The effect of pre-heat parameters and final-heat parameters on microstructure evolution and mechanical properties of Mg-Gd-Y-Zn-Zr alloy

In this study, an ultra-high strength Mg-13Gd-4Y–0Zn-0.5Zr (wt.%) alloy with tensile yield strength (TYS) of 456 MPa and ultimate tensile strength (UTS) of 486 MPa was prepared by using different pre-heat parameters and subsequent final-heat treatment combined with backward extrusion (BE) deformation process. It was shown that the pre-heat teratment could significantly enhance the UTS and TYS of the secondary backward extruded (BEed) alloy compared with none treatment. The reason was that the pre-heat treatment promoted the particle stimulated nucleation (PSN) mechanism and continuous dynamic recrystallization (CDRX), significantly increasing the fraction of dynamic recrystallization (DRX). The strengthening of the secondary BEed alloy was attributed to the unique microstructure characteristics: (I) the fine DRXed grains pinned by dynamic precipitates, (II) the dense dynamic precipitation phases, (III) the effect of the fine-grain strengthening. However, the overly densely distributed second phase, which increased the local dislocation density due to the narrower spacing, made it difficult to prevent crack propagation and caused premature fracture of the alloy. In addition, the subsequent final-heat treatment also facilitated the precipitation of the second phases of the secondary BEed alloys and brought in the ultra-high mechanical properties.