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

Abstract

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 $\beta$-phase field. Microstructure and crystallography were characterized by X-ray diffraction, electron microscopy, and reciprocal-space mapping. A 2D-XRD orientation simulation approach was applied to distinguish all 12 crystallographically equivalent $\alpha"$ martensitic variants originating from a single prior $\beta$ grain, enabling detailed diffraction analysis. This method further allowed quantitative evaluation of the atomic shuffle parameter y, describing the $\beta\rightarrow\alpha"$ transformation. The results demonstrate that Nb primarily governs $\alpha"$ martensite evolution. Increasing Nb stabilizes the $\beta$ phase and shifts the $\alpha"$ structure toward higher symmetry, as reflected by systematic changes in lattice parameters and increasing shuffle parameter y, indicating suppression of transformation toward the hexagonal $\alpha'$ phase. Oxygen, in contrast, modifies transformation pathways. At lower Nb contents, it suppresses the $\omega$ phase formation and promotes $\beta\rightarrow\alpha"$ transformation, while at higher Nb levels it inhibits long-range martensitic transformation, resulting in retained $\beta$ or competing $\omega$ phase. These effects are attributed to local lattice distortions induced by interstitial oxygen.