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Ti2−xNiCoSnSb (x = 0.125, 0.250, 0.375, and 0.500) half Heusler (HH) high-entropy thermoelectric alloys were synthesized by the arc melting—ball milling—spark plasma sintering route. The impact of secondary phase content on the thermoelectric properties in these alloys was examined. Ni-rich intermetallic (Ni3Sn2, Ni3Sn4) compounds were observed; the intermetallic content increased for lower Ti content, e.g., Ti1.5NiCoSnSb. A Ni-rich full Heusler (FH) secondary phase was also observed. These results were consistent with first-principles calculations that show that the formation enthalpy of Ti1.5NiCoSnSb was higher than that of Ti2NiCoSnSb and the full Heusler (FH) TiNi2Sn phase. In lower Ti content samples, the electrical conductivity increased, and lattice thermal conductivity decreased at the expense of thermopower owing to higher FH and the Ni3Sn2 phase content. Ti1.5NiCoSnSb exhibited lower lattice thermal conductivity of 3.5 W/mK, compared to 5.4 W/mK at 823 K for Ti2NiCoSnSb due to increased phonon scattering at HH/Ni3Sn2 interfaces. But considering the decreasing power factor with lower Ti content, the maximum ZT obtained in Ti1.875NiCoSnSb (0.171 at 973 K) was only marginally higher than the value for Ti2NiCoSnSb. Further, compositional tuning is hence necessary to maximize the power factor. [ABSTRACT FROM AUTHOR]

Sn-doped TiFe0.5Ni0.5Sb1−xSnx (x = 0, 0.05, 0.1, 0.2) were synthesized by vacuum arc melting (VAM). In addition to the half-Heusler phase, secondary phases of Fe–Sb-rich compound and Ti-rich compounds were obtained after VAM. The alloys were then subjected to ball milling for 1 h and 5 h. Ball milling for 1h led to microcrystalline grains, while that for 5 h led to nanocrystalline grains. Ball milling followed by spark plasma sintering (SPS) at 1173 K led to significant reduction in size of secondary phases in the microstructure. The undoped sample exhibited a ZT of 0.008 at 873 K for both 1h and 5h BM-SPS samples. [ABSTRACT FROM AUTHOR]

In the present work, the concept of mechanically activated annealing (MAA) has been applied to produce nanocrystalline Al0.3CoCrFeNi high-entropy alloys (HEAs) with reduced contamination levels. Phase evolution during conventional mechanical alloying (MA), MAA and subsequent consolidation by spark plasma sintering (SPS) have been studied in detail. Complete alloying is obtained after 15 h of MA, while milling time of 5 h and annealing at 1100 °C for 1 h have been used to achieve alloy formation during MAA. Both the MA–SPS and MAA–SPS routes have shown major FCC phase. The contamination of WC observed during MA was successfully eliminated during MAA, while the volume fraction of Cr7C3 reduced from 20% during MA–SPS to 10% after MAA–SPS. This method can serve as an effective way to produce nanostructured HEAs with minimum contamination. [ABSTRACT FROM AUTHOR]

A new single phase high entropy alloy, Ti2NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S2σ) of 0.57 and 1.02 mWm−1K−2, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy. [ABSTRACT FROM AUTHOR]

The sintering behavior and structural stability of two-dimensional (2D) WS 2 nanosheet powder under high-temperature pressure-assisted spark plasma sintering (SPS) were investigated. Transformation of the 2D particle morphology to coarse WS 2 platelets was observed during sintering at a higher temperature of and above 1000 oC. Particle coarsening was caused by diffusion bonding of multiple 2D-WS 2 nanosheet particles on the basal plane surface. Consolidation of the 2D-WS 2 nanosheet particles at lower sintering temperatures primarily involved particle-particle interlocking and limited particle-particle diffusion bonding. About 94% of the theoretical density was achieved without any sintering additives on SPS at 1200 oC. The WS 2 stoichiometry was retained even at 1200 °C with no indication of any thermal degradation. A fraction of the hexagonal 2H-type structure of the 2D-WS 2 transformed to rhombohedral 3R phase during SPS at and above 800 °C. The as-sintered 2D-WS 2 showed a low coefficient of friction in the range of 0.06–0.1. The observed tribological behavior was correlated with its thermo-mechanical properties and sintering characteristics at different temperatures. [ABSTRACT FROM AUTHOR]