Effect of Phase Stability on Some Physical and Mechanical Properties in β-Ti Single Crystal for Biomedical Applications

Abstract

The electron-atom ratio (e/a) dependence of the appearance of the lattice modulation and physical properties in β-phase Ti-xNb alloys (x = 28, 30, 34 and 40) were investigated by using some physical properties measurements, compressive test and transmission electron microscope observations (TEM observations), focusing on the β-phase stability. The microstructure, physical properties, deformation mode depend on the e/a ratio which is closely related to the β-phase stability in Ti-Nb alloys. The e/a ratio is defined by the average electrons per atom in free atom configuration. Athermal ω-phase is suppressed in Ti-30Nb alloy single crystal with low e/a ratio. The Ti-30Nb alloy single crystal also exhibits a lattice modulation and low Debye temperature. These results imply that the β-phase stability in β-phase Ti alloys decreases with decreasing the e/a ratio and are related to the softening of elastic stiffness, c′. Consequently, a decrease in the e/a ratio leads to the softening of c′ and a significant reduction in modulus along the [100] direction in β-phase Ti alloys single crystal. In fact, the Young’s modulus along [100] of the Ti-15Mo-5Zr-3Al alloy (wt.%) single crystal with low e/a ratio exhibits as low as 45 GPa, which is comparable to that the human cortical bone. That is, controlling the e/a ratio is an ultimate strategy to develop the future superior biocompatible implant materials with extremely low Young’s modulus and good deformability.