Low Young’s modulus of Ti–Nb–Ta–Zr alloys caused by softening in shear moduli c′ and c44 near lower limit of body-centered cubic phase stability

The composition and temperature dependence of the elastic properties and phase stability of quaternary Ti–Nb–Ta–Zr β-phase alloys with a body-centered cubic structure, developed for biomedical applications, were investigated using their single crystals, in order to clarify the origin of the low Young’s modulus in polycrystals. Transmission electron microscopy observations clarified that α″ martensitic transformation occurred in a temperature range that depended on the β-phase stability below room temperature. Electromagnetic acoustic resonance measurements clarified that the shear moduli c′ and c44 of single crystals softened upon cooling from room temperature and became rather low near the martensitic transformation start temperature, i.e. the lower limit of β-phase stability. An analysis by the Hill approximation indicates that low c′ and c44 caused the low Young’s modulus, and thus it is probable that the softening in c′ and c44 is the origin of the low Young’s modulus.