Impact of Zn/Sn variations on the microstructure, mechanical properties, and biocorrosion of T4-Treated Mg-0.5Ca-Zn-(Sn) alloys for orthopaedic implants

This study investigates the influence of Zn and Sn on the microstructure, mechanical properties, and corrosion resistance of T4-treated, squeeze-cast Mg-0.5Ca-Zn-(Sn) alloys fabricated under 300 MPa pressure. The alloys-Mg-0.5Ca-1Zn (A), Mg-0.5Ca-1Zn-0.5Sn (B), Mg-0.5Ca-1Zn-1Sn (C), and Mg-0.5Ca-2Zn-1Sn (D)- underwent T4 solution heat treatment and were characterized using XRD and FE-SEM. The T4-treatment resulted in the formation of Mg2Zn11, with Ca and Sn retained in solid solution, contributing to solid solution strengthening. Increased Zn and Sn content enhanced mechanical properties, with alloys Mg-0.5Ca-1Zn-1Sn and Mg0.5Ca-2Zn-1Sn demonstrating higher microhardness (74.8 f 3.9 HV and 84.8 f 3.6 HV, respectively) and compressive strength (247.75 f 6.44 MPa and 282.61 f 7.91 MPa, respectively). Corrosion tests in Hank's Balanced Salt Solution (HBSS) indicated that Sn-modified alloys exhibited lower corrosion rates, with Mg0.5Ca-1Zn-1Sn and Mg-0.5Ca-2Zn-1Sn showing the lowest initial corrosion current densities (0.328 f 0.011 and 0.240 f 0.002 mu A cm-2, respectively). Post-corrosion analysis confirmed that Mg-0.5Ca-2Zn-1Sn alloy developed a compact Mg(OH)2 layer, followed by Mg-0.5Ca-1Zn-1Sn, contributing to improved corrosion resistance. These results highlight that Sn-modified, T4-treated Mg-Ca-Zn alloys, particularly Mg0.5Ca-1Zn-1Sn and Mg-0.5Ca-2Zn-1Sn with 1 wt% Sn, exhibit superior mechanical strength and corrosion resistance, making them promising candidates for biodegradable orthopaedic implants.