Effect of Zn addition on the microstructure and mechanical properties of as-cast BCC Mg-11Li based alloys

Abstract

Effect of Zn addition on the microstructure and mechanical properties of as-cast BCC Mg-11Li based alloys were investigated. The results revealed that a certain amount of θ (MgLiZn) with coherent α-Mg phase precipitated along grain boundaries (GBs) and a high density of dispersing spherical small-sized θ′ (MgLi2Zn) precipitates distributed inside grains were contained in β-phase Mg-11Li-6Zn (in wt%, LZ116) alloy, and the volume fraction of both phases increased with the elevating Zn content under the lever of less than 6.2 wt%. After solution treatment, the θ / α-Mg phases located at the GBs and the spherical θ’ phase inside the grains were dissolved, whilst a high density of fine-laminar θ’ phase precipitated concomitantly in LZ116 alloy. The as-cast LZ116 alloy exhibited high ultimate tensile strength and ductility of 184 MPa and 30% at room temperature, respectively, which were much higher than those (72 MPa and 19%) of as-cast Mg-11Li (L11) alloy. The strength of the LZ116 alloy was further elevated to 235 MPa via solid-solution treatment at 350 °C for 4 h (the treated alloy is denoted as S-LZ116), but the elongation was dramatically reduced to 1.2%. As a result, the softening θ/α-Mg precipitates distributed at GBs enhanced the plasticity and the hardening θ′ phase elevated the strength of as-cast LZ116 alloy. Meanwhile, the intergranular fracture and dimple feature was observed in as-cast LZ116 alloy, and the fracture behavior is correlated with the softening GBs with θ/ α-Mg phases and the hardening grains reinforced by dispersed θ′ phase particles.