) was prepared by conventional permanent mold casting. The microstructure evolution and mechanical properties in as-cast, asannealed, as-extruded and extruded-T5 alloy were systematically investigated by OM,SEM,TEM and electronic universal testing machine. The results show that the as-cast microstructure of the alloy consists of the alpha-Mg matrix, (Mg, Zn),Gd eutectic and lamellar 14H-LPSO phase. Annealing at 510 C results in the transformation of (Mg, Zn)(3) Gd into the block-shaped 14H-LPSO phase. According to the distribution and morphology of the 14H-LPSO phase observed in the microstructure after annealing, the peritectoid transformation (Mg, Zn)(3)Gd alpha-Mg -> 14H-LPSO possibly occurs during annealing. The lamellar 14H-LPSO and beta(p)' precipitates have also been observed in the alloy during extruded-T5 treatment. Under the combined action of 14H-LPSO toughening and precipitation strengthening, the aged sample shows the best tensile properties at room temperature : ultimate tensile strength of 361 MPa, yield strength of 216 MPa and elongation of 6. 9%.