Microstructure Evolution of Ni-rich NiTi(-Hf) Alloy During Thermal Oxidation at Different Temperatures

The oxidation behavior of Ni-rich NiTi(-Hf) alloy at the temperature range from 400 to 950 degrees C is investigated in this paper. For Ni-rich NiTi alloy, the results show that the oxidation kinetics follow the logarithmic law at 400 degrees C and obey the parabolic law from 500 to 950 degrees C. After high-temperature oxidation, the observed multilayered scale structure consists of TiO2+Ti2O3+NiTiO3, Ni(Ti)+Ti2O3+rich Ti, and Ni3Ti from outside to the inside. The addition of the Hf element improves the high-temperature oxidation resistance. A schematic oxidation mechanism of Ni-rich NiTi(-Hf) alloy is proposed to explain the observed results.