High-temperature corrosion of NiTi-shape-memory alloys

Semi-finished products and components made of NiTi-shape-memory alloys (NiTi-SMA) are often subjected to heat treatment after their fabrication. During this heat treatment, oxide layers begin to form which contain a high amount of titanium. In this investigation special attention was drawn to the selective oxidation of Ti because a TiOX-layer can represent a Ni-barrier and may therefore be of special use for medical applications. A comparison of the following three samples was carried out: A sample oxidised at room temperature, another that was heat-treated in ambient air (600degreesC/1 min) and a third sample that was subjected to a heat treatment (600degreesC/1 min) in an atmosphere that oxidises titanium but reduces NiO in order to achieve a selective oxidation of the titanium. The analysis of the oxide layers was carried out by means of x-ray photoelectron spectroscopy (XPS). It was shown that the ratio of titanium to nickel in the oxide layer can be substantially increased when performing the annealing treatments in a partial reducing atmosphere. Furthermore, a thermo-gravimetric investigation of the material was carried out at 600degreesC in dry air in order to estimate the growth of the oxide layers.