The effect of oxygen on α″ martensite and superelasticity in Ti–24Nb–4Zr–8Sn

β Phase elasticity, stress-induced α″ transformation and superelasticity in hot-forged Ti–24Nb–4Zr–8Sn–(0.08–0.40)O (wt.%) has been investigated by in situ synchrotron X-ray diffraction, Rietveld refinement and texture analysis. The β elastic constants were determined by Eshelby–Kröner–Kneer self-consistent modelling. A micromechanical model based on texture, martensite crystallography and Schmid law nucleation criteria was used to analyse the generation of non-linear strain. Oxygen increased the C′ cubic shear constant. The β phase showed increased compliance along 〈1 1 0〉β as precursor to transformation, which occurs above 0.15 wt.% O by growth of nanosized nuclei, and is nearly suppressed at 0.40 wt.% O by structural convergence of parent/martensite phases. The shape of the stress–strain curve is due principally to stress-induced martensitic transformation with high-symmetry {7 5 5}β habit planes, and is controlled by oxygen through its effect on the transformation strains and critical shear stress.