Mechanical and Corrosion Properties of Two Precipitation-Hardened Mg-Y-Nd-Gd-Dy Alloys with Small Changes in Chemical Composition

Abstract

Precipitation hardening in Mg-Y-Nd alloys is based on finely dispersed particles an effective strengthening mechanism to achieve high strength at moderate ductility. However, these particles often affect corrosion by being more noble than the matrix. Biodegradable implant materials should show a corrosion rate fit to its application but should be free of pitting corrosion. Especially deep and narrow pits act as notches and cause increased mechanical stress leading into early failure. WE43 has already shown to have an acceptable biological response. In this study two Mg-Y-Nd-Gd-Dy alloys, WE32 and WE33, in extruded, solution and precipitation heat-treated conditions have been investigated. The difference in alloy composition is not very high. Solution heat treatment causes grain growth and strength loss. The ageing response to peak hardness depends on the temperature. A rather short ageing response was observed for 250 °C and for 200 °C highest hardness has been found at longer ageing time, but higher hardness compared to 250 °C. Grain growth during ageing is not significant. The higher alloyed alloy WE33 shows better mechanical strength, but less ductility. Corrosion was evaluated with immersion and potentiodynamic polarization in Ringer Acetate solution. The corrosion rate strongly depends on the alloy and heat treatment condition as well as on the test method. The highest corrosion rate is observed in the T4 condition. The peak aged alloy shows the lowest corrosion rate, but non-uniform corrosion – in this evaluated by the pitting factor.