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Young's modulus and tensile strength were investigated in relation to phase transformation and microstructural changes occurring during cold rolling and subsequent heat treatment using (Ti-35 mass%Nb)-4 mass%Sn and (Ti-35 mass%Nb)-7.9 mass%Sn alloys. Stress-induced 00 martensite is generated on cold rolling of (Ti-35Nb)-4Sn whose martensitic transformation start temperature is around room temperature. Young's modulus in the rolling direction is lowered by the generation of stress induced 00 phase with preferred texture, while it is recovered by the reverse martensitic transformation to at 523 K. The reverse transformation yields fine grains which are elongated approximately along the rolling direction and have an average grain size in width of less than 1 mm. This fine microstructure leads to high strength over 800 MPa with keeping low static Young's modulus of 43 GPa. In contrast, mechanical properties of (Ti-35Nb)-7.9Sn in which matensite is not stress-induced are not so significantly improved by cold rolling and heat treatment.

Composition dependence of Young's modulus in Ti-V and Ti-Nb binary alloys and Sn-added ternary alloys quenched from phase region was investigated at room temperature in relation to the stability of phase. A minimum of Young's modulus in the binary alloys appears at such a composition that athermal ! phase transformation is almost completely suppressed. Formation of isothermal ! phase by aging after quenching increases Young's modulus. Sn addition to the binary alloys suppresses or retards ! transformation, thereby decreasing Young's modulus. Optimization of alloy composition in Ti-Nb-Sn alloys leads to low Young's modulus of about 40 GPa. The composition dependence of Young's modulus obtained experimentally in this study can be qualitatively explained by the theoretical discrete-variational X cluster method.

Martensitic transformation and tensile properties of 4 to 5 mol%Sn-doped Ti-16 mol%Nb alloys consisting of biocompatible elements were investigated to provide superelasticity for biomedical applications as a function of heat treatment and Sn content. Martensitic transformation (bcc to orthorhombic structure) is accelerated at such quenching conditions that the bcc parent phase is slightly decomposed. Martensitic transformation temperature decreases rapidly with increasing Sn content. In-situ optical microscopic observation on cooling and heating indicates that the martensite is thermoelastic, corresponding to small temperature hysteresis between the martensitic and the reverse transformations, which is determined by differential scanning calorimetry. By controlling the heat treatment condition and Sn content, large superelastic strain is obtained at room temperature.

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