Effect of Nb on the properties of Ti-Nb random alloys from first-principles

The effect of Nb on the equilibrium lattice parameters and relative stability between β and ω phases of Ti_1-xNb_x (0 < x ≤ 0.4) random alloys as well as their mechanical properties in body-centered-cubic crystallographic phase was investigated using the exact muffin-tin orbitals method in combination with the coherent potential approximation. It has been found that the calculated lattice parameters of the β phase agree well with the experimental data. For ω phase, the value of a increases almost linearly with increasing Nb concentration, while the opposite situation presented for c/a. Both Nb addition and increasing temperature enhanced the stability of β phase relative to ω phase. The critical Nb concentration for the complete stabilization of β phase at 300 K, 673 K and 1273 K was 22 at.%, 17 at.% and 9 at.%, respectively. The polycrystalline bulk modulus B, Young's modulus E and shear modulus G increased monotonously with Nb addition and reducing the Nb concentration below 30 at.% resulted in lower E compared to that of Ti-6Al-4V. The calculated G/B values demonstrate that the bcc Ti_1-xNbx (0 < x ≤ 0.4) random alloys should be intrinsically ductile.