First-principles study of phase stability and elastic properties in metastable Ti-Mo alloys with cluster structure

This paper provided a novel approach for evaluating phase stability and elastic properties in metastable Ti-Mo alloys with low Mo content by first-principles combined with cluster structure. In 54-atom body-centered-cubic supercell by substituting Ti atoms with 2-7 Mo atoms (7.1-23.0 wt% Mo), individual cluster structure of beta-phase was constructed by '-Mo-Ti-Mo-' cluster unit having the lowest cohesive energy. The distorted supercell was more stable than undistorted one at a low Mo content. With increasing Mo content, the density of state at Fermi level decreased, and bonding electron number increased, indicating beta-phase stability was gradually promoted. Tetragonal shear elastic constant (C ' = (C-11 - C-12)/2), shear modulus (G(111)) and anisotropy factor (A = C-44/C ') exhibited a fluctuation with Mo addition, while the change trend of A was opposite to C ' and G(111). Calculated Young's modulus exhibited similar changing trend to the C ', implying that the softening of C ' resulted in low Young's modulus of beta-phase. Measured Young's modulus exhibited significant difference from calculated one, which was mainly caused by formation of alpha ''-martensite and omega-phase. The values of C ', G(111) and A were considered to associate with not only elastic properties of beta-phase itself but also transition from beta-phase to alpha ''-martensite and/or omega-phase.