Effect of Processing Routes on the Microstructure and Thermoelectric Properties of Half-Heusler TiFe 0.5 Ni 0.5 Sb 1x Sn x (x = 0, 0.05, 0.1, 0.2) Alloys Anirudha Karati, Sanyukta Ghosh, Ramesh Chandra Mallik, Rajashekhara Shabadi, B.S. Murty, and U.V. Varadaraju Submitted: 21 April 2021 / Revised: 26 July 2021 / Accepted: 4 August 2021 / Published online: 23 September 2021 Sn-doped TiFe 0.5 Ni 0.5 Sb 12x Sn x (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. Keywords intermetallic alloys, powder metallurgy, semiconductors, sintering 1. Introduction The ever-increasing demand of fossil fuels coupled with its fast depletion has channeled research interests towards clean, renewable and earth abundant resources. Among the many readily available alternative energy sources, thermoelectricity, which involves heat recovery to produce electricity, has been a topic of intense research over the last few decades. Thermo- electric materials that are used in devices have advantage over conventional heat engines owing to its lack of moving parts, less impact to environment, noiseless operation and high reliability (Ref 1–3). The efficiency of thermoelectric materials is denoted by the dimensionless figure of merit (ZT), which can be expressed as: ZT ¼ S 2r j T where S represents the Seebeck coefficient, r is the electrical conductivity, j is the thermal conductivity comprising of j e (electronic) and j l (lattice), and T is the absolute temperature. For a material to exhibit good thermoelectric property, it should possess high S, high r and low j to obtain a high thermoelectric figure of merit (ZT). However, high r increases the j e and thus optimization of both r and j l is required to enhance the overall r/j ratio (Ref 4). Thermoelectric materials operate in different temperature regimes, and thus there exist different classes of materials such as chalcogenides (Ref 5), skutterudites (Ref 6) and Zintl phase compounds (Ref 7) operating in low- (300-500 K), mid- (500-800 K) and high- temperature (800-1100 K) regimes. One such class of com- pounds that has demonstrated high ZT in mid- to high- temperature regimes are half-Heusler alloys. Half-Heusler alloys have gained prominence owing to their good mechanical and thermal robustness (Ref 8), excellent electronic properties due to narrow band gap of 1eV, high Seebeck coefficient and high electrical conductivity. Additionally, it is made of elements that are less toxic and more earth abundant (Ref 9). Half-Heusler alloys adopt the MgAgAs type of structure crystallizing adopting the space group F 43m (Ref 10). The structure comprises three interpenetrating fcc sub-lattices. Compositions in half-Heusler alloys that have a valence electron count of 18 are found to have the Fermi energy level (E f ) to be present in the center of the band structure. Such systems tend to be semiconducting and hence are suitable can- didates for thermoelectric applications (Ref 11). TiNiSn and TiCoSb are the two well-studied systems among half-Heusler alloys (Ref 9). TiCoSb is an intrinsic n-type semiconductor. It has high room-temperature Seebeck coefficient and electrical resistivity values of  265 lV/K and 4.5 9 10 -2 Xcm, respec- tively (Ref 12). However, it has a very high thermal conduc- tivity of 24 Wm 1 K 1 at 373 K (Ref 13). This high thermal conductivity serves to be one of the biggest bottlenecks in the application of half-Heulser alloys as thermoelectric materials. Methods like nanostructuring to introduce grain boundary scattering (Ref 14), in situ formation of second phase (Ref 15), ex situ addition of second phase (Ref 16) and point defects generated due to solid solution formation (Ref 17) are some of the strategies that have been generally employed to reduce j by reducing the lattice part of thermal conductivity (j l )in half- Heusler alloys. Kim et al. reported a much reduced j l of 2.4 Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s11665-02 1-06207-z. Anirudha Karati and U.V. Varadaraju, Department of Chemistry, Indian Institute of Technology Madras, Chennai, India; Sanyukta Ghosh and Ramesh Chandra Mallik, Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of Science, Bangalore, India; Rajashekhara Shabadi, Faculty of Science and Technology, UMET, University of Lille, Villeneuve-dÕAscq, France; B.S. Murty, Department of Chemistry, Indian Institute of Technology Madras, Chennai, India; Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, India; and Indian Institute of Technology Hyderabad, Kandi, India. Contact e-mail: murty@iitm.ac.in. JMEPEG (2022) 31:305–317 ÓASM International https://doi.org/10.1007/s11665-021-06207-z 1059-9495/$19.00 Journal of Materials Engineering and Performance Volume 31(1) January 2022—305