Microscopic and mesoscopic deformation behaviors of dual-phase Mg-Li-Gd alloys

Abstract

The Mg-Li dual-phase alloys, comprised of hexagonal (HCP) and body-centered cubic (BCC) phases, exhibit a better combination of strength and ductility than Mg single-phase alloys. In this work, the deformation behaviors of Mg-6Li-2Gd and Mg-2Gd alloys, representatives of dual-phase and single-phase alloys, have been studied at both microscale and mesoscale to elucidate the underlying mechanisms. Nanoindentation results show that the α-Mg phase in the Mg-6Li-2Gd alloy is harder than the β-Li phase. The intergranular deformation incompatibility, which arises from the elastic-plastic interactions, different strain accommodation behaviors, and strain hardening behaviors between the hard α-Mg phase and the soft β-Li phase, leads to pronounced hetero-deformation induced (HDI) stress of the Mg-6Li-2Gd alloy. The HDI stress strengthens the two phases simultaneously, so that the yield strength of the dual-phase Mg-6Li-2Gd alloy is higher than the Mg-2Gd alloy as well as the harder α-Mg phase in the Mg-6Li-2Gd alloy. Due to the decreased strength difference between the two phases caused by the HDI stress strengthening, the dual-phase alloy exhibits homogeneous plasticity at the mesoscale, which benefits the elongation of the Mg-6Li-2Gd alloy. The HDI strengthening magnitude in the Mg-6Li-2Gd alloy is further quantified. Based on the equal strain upper bound and equal stress lower bound approximations, the yield strength improved by the HDI stress is estimated to be 18–37 MPa, which is in the same range as the elastic visco-plastic self-consistent (EVPSC) simulation results. As the tensile strain is larger than ∼3 %, the HDI strengthening magnitude for the Mg-6Li-2Gd alloy reaches 50–65 MPa, accounting for 35 % of the corresponding flow stress.