Lattice parameters and relative stability of α″ phase in binary titanium alloys from first-principles calculations

The crystallographic structure and stability of the α″α″ phase relative to the αα and ββ 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-α″α″ phase evolutes from the hcp-αα to the bcc-ββ phase, i.e., the lattice parameters b/a and c/a as well as the basal shuffle y   decreases from those corresponding to the αα phase to those of the ββ phase. The compositional α/α″α/α″ and α″/βα″/β phase boundaries are determined by comparing the total energies of the phases. The predicted α/α″α/α″ 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 α″/βα″/β 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 α″α″ phase is controlled by the compromise between the strength of the covalent and metallic bonds.

► α″α″ structure involves from αα to ββ structure during alloying. ► The compositions that identify phase boundary of α″−αα″−α are determined. ► Covalent and metallic bondings control the stability of α″α″ phase.