OXIDATION OF AN NITI ALLOY

In this study, the oxidation of the surface of a 50/50 (at.%) NiTi alloy was studied by XPS, AES and SEM. Samples of the NiTi alloy were oxidized at 23°C and high temperatures at an oxygen pressure of 10−4 Torr and in atmosphere. At 10−4 Torr of O2 and 23°C, titanium was oxidized preferentially while nickel remained in the metallic form even after 40 min of exposure. At the same oxygen partial pressure and 400°C, a mixture of TiOx, Ti2O3 and TiO2 was formed, and nickel remained metallic. Eventually, after 40 min of O2 exposure, the whole surface was covered with a layer of TiO2. In atmosphere and at 23°C, titanium again was oxidized preferentially, but NiO was detected on the surface after the first minute of air exposure. At a higher temperature of 450°C, areas comprising a mixture of NiO and TiO2, and areas consisting of TiO2 only were found. Below the top surface layer, a complete layer of TiO2 was detected. Underneath the TiO2 layer, a nickel‐rich layer was present. Copyright © 1990 John Wiley & Sons Ltd.