The effect of Nb and O on the martensitic transformation in Ti-Nb-O alloys.

This study examines the influence of niobium and oxygen on phase stability, crystal structure and martensitic transformation pathways in Ti-Nb-O alloys. A series of Ti-(8-28)Nb-(0-3)O (at.%) alloys were prepared and solution-treated in the β-phase field. The microstructure and crystallography were characterized by X-ray diffraction (XRD), electron microscopy and reciprocal-space mapping. A 2D-XRD orientation simulation approach was applied to distinguish all 12 crystallographically equivalent α″-martensitic variants originating from a single prior β grain, enabling detailed diffraction analysis. This method further allowed quantitative evaluation of the atomic shuffle parameter y, describing the β→α″ transformation. The results demonstrate that Nb primarily governs α″-martensite evolution. Increasing Nb stabilizes the β phase and shifts the α″ structure toward higher symmetry, as reflected by systematic changes in lattice parameters and increasing shuffle parameter y, indicating suppression of the transformation to the hexagonal α' phase. Oxygen, in contrast, modifies the transformation pathways. At lower Nb contents, it suppresses ω-phase formation and promotes β→α″ transformation, while at higher Nb levels it inhibits long-range martensitic transformation, resulting in retained β or competing ω phase. These effects are attributed to local lattice distortions induced by interstitial oxygen.