Microstructure and mechanical properties of Mg-Zn-Ca-Zr alloy fabricated by hot extrusion-shearing process

Abstract

In this study, ultra-fine grain Mg-3Zn-xCa-0.6Zr (x = 0, 0.6, 1.2, 1.8) alloy was obtained by a novel extrusion-shearing (ES) deformation process combining initial forward extrusion and subsequent equal channel angular extrusion (ECAP) shearing. The purpose of this paper is to improve the strength and plasticity of the alloy by ES process. The results of as-cast and homogenized alloy show that the ternary Ca2Mg6Zn3 phase with excellent thermal stability and refined grain size was formed in the alloy by adding Ca. In the ES process, the ternary phase was broken into ultra-fine particles pinned to the dynamic recrystallization (DRX) grain boundary, which introduced strong particle stimulated nucleation (PSN). Moreover, large number of MgZn phases in Ca-containing alloys were dynamically precipitated at DRX grain boundaries and within grains. In addition, in the early stage of the ES process, Ca2Mg6Zn3 phase promoted the accumulation of dislocations at the grain boundaries, and part of the grain boundary was transformed into low-angle grain boundaries (LAGBs) and high-angle grain boundaries (HAGBs). After the die corner shearing, the accumulated LAGBs gradually transformed into HAGBs due to continuous dynamic recrystallization (CDRX). Through the novel ES process and the addition of Ca element, the basal plane texture was significantly weakened and the DRX grain was refined. With the increase in Ca content, the strength and toughness of ES alloy increased obviously, and the yield asymmetry decreased gradually. When the content of Ca was 1.2 wt%, ES alloy exhibited the best combination of strength and toughness (UTS = 342 MPa, δ = 20.9%) due to nano-scale Ca2Mg6Zn3 phase, grain refinement and texture weakening. This property is of great significance for the development of magnesium alloys for biological applications.