Influence of heat treatment on microstructure and mechanical properties of Mg-10Gd-3Y-1.2Zn-0.4Zr alloy

Microstructure evolution and mechanical properties of the cast Mg-10Gd-3Y-1.2Zn-0.4Zr (mass fraction, %) alloy during annealing at 798 K for different time were investigated. In the as-cast state, the microstructure consists of alpha-Mg, Mg-5(Gd, Y, Zn) eutectic compounds and stacking faults(SF) of basal plane distributed from grain boundary to inner grain. During heat treatment at 798 K, the SF and parts of eutectic compounds dissolve into the matrix gradually, simultaneously, a new straight lamellar phase with 14H type of long period stacking ordered(LPSO) structure comes into being. Microstructure observation indicates that the lamellar phase is transformed from the un-dissolved eutectic compounds and shows a coarsening and disappearing process with annealing time prolonging. Three alloy specimens with typical microstructure after being annealed for 6, 36 and 72 h were selected to age at 498 K to peak hardness. Room and elevated temperature tensile tests show that the LPSO structured phase contributes to the change of high temperature tensile properties.