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 Mg-0.5Ca–2Zn–1Sn demonstrating higher microhardness (74.8 ± 3.9 HV and 84.8 ± 3.6 HV, respectively) and compressive strength (247.75 ± 6.44 MPa and 282.61 ± 7.91 MPa, respectively). Corrosion tests in Hank's Balanced Salt Solution (HBSS) indicated that Sn-modified alloys exhibited lower corrosion rates, with Mg-0.5Ca–1Zn–1Sn and Mg-0.5Ca–2Zn–1Sn showing the lowest initial corrosion current densities (0.328 ± 0.011 and 0.240 ± 0.002 μA 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 Mg-0.5Ca–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.