Effect of Alloying Element X on Transformation Strains and Phase Stabilities between α" and β Ti-Nb-X (X = Al, Sn, Zr, Ta) Ternary Alloys from First-Principles Calculations

The effect of alloying element X (X = Al, Sn, Zr, Ta) on the transformation strains and phase stabilities of Ti-12.5 at%Nb and Ti-25 at%Nb alloys was investigated. The principle strain, eta(2), was calculated as a function of the X concentration. The value of eta(2) of Ti-12.5 at%Nb-6.25 at%Zr was larger than that of Ti-12.5 at%Nb-6.25 at%Ta. This is consistent with the experimental results. The difference of solution energy, Delta H-sol(alpha ''-beta), was also calculated. The value of Delta H-sol(alpha ''-beta) increased with the addition of Al, Sn, Zr, and Ta to Ti-12.5 at% Nb-6.25 at%X alloys. We found a good linear correlation between the experimental values for the decrease of the martensitic transformation start temperature (Ms), dMs/dc, and the calculated values of Delta H-sol(alpha ''-beta). The effect of the alloying element on the shape-memory properties of Ti-Nb-based alloys were evaluated from the first-principles calculations.