TEM observation of coexistent Heusler and half-Heusler phases in TiNi1.5Sn

Half-Heusler compounds are considered to be a prospective material for thermoelectric conversion. The thermoelectric properties are improved and controlled by the microstructure and the atomic configuration, which are easily changed by the heat-treatments or the composition. In the present work, the microstructure of thermoelectric compound TiNi1.5Sn with the Ni-excessive half-Heusler composition was measured and observed by using an X-ray diffraction technique and transmission electron microscopy. The X-ray diffraction pattern was indexed by assuming a mixture of the Heusler and the half-Heusler phases with the lattice constants of 0.604 and 0.594 nm, respectively. Crystal grains with about 3 μm dimension were observed in a bright field TEM image. It was clarified from a selected-area electron diffraction and a TEM–EDX measurement that the Heusler and the half-Heusler domains were separated in each crystal grain and were alternately aligned with a strong lattice coherency. Using a Fourier-transformed image processing for high-resolution TEM image, edge dislocations aligned at the regular intervals were observed at the boundary between the Heusler and the half-Heusler domains. The dislocations were introduced in order to retain the lattice coherency between the two phases.