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 beta and omega phases of Ti1-xNbx (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 beta phase agree well with the experimental data. For omega 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 beta phase relative to omega phase. The critical Nb concentration for the complete stabilization of beta 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 Ti1-xNbx (0 < x <= 0.4) random alloys should be intrinsically ductile.