Stress Corrosion Cracking of Fe35Mn

Abstract

Recent studies have shown that Fe35Mn alloy has the potential of becoming one of the alternatives for biodegradable metal implants because of its magnetic compatibility and corrosion properties. For further development of this new class degradable metallic biomaterial, in depth study on the stress corrosion susceptibility (SCC) behaviour is essential as the combination of mechanical loading and corrosive physiological environment can cause premature failure when it is used as implants. In this present study, Fe35Mn alloy was prepared via a powder metallurgy route. The eff ects of mechanical milling on the mechanical properties and SCC behaviour were investigated and the results were compared with the normal blending alloy. Changes in the microstructure, porosity, homogeneity and mechanical properties after ball milling was characterised using optical micrography, XRD, SEM, EDS and hardness tests. These samples were then tested using the slow strain rate tensile testing to evaluate its susceptibility to SCC in environments that have different concentration of chloride ions. The effect of strain rates on the SCC susceptibility of Fe35Mn alloys was also investigated. Results show that the presence of fine and dispersed pores as well as the small grain size in the ball milled Fe35Mn alloy were identified to be the major factors that led to better mechanical properties and thus higher resistance to SCC. It was also confirmed that the increase in porosity of Fe35Mn alloys led to a severe SCC effect. Together with fractrography, the slow strain rate tensile testing results showed that the exposure to more chloride ions made the alloys more susceptible to SCC. In fact, the alloys that were exposed to the corrosive environment for a longer duration (i.e. lower loading rates) allowed more time for the initiation cracks formed in the pores to propagate, which eventually failed the alloys at a lower load. These results provide useful information on the SCC behaviour of Fe35Mn alloy that is essential for the development of this new class degradable metallic biomaterial as implants.