Determining critical Zn/Ca atomic ratio and its role in mechanical and corrosion properties of biodegradable Mg-Ca-Zn-Mn alloys

Abstract

Mg-Ca-Zn-Mn alloys are promising for applications in biodegradable bone fixation devices. The Zn/Ca atomic ratio in the compositions of these alloys is important to their corrosion and mechanical properties. This paper investigated the Zn/Ca ratio using a phase-focused approach based on CALPHAD (CALculation of PHAse Diagrams) modeling and experimental validation. Six Mg-0.5Ca-xZn-0.5Mn (all wt.%) alloys were cast with x = 0.96, 1.15, 1.47, 1.69, 1.94, and 3.81 so that the Zn/Ca atomic ratio spanned from 1.18 to 4.66. The microstructure is studied in the as-cast, solution-treated, and as-rolled conditions. A critical ratio was determined to be 2.77, above which Mg₂Ca phase can be suppressed in as-cast microstructure. In the solution-treated condition, a Zn/Ca ratio of less than 2.0 was required to dissolve the Ca₂Mg₆Zn₃ phase. Alloys below 2.0 Zn/Ca were found to have yield strength of 300 MPa and a corrosion rate of 0.25 to 0.3 mg/cm²/day as measured by both weight loss and hydrogen evolution. In alloys above 2.0 Zn/Ca, the yield strength decreased to 280 MPa and the corrosion rate measured by weight-loss increased to 0.5 mg/cm²/day. Above the critical ratio, the yield strength was the highest at 347 MPa but a corrosion rate of 0.4 mg/cm²/day. The Zn/Ca region with the best combination of corrosion resistance and mechanical properties is between 1.18 and 1.8 (in rolled sheet condition), which provides important guidance for biomedical Mg-Ca-Zn alloy design and optimization.