The aim of this study was to investigate the effect of air heat-treatment temperature on the structural, morphological, compositional, and phase evolution of the surface oxide layer formed on a cast Ti-50.2Ni alloy. NiTi samples were heat-treated in air at 300–500 ◦ C for 60 min. Surface morphology and microstructure were characterized using scanning electron microscopy (SEM), while elemental composition was assessed by energydispersive X-ray spectroscopy (EDX). Surface roughness parameters were quantified using atomic force microscopy. Raman spectroscopy combined with Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS) analysis was employed to identify and evaluate the relative contribution of TiO 2 polymorphs in the oxide layers formed during heat treatment. EDX analysis indicates progressive oxidation with increasing treatment temperature, evidenced by higher oxygen content and a corresponding decrease in titanium and nickel concentrations in the near‑surface region. SEM observations reveal a gradual evolution of surface morphology associated with oxide layer growth and densification. Surface roughness exhibits a non‑monotonic temperature dependence, reflecting the interplay between smoothing processes and the development of the oxide scale. Raman spectroscopy showed that the oxide layer contained anatase and rutile phases, with rutile becoming increasingly dominant as the treatment temperature increased. MCR-ALS analysis confirmed a pronounced temperatureinduced shift toward a rutile-dominated surface oxide and reduced spectral variability at higher temperatures, indicating greater phase homogeneity within the analyzed surface region. The scientific novelty of this work lies in the systematic integration of Raman spectroscopy, MCR-ALS analysis, SEM, EDX, and atomic force microscopy to correlate temperature-dependent oxide formation, surface roughness, and anatase–rutile evolution on cast Ti-50.2Ni alloy subjected to 60 min air heat treatment. The resulting correlations provide a foundation for understanding thermally induced surface transformations in NiTi and for selecting controlled oxidation regimes aimed at producing structurally stable, rutile-dominated surface oxide layers relevant to biomedical surface design.
https://www.sciencedirect.com/science/article/pii/S2468023026016457