Revealing the effect of minor Ca and Sr additions on microstructure evolution and mechanical properties of Zn-0.6 Mg alloy during multi-pass equal channel angular pressing

Abstract

In this study, the microstructure evolution and mechanical properties of Zn-0.6 (wt.%) Mg alloys with minor calcium (Ca) and strontium (Sr) additions (0.1 wt%) during multi-pass equal channel angular pressing (ECAP) were comparatively investigated. The obtained results illustrate that in addition to α-Zn matrix and network-shaped ternary α-Zn + Mg2Zn11 + MgZn2 eutectic structure, CaZn13 and SrZn13 particles are formed in as-cast Zn-Mg-Ca and Zn-Mg-Sr alloys, respectively. The CaZn13 particles exhibit cuboid shape, and their sizes are much larger than the ellipsoidal SrZn13 particles. After multi-passes ECAP, both α-Zn grains and eutectic structure are obviously refined owing to the dynamic recrystallization (DRX) process and mechanical crushing effect, while the CaZn13 and SrZn13 particles exhibit no obvious change. Moreover, the existence of cuboid CaZn13 particles with sharp corners are more effective to refine α-Zn matrix via the particle stimulated nucleation DRX mechanism, resulting in finer microstructure of Zn-Mg-Ca alloy. Since the CaZn13 particles could induce large stress concentration at their shape corners during deformation, which even causes crack of the particles in ECAP alloys, the ductility of Zn-Mg-Ca alloys is significantly deteriorated. Overall, the 12p ECAP Zn-Mg-Sr alloy exhibits optimal mechanical properties with ultimate tensile strength above 300 MPa and elongation higher than 20%. Nevertheless, the deformed Zn-Mg-Ca alloys possess poor ductility owing to the existence of CaZn13 particles, which should be avoided in future design of Zn-based biodegradable metals.