Effects of Yb concentration on recrystallization, texture and tensile properties of extruded ZK60 magnesium alloys

Abstract

Microstructures and mechanical properties of as-extruded ZK60 alloys with different Yb concentrations (0, 1.0 and 2.0 wt%) were investigated. The results showed that Yb addition effectively refined the recrystallized grains and resulted in dense Mg–Zn–Yb nano-precipitates. With increasing Yb concentration, sufficient boundary drag was provided by solute clustering and high-density precipitates, inhibiting the grain boundary bulging. Meanwhile, the decreased stacking fault energy retarded dynamic recovery and led to an increased population of shear bands, extension twins and dislocations, promoting the formation and rotation of sub-grains. These facilitated the transition of the dominant nucleation mechanism from discontinuous dynamic recrystallization (DRX) to continuous DRX. Moreover, Yb addition disrupted the tendency for developing an intense basal texture during extrusion and the “Rare-Earth” (RE) texture was generated in the ZK60–2wt% Yb (ZYbK620) extrudate. It demonstrated that with Yb solute, the comprehensive effects including nucleation along extension twin boundaries preserving an initial “Rare-Earth” orientation, preferential grain growth along the non-basal planes and the opposite lattice rotation triggered by increased activity of pyramidal <c + a> slip contributed to the c-axis rotation of DRX grains to the extrusion direction. Favored by the grain refinement, nano-precipitates dispersion and RE texture, the tensile properties of extruded ZYbK620 alloy were considerably improved in comparison with the Yb-free counterpart, achieving a good strength-ductility balance with the ultimate tensile strength of 378 ± 1.5 MPa, tensile yield strength of 237 ± 2.3 MPa and elongation to failure of 17.6 ± 2.0%, respectively.