Surface oxidation of NiTi during thermal exposure in flowing argon environment

Surface oxidation at elevated temperatures has been a phenomenon of interest for NiTi alloys. On one hand it has been deliberately used to provide a surface protective layer to prevent Ni release for biomedical applications, and on the other it is to be minimised in metal processing and production. This study investigated the use of flowing argon as a protective gas for heat treating NiTi at elevated temperatures. It was found that under flowing argon protection, complex oxidation reactions still take place within the surface layers of NiTi. This includes the formation of transient oxides of Ni₂Ti₄O and TiO at lower temperatures, followed by formation of TiNi₃ underneath the oxide layers, and conversion of TiO to TiO₂ at higher temperatures. The final product contained a rutile TiO₂ outer surface, a composite middle layer consisting of Ni(Ti) + TiO₂, a TiNi₃ inner layer and a Ti-depleted zone within the body of the alloy. For heating at 1223 K for 24 h, the surface layers have a combined thickness of ~ 15 μm and a Ti-depleted zone of ~ 400 μm within the body of the alloy. The Ti-depleted zone appeared to play a major role in affecting the transformation and mechanical properties of the alloy.