First principles calculations on the effect of interstitial oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys

The effect of oxygen on phase stability and β–α″ martensitic transformation in Ti–Nb alloys has been studied using first principles calculations. Three stable atomic configurations of Ti–Nb (Ti-12.5, 16.6, and 25 at.% Nb) systems, which can transform from β-phase to α″-phase without changing the local atomic position of Nb atoms, were first identified using the cluster expansion method. Phonon calculations indicated that these structures were stable. Next, the martensitic transformation behavior of Ti–Nb–O system was studied using these structures. We observed a significant lattice distortion around oxygen atoms occupying octahedral interstitial sites that resembles a bcc type of stacking. Our results conclusively revealed that while the oxygen interstitials can oppose the atomic shuffle required for martensitic transformation, they can also cooperatively stabilize the β-phase even at 1 at.% oxygen concentrations by inducing local elastic shear strains. Interestingly, the canceling of these fields can stabilize the β-phase by suppressing the β to α″ transformation which decreases the martensitic start temperature (M_s). Our study revealed that the reduction in M_s is higher at lower Nb concentration. The stabilization of β-phase increases with oxygen concentration.