Effect of Zn content on mechanical properties and corrosion kinetics of Mg-xZn-0.4Ca alloys

Abstract

Four Mg-0.4 wt% Ca alloys with different Zn contents (0.4, 1.1, 1.8 and 2.4 wt%) were fabricated by induction melting and casting. Microstructural examinations revealed that all the alloys exhibit α-Mg dendritic grains, with second-phase particles preferentially located along the grain boundaries and within the interdendritic positions. Increasing Zn content gives rise to a higher volume fraction of particles and a corresponding reduction in the grain size. Nonetheless, while the Mg-0.4Zn-0.4Ca alloy contains binary Mg2Ca particles enriched with Zn and the Mg-1.8Zn-0.4Ca and Mg-2.4Zn-0.4Ca alloys contain particles of a Zn-rich ternary phase, the Mg-1.1Zn-0.4Ca alloy features both binary and ternary particles. Mechanical characterisation showed that microhardness, tensile stress and ductility display a relatively inconsistent trend, attributed to the risk of premature cracking during plastic deformation introduced by the large and brittle particles present in the alloys. In contrast, the yield stress, which is more relevant for biomedical applications, gradually increases with increasing Zn content, and this can be rationalised in terms of particle and grain boundary strengthening. Corrosion tests in Hanks’ solution indicated that the corrosion rate tends to rise with Zn content due to the higher galvanic activity associated with the increasing fraction of particles. However, the Mg-1.1Zn-0.4Ca alloy exhibits the highest corrosion rate, which is believed to result from galvanic coupling between the coexisting binary and ternary particles.