Cyclic and isothermal oxidations of nitinol wire at moderate temperatures

Both isothermal and cyclic oxidation behaviours of a commercial nitinol wire at 530-650 °C/10-240 min in air are studied. Chemical and phase compositions of scales and substrate after oxidation are determined using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, Raman spectrometry and X-ray photoelectron spectroscopy. It is found that the cyclic and isothermal oxidation rates are similar at the experimental temperatures, because the scale spalling is not significant. The isothermal oxidation obeys the parabolic law with an activation energy of 158 kJ/mol. The main oxidation product even after early 10 min oxidation is rutile containing a few at.% Ni. The grain growth of rutile at 530-650 °C is characterized by the activation energy and grain growth exponent of 45.7 kJ/mol and 0.29-0.34, respectively. The preferential oxidation of titanium causes the formation of an Ni-enriched and Ti-depleted zone containing Ni3Ti phase. XRD reveals that the presence of cubic B2 NiTi phase in the base alloy is not significantly affected by oxidation at 530-650 °C/240 min.