Effects of nanoprecipitates and LPSO structure on deformation and fracture behaviour of high-strength Mg-Gd-Y-Zn-Mn alloys

In this work, a high-strength Mg-12Gd-2Y-1Zn-Mn alloy with an ultimate tensile strength of 509 MPa and a fracture elongation of 5% at room temperature tensile was developed by using hot extrusion and subsequent ageing. Progressive observations of crack initiation and early propagation of the as-extruded and peak-aged alloys were conducted during the tensile process. The results show that the activation of fine-grain boundary sliding and multiple non-basal slip systems in coarse unDRXed grains is the dominant deformation mechanisms of the as-extruded alloy, which eventually lead to crack initiation and propagation. After peak-ageing treatment, dense β′ precipitates are formed within the bimodal grains, resulting in changes in deformation mechanisms and crack initiation and propagation behaviours during the tensile process. The grain boundary sliding is suppressed and the grain boundary strength is enhanced due to the precipitation of β′ phases on the hindering effect of dislocation mobility. Therefore, no-basal slip and cross-slip in coarse unDRXed grains become the major roles in crack propagation. The intra-grain LPSO (long-period stacking ordered) phase can suppress crack initiation and propagation in both as-extruded and peak-aged alloys. For the thick LPSO phase at the grain boundaries of the as-extruded alloy, it inhibits crack propagation to some extent. While the thick-deformed LPSO phase at the grain boundaries of the extruded-peak-aged alloy more easily causes crack initiation with increasing tensile strains.