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