Factors determining solid solution phase formation and stability in CoCrFeNiX 0.4 (X¼Al, Nb, Ta) high entropy alloys fabricated by powder plasma arc additive manufacturing Yupeng Zhang a , Xizhang Chen a, * , S. Jayalakshmi a , R. Arvind Singh a , Vladislav B. Deev b , Evgeny S. Prusov c a College of Mechanical and Electrical Engineering, Wenzhou University, Wenzhou, 325035, China b Department of Metal Forming, National University of Science and Technology “MISiS”, Moscow, 119049, Russian Federation c Department of Functional and Constructional Materials Technology, Vladimir State University, Vladimir, 600000, Russian Federation article info Article history: Received 20 July 2020 Received in revised form 15 October 2020 Accepted 15 October 2020 Available online xxx Keywords: Equal atomic radii Mixed enthalpy Electronegativity VEC DSC Microstructure abstract In high entropy alloys (HEAs), the formation of solid solution phase is governed by main factors such as mixing entropy, mixing enthalpy, atomic radii, and atomic size difference. However, factors such as electronegativity, valence electron concentration, and melting point, also significantly influence the formation of the solid solution phase, individually or in combination in specific alloys, and are often less studied and reported. In this work, CoCrFeNiX 0.4 (X ¼ Al, Nb, Ta, elements with equi-atomic radii) high entropy alloys have been prepared by powder plasma arc additive manufacturing (PPA-AM). The effect of equi-atomic radii element addition on the microstructural evolution was studied. The results showed that although Al, Nb, and Ta were equi-atomic radii element additions, the resulting HEAs had variations in their phase formation and mechanical properties. Following observations were made: (i) Al addition: FCC þ BCC þSigma phase formation with lowest hardness and (ii) Nb and Ta addition: FCC þ Laves phase þ Eutectic phases formation with relatively high hardness (>85%e110% increase). Considering other factors that determine the formation of solid solution phase, it has been identified that: (i) high melting point of the metal has a superior influence on the formation of topologically closed packed phases (TCP) solid solution phase and (ii) elements with large electronegativity differences tends to be rich in the second solid solution phase. The HEAs deposited by PPA-AM have similar/better mechanical stability when compared to the as-cast alloys. Using an innovative AM technology to fabricate HEAs, this work emphasizes the importance of the control of multiple variables in manipulating the solid solution phase formation and mechanical properties of HEAs. © 2020 Elsevier B.V. All rights reserved. 1. Introduction High-entropy alloys are multicomponent alloys with more than 4 elements having concentrations between 5% and 35%, which tend to form a simple disordered solid solution rather than forming brittle intermetallic compounds as seen in traditional alloys [1 ,2]. The disordered solid solution that makes high-entropy alloys exhibit high strength, high toughness, and good corrosion resis- tance, has attracted much research interest in recent years [3e5]. High entropy alloy formation criteria are much under debate and intensely researched, and various factors that influence the for- mation of HEAs have been proposed. Guo et al. [6] opined that the atomic size difference, mixing entropy (DS mix ), and mixing enthalpy (DH mix ) together determine the formation of solid solu- tion. Chanda et al. [7] proposed that higher atomic radii difference can promote the formation of a two-phase solid solution in eutectic HEAs (EHEAs) by increasing the lattice stress leading to lattice distortion. A new mathematical model based on the atomic size was reported [8] to better distinguish solid solution, intermetallic compound, and metallic glass. The large entropy change at high temperature is conducive to the formation of single-phase solid solution, and an adequately negative enthalpy value is conducive to the formation of two-phase solid solution, ensuring the stability of EHEAs [7]. Yang et al. [9] proposed the relationship between the * Corresponding author. E-mail address: chenxizhang@wzu.edu.cn (X. Chen). Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom https://doi.org/10.1016/j.jallcom.2020.157625 0925-8388/© 2020 Elsevier B.V. All rights reserved. Journal of Alloys and Compounds xxx (xxxx) xxx Please cite this article as: Y. Zhang, X. Chen, S. Jayalakshmi et al., Factors determining solid solution phase formation and stability in CoCrFeNiX 0.4 (X¼Al, Nb, Ta) high entropy alloys fabricated by powder plasma arc additive manufacturing, Journal of Alloys and Compounds, https://doi.org/ 10.1016/j.jallcom.2020.157625