Microstructure and properties of NiTi shape memory alloy with thermal oxidation treatment

Scanning electron microscopy,X-ray photoelectron spectroscopy,transmission electron microscopy,and nanoindenter were used to study the microstructure and nanomechanical properties of the surface oxide layer of NiTi alloy under different thermal oxidation temperatures and times.The results show that the oxidation at 500 ℃and 600 ℃can ensure the formation of a complete and continuous TiO₂ oxide layer on the alloy surface.The thickness of TiO₂ oxide layer is up to 1.1 μm.The growth of TiO₂ oxide layer is mainly controlled by the outward diffusion of Ti elements,and the growth direction of TiO₂ is almost perpendicular to the substrate surface.The deformation behavior of NiTi alloy surface after thermal oxidation is still dominated by plastic deformation.After oxidation at 500 ℃for 16 hours,NiTi alloy had the best anti-plastic deformation ability,and its highest Hardness/Elastic modulus and (Hardness)³/(Elastic modulus)² were the highest,which were 0.071 and 0.035,respectively.The dense TiO₂ oxide layer formed on the alloy surface effectively improves the surface hardness and resistance to plastic deformation of the alloy.