Temperature-induced martensitic phase transitions in gum-metal approximants: First-principles investigations for Ti3Nb

We present a first-principles investigation of the structures and the dynamical stability of the austenite and martensite phases of binary Ti3Nb alloys, used as a model system for the superelastic and superplastic gum-metal alloy. For the body-centered cubic high-temperature beta phase, structural models are constructed by optimizing the chemical decoration of a large supercell and by a cluster expansion method. The energetically most favorable structure is found to be elastically stable but dynamically unstable in the harmonic approximation. At finite temperature anharmonic phonon-phonon interactions treated in a self-consistent phonon approximation stabilize the structure already at room temperature. For the low-temperature alpha', omega, and alpha '' phases stable structure models have been constructed. For the hexagonal alpha' phase a model is generated by optimizing the chemical decoration of a supercell based on the hexagonal B-h lattice. The hexagonal omega structure may be derived from the body-centered cubic beta phase using the (111) plane collapse model. The structure of the orthorhombic alpha '' phase may be viewed as produced by a strain-induced transformation of the body-centered cubic beta phase, albeit with a different chemical decoration. The relaxed structures of the alpha', omega, and alpha '' phases were found to be both elastically and dynamically stable in the low-temperature limit. The martensitic temperatures for the beta -> alpha '', beta -> omega, and beta -> alpha' transitions were estimated by comparing the Helmholtz free energies as a function of temperature.