Effect of heat treatment temperature on oxidation behavior in Ni–Ti alloy

Variation in oxidation behavior with heat treatment temperature is investigated for a Ni–Ti alloy using X-ray diffraction, differential scanning calorimetry and Auger spectroscopy, and the effect of oxidation on transformation behavior and superelasticity is characterized. A cold-worked Ni–49.4 at.% Ti alloy is oxidized at 300–700 °C for 1 h in air. With an increase in heating temperature, the structure of TiO2 changes from amorphous (300 and 400 °C) to anatase (500 °C), and to rutile (700 °C). The activation energy of oxidation for NiTi is determined to be 51 kcal/mol when heating temperature is 500 °C or above. Since Ti reacts preferably with oxygen, Ni content increases between matrix and oxide, forming Ni3Ti compounds. Oxidation products decrease significantly the Ms and As temperatures in the specimen oxidized at 900 °C with B2 → M transformation path. An extra calorimetric peak is found on cooling between two peaks in the specimen with B2 → R → M which was oxidized at 900 °C and aged at 500 °C. Oxidation deteriorates superelasticity due to formation of a Ni-rich compound.