Effect of Ca addition on microstructure, mechanical properties, solidification behavior, and fluidity of cast Mg-4Zn-xCa-0.3Zr alloys

Abstract

Mg-Zn-(Ca-Zr) alloys are regarded as promising biomedical magnesium alloys due to their low cost, superior strength-toughness matching, and excellent biocompatibility. However, the correlation among Ca content, solidification characteristics, mechanical properties, and casting fluidity remains unclear, limiting the targeted performance optimization of cast Mg-Zn-(Ca-Zr) alloys. To address this issue, this study focused on Mg-4Zn-xCa-0.3Zr alloys (x=0, 0.5, 1, 1.5, 2, wt.%). In this study, XRD, SEM, EBSD, TEM, tensile testing, thermal analysis, and fluidity measurement were adopted to investigate the effects of Ca addition on the microstructure, mechanical properties, fluidity, and solidification path of the investigated alloys, while exploring the solidification behavior and the mechanism governing strength and ductility. Experimental results indicate that the addition of Ca promotes the formation of reticularly distributed lamellar coarse brittle secondary phases (Ca2Mg6Zn3 and Mg2Ca phases), which reduce grain boundary strength and deteriorate the mechanical properties of the alloys. Among them, the Mg-4Zn-0.3Zr alloy exhibits the optimal mechanical properties (UTS=261.7 MPa, YS=147.0 MPa, EL=8.8%). The addition of Ca significantly shortens the solidification temperature range of the investigated alloys and increases the content of secondary phases. Furthermore, under the combined effect of delayed dendrite coalescence, reduced overall and liquid-phase viscosity, and improved structure of the solid-liquid interface front, the fluidity of the alloys gradually increases with increasing Ca content. Specifically, the length of the fluidity sample for the Mg-4Zn-2Ca-0.3Zr alloy is 45.7% longer than that of the Mg-4Zn-0.3Zr alloy. Considering both formability and mechanical properties, the Mg-4Zn-0.5Ca-0.3Zr alloy exhibits the optimal comprehensive performance among all investigated alloys.