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Abstract: We investigated the phase stability from B2 to L10/B19/B19′ in equiatomic PtTi alloy using the density functional theory within the generalized gradient approximation. Their structural, energetic, electronic and the mechanical properties were determined to show the stability trend of the shape memory binary alloy. The lattice parameters are well reproduced and agree to within 2% with the available experimental data. We found that the calculated heats of formation and density of states predicted B19 and B19′ PtTi to be the most stable structures compared to B2 and L10. On the other hand, their mechanical stability predictions show consistency with regard to the elastic constants and phonon dispersion curves. The elastic instability (negative C ij) and soft modes were observed in the B2, B19 and B19′ structures and not found in the L10 structure. [Copyright &y& Elsevier]

Abstract: Composition and temperature dependence of the crystal structure of IrTi with Ti content of 46–61 at.% was investigated using high-temperature X-ray diffraction analysis. Single-point energy calculations and geometry optimization were carried out based on first-principles calculations to understand the phase stability and transformation. Using X-ray diffraction analyses, a CsCl-type B2 structure was found in Ir-61 at.% Ti at room temperature; on the other hand, an orthorhombic structure in the alloys with Ti content below 58 at.%. The atomic positions obtained through first-principles calculations suggested a distorted L10 structure refined as a new orthorhombic (Cmmm #65) structure. The orthorhombic structure changed to almost a tetragonal structure with rising temperature and finally to a distorted B2 structure above 873 K in Ir-46–58Ti alloys. [Copyright &y& Elsevier]

Abstract: The deleterious low-temperature tetragonal phases in prototypical Pt-based superalloys have variously been reported as taking the tI16-U3Si (DOc), tI16-Ir3Si (DOc′) and tP16-Pt3Ga structure-types in contrast to the high-temperature cubic cP4-Cu3Au (L12) phase. We have investigated the relative stability of these four structure-types at absolute zero by using density functional theory. We find that the ground state of stoichiometric Pt3Al is tP16-Pt3Ga and that the other three lattices are mechanically unstable at absolute zero. Experiments are needed to measure the internal displacement parameters of these three competing tetragonal phases. [Copyright &y& Elsevier]