Effect of heat treatment on the microstructures and corrosion behaviour of quaternary mg-2.2zn-3.7re-0.5ca alloys for biomedical applications

Abstract

Some of magnesium alloys have poor corrosion resistance in ambient air or aqueous environment due to their electrochemically active state. Different surface coating techniques, alloying and heat treatment are the ways to enhance the corrosion performance of magnesium alloys. The goal of this research is to study the characteristic and corrosion behaviour of quaternary Mg-Zn-RE-Ca alloys due to thermal treatment. The effects of two types of heat treatments, T4 (solution treatment) and T6 (aging treatment) on corrosion behaviour of quaternary Mg-2.2Zn- 3.7RE-0.5Ca alloys were studied. Microstructural evaluations were characterized using optical microscope and scanning electron microscope (SEM). The compositions of the material were determined by X-ray diffractometer (XRD) and energy dispersive X-ray spectrometer (EDS). The corrosion features were examined in-vitro by potentiodynamic polarization, pH variation and immersion test in Hank?s solution at room temperature (27°) with pH 7.4. It was found that a-Mg matrix, eutectic phase Mg12RE and Mg29Zn25RE with intermetallic IM1 precipitates formed in the quaternary Mg-Zn-RE-Ca system. T4 treatment produced supersaturated a-Mg while the secondary phases were mostly dissolved in the matrix. The lamellar structure which composed of IM1 precipitated after T6-treated were detected along the grain boundaries. The hardness of quaternary magnesium based alloy decreased significantly after T4 treatment and increased after T6 treatment. The pH value for T6 treatment shows the lowest after 96 hours immersion in Hank?s solution. Electrochemical measurement shows that T4 and T6 treatments increased the corrosion resistance of the quaternary magnesium alloy. It was found that the corrosion products consists mainly pure magnesium, magnesium hydroxide (Mg(OH)2), and hydroxyapatite (HA).