Effect of homogenization heat treatment on the microstructure of AZ91 magnesium alloy at different temperatures and ageing times

Abstract

Magnesium (Mg) alloys are promising biodegradable implant materials. If successful, they do not require a second surgiabscal operation for their removal. These alloys possessed microstructure, which contained alpha (α-Mg) phase, eutectic (α + β) phase and intermetallic or beta (β-Mg17Al12) phase. The bio-corrosion rate of any intermetallic alloy is related to its β phase volume fraction (VF). However, this β phase contributes galvanic corrosion which hinders the clinical application of Mg alloys. Thus, this study is focused to reduce the VF of the β phase and select the appropriate temperature for Mg homogenization heat treatment (HHT). HHT was carried out on Mg AZ91 alloy samples by using a high-temperature rotary furnace at 350 °C for 10 h, 370 °C for 10 h, 390 °C for 5 h and10h, 410 °C for 5 h and10h and 430 °C for 5 and 10 h. The influence of varying temperature with time was investigated on the microstructure and microhardness of Mg AZ91 alloy by using a field emission scanning electron microscope equipped with an energy-dispersive spectroscope and image Digimizer. The results revealed that the higher the HHT or the longer the homogenization heat dwelling time, the more the homogenous distribution of the large β phase over the microstructure, hence containing little or without a β phase. The microhardness increased with an increase in temperature and ageing time with the peak hardness at 410 °C and 430 °C for 10 h.