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

• Published in: Acta materialia 2011, Vol.59 (1), p.112-125
• Main Authors: Obbard, E.G., Hao, Y.L., Talling, R.J., Li, S.J., Zhang, Y.W., Dye, D., Yang, R.
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Martensite; Martensitic phase transformation; Martensitic transformations; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Nanostructure; Phase transformations; Shape memory alloys; Surface layer; Synchrotron radiation; Texture; Titanium; Titanium alloys; Transformations; X-ray diffraction
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2010.09.015

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.