Microstructure Evolution of Biphasic TiNi1+x Sn Thermoelectric Materials

Abstract

The effects of thermal treatment on the microstructure of bi-phasic materials comprising halfHeusler (hH) and full-Heusler (fH) phases, as well as on their associated thermal conductivity, are discussed. The focus is on a biphasic hH/fH alloy of nominal stoichiometry TiNi 1.2 Sn, synthesized by containerless (magnetic levitation) induction melting. The alloy samples were exposed to various heat treatments to generate microstructures containing second phase precipitates ranging from ~10 nm to a few micrometers. The materials were characterized with regard to morphology, size, shape and orientation relationship of the fH and hH phases, both of which are present as precipitates within larger regions of the counterpart phase. The solidification path of the alloy and its implications for the subsequent microstructure evolution during heat treatment were elucidated, and relationships with the ensuing thermal conductivity were characterized.

Enhancing Thermoelectric Properties through Control of Nickel Interstitials and Phase Separation in Heusler/Half-Heusler TiNi1.1Sn Composites

“…Low temperature annealing of the homogeneous TiNi Sn phase forms nanoscale Heusler precipitates with a high aspect ratio, shown in Figure 6 c. This microstructure has been observed by Verma et al [ 28 ]. These precipitates are semicoherent with the half-Heusler matrix, forming with a cube-on-cube orientation relationship along the 〈100〉 directions of the half-Heusler.…”

“…The TEM micrograph in Figure 6 a confirms the micron-scale phase separation of the full- and half-Heusler after the homogenization treatment. The semicoherent interface between half-Heusler precipitates and the Heusler phase shows evidence of misfit dislocations [ 28 ]. The 3% lattice mismatch between the full- and half-Heusler phases produces strain fields extending in to each phase, which contribute to phonon scattering [ 27 ].…”

“…At elevated temperatures, the Ni interstitials that were trapped in the half-Heusler structure are able to diffuse, changing the nickel distribution. Due to the effect of high temperature measurements on samples, measurements were taken between 300 K and 310 K [ 28 ].…”

“…The two phases are immiscible at moderate temperatures. However, the phase diagram presented by Verma et al suggests that both regions of phase separation and phase solubility can be accessed at the composition TiNi Sn [ 28 ]. At high temperatures, the excess Ni fills the tetrahedral voids in the half-Heusler crystal structure, shown in Figure 1 [ 27 , 29 , 30 ].…”

Characterisation of Misfit Dislocations at Semicoherent Interfaces in Biphasic Functional Heusler Intermetallics

“…Rather than externally applied strain, strain fields can be built in to these systems through phase separation between the full Heusler (fH, XY2Z) phase and the half Heusler (hH, XYZ) phase. Biphasic Heuslers exhibit complex microstructures, which can be tailored to control precipitate shape, size, and volume fraction, ultimately affecting the physical properties of the composite [2,3].…”