crystals Review A Review of the Latest Developments in the Field of Refractory High-Entropy Alloys Muthe Srikanth 1,† , A. Raja Annamalai 1,† , A. Muthuchamy 2 and Chun-Ping Jen 3, *   Citation: Srikanth, M.; Annamalai, A.R.; Muthuchamy, A.; Jen, C.-P. A Review of the Latest Developments in the Field of Refractory High-Entropy Alloys. Crystals 2021, 11, 612. https://doi.org/10.3390/ cryst11060612 Academic Editors: Patrice Berthod and Cyril Cayron Received: 25 March 2021 Accepted: 24 May 2021 Published: 28 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Centre for Innovative Manufacturing Research, VIT, Vellore 632 014, Tamil Nadu, India; muthe.srikanth@vit.ac.in (M.S.); Raja.annamalai@vit.ac.in (A.R.A.) 2 Department of Metallurgical and Materials Engineering, National Institute of Technology Tiruchirappalli, Tiruchirappalli 620 015, Tamil Nadu, India; a.muthuchamy@vit.ac.in 3 Department of Mechanical Engineering and Advanced Institute of Manufacturing for High-Tech Innovations, National Chung Cheng University, Chia-Yi 62102, Taiwan * Correspondence: chunpingjen@alum.ccu.edu.tw † Equally contributed as 1st Author. Abstract: This review paper provides insight into current developments in refractory high-entropy alloys (RHEAs) based on previous and currently available literature. High-temperature strength, high-temperature oxidation resistance, and corrosion resistance properties make RHEAs unique and stand out from other materials. RHEAs mainly contain refractory elements like W, Ta, Mo, Zr, Hf, V, and Nb (each in the 5–35 at% range), and some low melting elements like Al and Cr at less than 5 at%, which were already developed and in use for the past two decades. These alloys show promise in replacing Ni-based superalloys. In this paper, various manufacturing processes like casting, powder metallurgy, metal forming, thin-film, and coating, as well as the effect of different alloying elements on the microstructure, phase formation, mechanical properties and strengthening mechanism, oxidation resistance, and corrosion resistance, of RHEAs are reviewed. Keywords: refectory high-entropy alloys (RHEAs); powder metallurgy; casting; thin film; coatings; mechanical properties; oxidation resistance; corrosion resistance 1. Introduction In day-to-day life, the demand for new materials is greatly expanding for various applications. Different materials have been developed to increase the ability to withstand challenging environments, efficiency, and safety of materials. Refractory high-entropy alloys were developed in 2010 [1] with the primary purpose of withstanding high tem- peratures and to act as replacements for Ni-based superalloys and other applications, such as anodes for X-ray production gas turbine blades, armor, aerospace, and structural applications [2]. From the thermodynamic perspective, configurational entropy increases with an increasing number of elements, leading to high-entropy alloys (HEAs) [3]. The presence of multiple HEA elements leads to a decreased diffusion rate and an increased lattice distortion, which results in high-temperature stabilized phases, thus making HEAs useful for high-temperature applications [4]. Refractory elements like rhenium (Re), tungsten (W), molybdenum (Mo), tantalum (Ta), Niobium (Nb), and zirconium (Zr) are the main constituents of HEAs; along with them, other metals and materials are being used for the production of alloys named refractory high-entropy alloys (RHEAs). These alloys consist of multiple elements, and multiple elements have different crystal structures, such as face-centered cubic (FCC), body-centered cubic (BCC), hexagonal close-packed (HCP), and intermetallic compounds (B2, L1 2 , C14, and C15). The atomic positions of some intermetallic compounds are explained as follows. In the B2 intermetallic compound, the atomic position is the same as the body-centered cubic structure, but one type of atom occupies body-centered positions and another type of Crystals 2021, 11, 612. https://doi.org/10.3390/cryst11060612 https://www.mdpi.com/journal/crystals