Orthorhombic martensitic phase in Ti–Nb alloys: A first principles study

The structure of orthorhombic martensitic phase ( α ″ ) has been investigated using first principles density functional theory (DFT) within generalized gradient approximation (GGA). Formation energy/atom, electronic properties, lattice constants and Wyckoff positions of the α ″ phase have been calculated for binary Ti–xNb (x = 0–25 atom%) alloys. The instability of the α ″ phase increases with increase in Nb concentration as the formation energy/atom increases with increase in Nb content. The structural details of these results are compared with the experimental data obtained by Rietveld refinement of the α ″ phase of Ti–8Nb, Ti–12Nb and Ti–16Nb alloys. A limit of Wyckoff positions obtained by symmetry of Cmcm space group and Rietveld refinement has been verified. The β → α ″ transformation is related with smaller strains along three principal axes than that of the β → α′.