In-vitro corrosion study and mechanical properties assessment of Mg-Zn-Zr alloys

Abstract

Commercial metal implants, such as 316L Stainless Steel, Cobalt-Chromium, Titanium alloys that are presently used remain permanently in the body and cause various side effects. In order to overcome this problem, next generation biodegradable (BD) alloys (Magnesium, Zinc & Fe-Mn) are being developed. Among these alloys, Magnesium is studied extensively and found that it is an attractive candidate for BD implants such as stents, screws, and soft-tissue anchors, due to its excellent biocompatibility, biodegrability and MRI compatibility. However, the alloy exhibits lower strength, higher corrosion rate, higher hydrogen evolution that limits its usage as bio-implants. The purpose of this research is to study effects of alloying addition of Zinc and Zirconium in Magnesium and observing the degradation property and mechanical strength suitable for bio-implant material. In this research binary Mg-3Zn alloy and ternary Mg-3Zn-xZr (x = 0.3, 0.6, 0.9 wt%) alloys have been prepared by vacuum induction melting process under an argon atmosphere. The microstructure consisted of α-Mg and MgZn intermetallic phases are observed in as cast Mg-3Zn alloy with an average grain size of 75.6μm. The presence of base-centered monoclinic MgZn intermetallic phase is confirmed by XRD. Solution heat treatment (T4) has been carried out to dissolve secondary phases and homogenize the as-cast samples for uniform composition. ICP-OES analysis was carried out to confirm the uniform chemical composition. Hardness decreased as holding time of solution treatment increased from 4 to 24 hrs. The higher hardness of Mg-3Zn alloy at lower holding times when compared to as cast is due to the formation of regions of short-range order. SEM analysis was carried out to find out the microstructure and secondary phases. The addition of Zr (0.9wt%) to Mg3Zn resulted in lower grain size (43 μm) due to formation of Mg (Zn,Zr) precipitates at grain boundaries, which pin the grain growth and also increase the hardness. Degradation rate has been measured in 3.5 wt% NaCl and Hank’s solution. Corrosion rate of Mg alloy decreased with addition of Zn and Zr of Mg, Mg-3Zn and Mg-3Zn-0.9Zr respectively. Tensile has been carried out for Mg-3Zn-xZr (x =0.3, 0.9 wt%). Improvement in yield strength has been observed in 0.9 wt% Zr sample compared to 0.3 wt% addition. However lower ductility in 0.9 wt% Zr sample has been obtained.