Peculiar elastic behavior of Ti–Nb–Ta–Zr single crystals

The cause of a low Young’s modulus was investigated in quaternary β-type Ti–Nb–Ta–Zr alloys, as the modulus is decreased to prevent bone absorption and degradation of bone quality when these alloys are implanted into human bones. This investigation was carried out using the alloys′ single crystals. Acoustic measurements and analysis by the Hill approximation revealed that a low Young’s modulus in a polycrystalline form is caused by the low shear modulus c′, related to the low β-phase stability, low c44, and relatively low bulk modulus B compared with those of binary Ti-based alloys. Furthermore, it was found that the single crystals had strong orientation dependence on Young’s modulus, where that in the 〈1 0 0〉-direction E100 is the lowest of all crystallographic orientations. For quaternary Ti–29Nb–13Ta–4.6Zr alloy (mass%), E100 is only ∼35 GPa, which is similar to Young’s modulus of human cortical bones as a result of the low B and c′. These results indicate that decreases in c′, c44 and B are essential for decreasing Young’s modulus of novel β-type Ti alloys which are expected to be developed in the near future.