Mg alloys are promising materials for bone implant applications mainly due to their low specific density, desirable stiffness and bioresorbability in the human body. Mg-Zn-Ca alloys are among the most promising materials for resorbable orthopedic fixation devices due to their superior biocompatibility. However, the mechanical and corrosion properties of the as-cast Mg-Zn-Ca alloys are insufficient. Heat treatment is a practical approach for strengthening Mg alloys especially after the fabrication of porous structures and 3D-printed components.
We have investigated heat treatment of these devices and have studied the resulting microstructure of Mg-1.6Zn-0.5Ca (wt. %) alloys by hardness, compression, scanning electron microscopy (SEM), and electrochemical and immersion corrosion tests. Mg-1.6Zn-0.5Ca alloy was prepared with high purity Mg, Zn and Ca by casting. The cast ingots were solution-treated at 510 °C for 3 h then quenched in water. The quenched ingots were age hardened in an oil bath at 200 °C for 2 h. Pure Mg, as-cast and heat-treated Mg-1.6Zn-0.5Ca alloy ingots were cut into coupons to characterize their mechanical and corrosion properties. In vitro corrosion tests were conducted in modified simulated body fluid (m-SBF) at pH 7.4 and 37 °C.
The hardness of the Mg-Zn-Ca alloy was significantly increased from 52.6 to be 66.8 HV after heat treatment. Also, the compression test results revealed that the heat-treated alloy has the highest compressive yield and ultimate strengths without significant change in stiffness and maximum strain. The mass loss of the Mg-Zn-Ca alloy by week 4 of the in vitro immersion test reduced from 174.6 mg/cm2 for the as-cast alloy to 101.7 mg/cm2 after the heat-treatment process. Heat-treatment was found to be a powerful post-shaping process not only to enhance the mechanical properties of the Mg-1.6Zn-0.5Ca (wt. %) alloy, but also to significantly improve its biocorrosion properties. Such heat-treated alloys can also be coated with biocompatible ceramics that provide additional protection from corrosion during the bone healing period (3–4 months).