Effect of La addition on microstructure, mechanical behavior, strengthening and toughening mechanisms of cast Mg-Gd-Zn alloy

Abstract

High strength cast Mg-RE (magnesium-rare earth) alloys usually show a poor balance between strength and ductility, typically high strength with low ductility. Grain coarsening during solution treatment is one of the reasons for the reduced ductility. La element has very limited solubility in Mg and the La-containing second-phase is expected to pin the grain boundaries and restrict the significant grain coarsening during solution treatment. Therefore, different amounts of La addition are introduced to the Mg-Gd-Zn casting alloy and the microstructure and mechanical properties are systematically studied. The addition of La results in the formation of the Mg12La second-phase along the grain boundaries during solution treatment, significantly increasing the number density of the β’ precipitates while slightly reducing their sizes. The Mg12La phase along the grain boundaries restricts grain coarsening, resulting in very low grain coarsening rates in the La-containing alloys. Compared with high strength cast Mg-RE alloys in literature, the present La-containing alloys indicate better balance between strength and ductility. The 0.4 La alloy exhibits the highest yield strength (YS) of 247 MPa and ultimate tensile strength (UTS) of 397 MPa with moderate elongation (El) of 5.8% in the under-aged condition; while the 0.2 La alloy with the highest product of strength and elongation (PSE) of 4.63 GPa·% presents a good combination of high strength and outstanding elongation (YS: 245 MPa; UTS: 383 MPa; El: 12.1%) in the peak-aged condition. Micro-alloying with La element is expected to be an effective strategy to improve the strength and ductility of cast Mg alloys simultaneously.