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

TL;DR

This study investigates how niobium and oxygen influence phase stability and martensitic transformation pathways in Ti-Nb-O alloys, revealing Nb's role in stabilizing the beta phase and oxygen's impact on transformation mechanisms.

Contribution

It introduces a 2D-XRD orientation simulation method to analyze martensitic variants and atomic shuffles, providing detailed insights into the effects of Nb and O on transformation pathways.

Findings

Nb stabilizes the beta phase and shifts the martensitic structure to higher symmetry.

Oxygen suppresses omega phase formation at low Nb and inhibits long-range transformation at high Nb.

Transformation pathways are significantly affected by local lattice distortions caused by oxygen.

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.