The Properties Evolution of Medical Mg–Zn Alloys Prepared by Semi-solid Powder Moulding

Abstract

In this study, medical Mg–xZn (x = 3, 6, 9 wt%) alloys were successfully prepared by a novel technique, semi-solid powder moulding. The effects of Zn content and temperature (580, 590, 600 ℃) on the relative density, microstructure, microhardness and degradation behaviour were studied. The microstructure evolution and refinement mechanism during the forming process were analysed. The results show as the temperature and Zn content increase, the relative density and microhardness gradually increase. When the temperature is 600 ℃, the relative density of Mg–3Zn, Mg–6Zn and Mg–9Zn is 92.3%, 97.2% and 97.8%, respectively. The corresponding microhardness is 101.2 HV, 105.6 HV and 106.3 HV, respectively. The prepared Mg–Zn alloys have fine microstructure with equiaxed grains, which consists of α-Mg matrix and second phase of MgZn₂ with a few of Mg₄Zn₇ and Mg₂Zn₁₁. As Zn content increases, the amount of second phase increases, and the microstructure becomes uneven at the Zn content of 9 wt%. Pseudo-transgranular liquation cracking is one of the grain refinement mechanisms. As the Zn content increases, the corrosion rate decreases firstly and then increases. Mg–6Zn prepared at 600 ℃ has the lowest corrosion rate of 4.8 mm/year after 9 days of dynamic immersion. Both the porosity and second phase influence the corrosion rate, but the porosity is the main factor controlling the degradation. Mg–6Zn alloy is the best composition based on the properties and microstructures. The main components of corrosion products are Mg (OH)₂, hydroxyapatite and a small amount of MgO and CaCO₃, which shows a good biocompatibility. Semi-solid powder moulding shows a fantastic potential to prepare medical Mg alloys.