First-principles study of transformation strains and phase stabilities in α″ and β Ti-Nb-X alloys

Effects of third alloying elements on the transformation strains and phase stabilities of Ti-Nb based alloys at 0 K were investigated using the first-principles calculations. In binary Ti-Nb alloys, the transformation strains and the body-centered cubic high-temperature beta (β) phase stabilities decrease with increasing Nb concentration. In Ti-12.5Nb-6.25X (at.%) alloys, the effects of 46 different alloying elements X on the transformation strains and phase stabilities were also investigated. The addition of Al, Be, Ca, Cu, Ga, Ge, Hf, La, Mg, Sc, Si, Sn, Sr, Y, Zn, and Zr increase the transformation strains, and all alloying elements except for Sc act as the β-phase stabilizing element. Using the calculated values of the effects of alloying elements, xM values were calculated for the different 46 alloying elements, where xM are the X contents of Ti-12.5Nb-X (at.%) ternary alloys where the martensitic transformation start temperatures were equal to 300 K. Then, the transformation strains of Ti-12.5Nb-xMX (at.%) alloys were predicted. The Ti-12.5Nb-17.8Hf (at.%) alloy shows the largest transformation strain value.