Elastic properties of single-crystalline ω phase in titanium

The elastic properties of single-crystalline ω (hexagonal) phase of titanium are studied. Understanding the elastic properties is important for the development of biomedical titanium alloys with a low Young’s modulus. However, the elastic properties of the ω phase have remained unclear because of the difficulty in preparing a large single crystal consisting of a single phase of the ω phase, even though the ω phase has been believed to exhibit a higher elastic modulus than the β (body-centered cubic) phase. In this work, pure titanium was severely deformed by high-pressure torsion processing, to obtain polycrystalline specimens consisting exclusively of the ω phase, which is metastable at room temperature. For the ω-phase polycrystal, the complete set of elastic stiffness components was measured by RUS combined with laser Doppler interferometery. By analyzing the elastic stiffness of the ω-phase polycrystal on the basis of an inverse Voigt–Reuss–Hill approximation, the elastic stiffness components of the single-crystalline ω phase were determined. The Young’s modulus of the ω phase along 〈0001〉 was found to be clearly higher than that along image, and the shear modulus also exhibited anisotropy. Importantly, the Young’s modulus and shear modulus of the metastable ω phase were higher than those of the β phase and also higher than those of the α (hexagonal close-packed) phase, which is stable at room temperature. Furthermore, analysis by a micromechanics model using the determined elastic stiffness deduced the effect of ω phase formation on the elastic properties of β-phase titanium alloys.