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

• Formation energy/atom and electronic properties of the α″ phase calculated.
• The instability of the α″ phase increases with increase in Nb concentration.
• A limit of Wyckoff positions obtained by symmetry of Cmcm space group verified.
• β → α″ transformation is related with smaller strains along three principal axes.

The structure of orthorhombic martensitic phase (α″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″α″ phase have been calculated for binary Ti–xNb (x = 0–25 atom%) alloys. The instability of the α″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″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″β→α″ transformation is related with smaller strains along three principal axes than that of the β → α′.