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-Kroener-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.