Alloys with low Young’s modulus will be useful in biomedical applications because they provide design opportunities to alleviate short-term/long-term design conflict, to reduce sensitivity of implant performance to individual factors and to incrementally improve current implant designs. Ti-24Nb-4Zr-8Sn (Ti2448) and Ti-30Nb-10Ta-5Zr (TNTZ) are two such low Young’s modulus β phase alloys. Both alloys are distinguished by their low modulus of 50-60 GPa, by nonlinear recoverable strain at room temperature and by the belief that α" martensite, which is the conventional mechanism of superelasticity in β titanium, does not account for their respective superelastic effects. Initial in-situ diffraction experiments were carried out at IMR, Shenyang, and at the ISIS neutron source in the United Kingdom, to elucidate these mechanisms, yet due to low achievable resolution still drew uncertain conclusions. Successful experiments were subsequently carried out at the European Synchrotron Radiation Facility in Grenoble, France using tensile testing and in-situ diffraction of high energy 88 keV synchrotron X-rays. Elastic properties of the β parent phases were investigated where possible by using the Eshelby-Kröner-Kneer elastic-plastic self-consistent model to determine the elastic constants. The β phase diffraction elastic moduli show a departure from cubic symmetry in the form of increased compliance along <110>β as a precursor to α" transformation. Stress-induced orthorhombic α" was observed in all samples under load, and the β-α" transformation is found to be unusual in that transformation strains in both alloys precisely conform to an invariant plane strain condition. For TNTZ, this fact is used to construct a micromechanical model, which is used to account for observed diffraction peak positions, and shows that α" was not previously observed because transformation texture leaves no diffraction peaks visible when the diffraction vector is perpendicular to the tensile direction. For Ti2448, Rietveld refinement is used to fit the diffraction patterns acquired at load and investigate the structure, texture and volume fraction of the α" martensite. The α" phase is structurally remarkably similar to the parent phase and oxygen has the effect of further lessening the distinction. A micromechanical model of α" nucleation based on critical resolved shear stress on the habit plane is used to provide a framework within which to interpret quantitative diffraction measurements, with the aim of rationalizing the effect of oxygen concentration on the shape of the non-linear load curve. It is shown that the distributions of maximum unidirectional Schmid factor is controlled by habit plane symmetry and parent phase texture, and that in Ti2448 this strongly affects the shape of the load curve.
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