The crystallographic structure and stability of the alpha" phase relative to the alpha and beta phases in Ti-x M (M=Ta, Nb, V, Mo) alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation. We show that, with increasing concentration of the alloying elements, the structure of the orthorhombic-alpha'' phase evolutes from the hcp-alpha to the bcc-beta phase, i.e., the lattice parameters b/a and c/a as well as the basal shuffle y decreases from those corresponding to the alpha phase to those of the beta phase. The compositional alpha/alpha" and alpha"/beta phase boundaries are determined by comparing the total energies of the phases. The predicted alpha/alpha" phase boundaries are about 10.2, 10.5, 11.5, 4.5 at% for Ti-V, Ti-Nb, Ti-Ta, and Ti-Mo, respectively, in reasonable agreement with experiments. The alpha"/beta phase boundaries are higher than the experimental values, possibly due to the absence of temperature effect in the first-principles calculations. Analyzing the electronic density of states, we propose that the stability of the alpha" phase is controlled by the compromise between the strength of the covalent and metallic bonds.
2013. Vol. 159, 70-75 p.
Titanium alloys, First-principles calculation, Composition dependence, Martensitic transformation