Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti50Ni30Pt20 (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.