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

The cause of a low Young's modulus was investigated in quaternary P-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 P-phase stability, low c(44), 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 Youngs modulus, where that in the < 100 >-direction E-100 is the lowest of all crystallographic orientations. For quaternary Ti29Nb-13Ta-4.6Zr alloy (mass%), E-100 is only similar to 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', c(44) and B are essential for decreasing Young's modulus of novel beta-type Ti alloys which are expected to be developed in the near future. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.