| Due to the importance of NiTi as a shape memory material and the uncertainty regarding its atomisitic martensitic transformation path, a thorough investigation to understand the structural stability governing this displacive phase transformation is warranted. We investigate elastic and shear stabilities of NiTi binary and ternary (with additions of Pd and Pt) alloys using first-principles calculations with the highly-precise full-potential linearized augmented plane wave (FLAPW) method. Ambiguities of the B2, R, B19, B19', and proposed B33 structures are resolved, and the phase stability of each structure is established by examining calculated formation energies. All single crystal elastic constants, Young's, bulk, and shear moduli, Poisson's ratio, and the Zener anisotropy of the B2, R, B19, B19', and B33 phases are calculated and presented. To investigate the susceptibility to shearing, generalized stacking fault energetics and cleavage energies are calculated for the {001}, {011}, and {111} slip planes of the B2 phase. Burgers vectors and shear resistance are established. By investigating various deformation mechanisms related to these stacking faults, we find an instability to h100i{011} slip in the B2 phase. Using this and reviewing previously proposed atomistic transformation paths, the mechanisms governing the direct martensitic transformation of NiTi between the austenite and the martensite are identified. Barrierless transformation paths from the B2 phase to the B19' phase and from the B2 phase to the B33 phase are proposed, and the ternary transformation path is investigated. Differences between binary and ternary alloys, which are known to raise transformation temperatures, are illustrated. To provide a theoretical foundation for this diffusionless structural phase transformation, we illustrate the changes in electronic structures which explain its martensitic behavior. Electronic structure evolution is illustrated throughout the proposed atomistic transformation paths by examining changes in the Fermi surface topology, band structures, generalized susceptibilities, and calculated phonon dispersions. Electronic topological transitions are observed in calculated densities of states using the rigid band method to determine the effects of stoichiometry changes. Finally, by carefully evaluating the Fermi surface as the martensitic transformation progresses, imaginary phonon modes and nesting in the Fermi surfaces are traced to atomic instabilities which drive the martensitic transformation. |
