Three-dimensional multimodal imaging and analysis of biphasic microstructure in a Ti–Ni–Sn thermoelectric material

Abstract

The three-dimensional microstructure of levitation melted TiNi1.20Sn has been characterized using the TriBeam system, a scanning electron microscope equipped with a femtosecond laser for rapid serial sectioning, to map the character of interfaces. By incorporating both chemical data (energy dispersive x-ray spectroscopy) and crystallographic data (electron backscatter diffraction), the grain structure and phase morphology were analyzed in a 155 μm × 178 μm × 210 μm volume and were seen to be decoupled. The predominant phases present in the material, half-Heusler TiNiSn, and full-Heusler TiNi2Sn have a percolated structure. The distribution of coherent interfaces and high-angle interfaces has been measured quantitatively.

Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn

“…Our experimental work and that of others indicates that up to ~8% excess Ni can be accommodated prior to the observation of FH phases in diffraction patterns [ 18 , 19 , 22 , 23 , 24 , 25 ]. Microscopy studies confirm the segregation into HH and FH phases with the arrangement of the excess Ni within the HH phase dependent on sample processing [ 16 , 17 , 19 , 26 , 27 , 28 , 29 ]. Our work on samples prepared using solid-state reactions indicates that most of the excess metals are present as randomly distributed interstitials with no evidence for large numbers of nanometer sized FH inclusions [ 23 , 30 ].…”

Interfacial structure and strain accommodation in two-phase NbCo1.2Sn Heusler intermetallics

“…This phenomenon has been recently explored in two-phase Heusler thermoelectric, M-Ni-Sn (M = Hf, Zr, Ti) [2,3,[12][13][14][4][5][6][7][8][9][10][11], and Nb-Co-Sn [15][16][17], where it is believed that a high density of semicoherent interfaces at phase boundaries increase the number of phonon scattering centers, thereby improving thermoelectric performance. These findings have triggered an interest in understanding and controlling the evolution and constitution of interfaces within two-phase Heusler microstructures, concerning precipitate morphology, size, and volume fraction [2,[5][6][7]15]. Detailed microstructure evolution characterization has up to now been performed for the Ti-Ni-Sn HC two-phase system, where the two-phase microstructure consists of TiNi2Sn FH precipitates embedded in TiNiSn HH matrix, exhibiting a lattice misfit δ of 2.5%.…”

Recent Developments in Femtosecond Laser-Enabled TriBeam Systems

“…4(b). In the case of Ti-6Al-4V, 69,90 a Heusler NiTiSn alloy, 91 Ti-Al (a material that often exhibits challenging pseudosymmetry issues 92,93 ) and a spot weld on a single-crystal NiAlMo alloy, a nearglancing-angle (< 10 to sample surface) Ga FIB beam is used to remove a thin layer from the femtosecond laser-ablated surface. Precise definition of orientation gradients is also enhanced by surface cleanup, which results in higher-quality EBSD patterns.…”