  Citation: Lam, T.-N.; Luo, M.-Y.; Kawasaki, T.; Harjo, S.; Jain, J.; Lee, S.-Y.; Yeh, A.-C.; Huang, E.-W. Tensile Response of As-Cast CoCrFeNi and CoCrFeMnNi High-Entropy Alloys. Crystals 2022, 12, 157. https:// doi.org/10.3390/cryst12020157 Academic Editor: Pavel Lukᡠc Received: 24 December 2021 Accepted: 18 January 2022 Published: 21 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). crystals Article Tensile Response of As-Cast CoCrFeNi and CoCrFeMnNi High-Entropy Alloys Tu-Ngoc Lam 1,2, *, Mao-Yuan Luo 1 , Takuro Kawasaki 3 , Stefanus Harjo 3 , Jayant Jain 4 , Soo-Yeol Lee 5 , An-Chou Yeh 6,7 and E-Wen Huang 1,7, * 1 Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan; owo8691.en08@nycu.edu.tw 2 Department of Physics, College of Education, Can Tho University, Can Tho 900000, Vietnam 3 J-PARC Center, Japan Atomic Energy Agency, Ibaraki 319-1195, Japan; takuro.kawasaki@j-parc.jp (T.K.); stefanus.harjo@j-parc.jp (S.H.) 4 Department of Materials Science and Engineering, Indian Institute of Technology, New Delhi 110016, India; Jayant.Jain@mse.iitd.ac.in 5 Department of Materials Science and Engineering, Chungnam National University, Daejeon 34134, Korea; sylee2012@cnu.ac.kr 6 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan; yehac@mx.nthu.edu.tw 7 High Entropy Materials Center, National Tsing Hua University, Hsinchu 30013, Taiwan * Correspondence: lamtungoc1310@nctu.edu.tw (T.-N.L.); ewhuang@g2.nctu.edu.tw (E.-W.H.) Abstract: In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs). Both of these HEA systems are single-phase, face-centered-cubic (FCC) structures. Specifically, we examined the tensile response in as-cast quaternary CoCrFeNi and quinary CoCrFeMnNi HEAs at room temperature. Compared to CoCrFeNi HEA, the elongation of CoCrFeMnNi HEA was 14% lower, but the yield strength and ultimate tensile strength were increased by 17% and 6%, respectively. The direct real-time evolution of structural defects during uniaxial straining was acquired via in situ neutron-diffraction measurements. The dominant microstructures underlying plastic deformation mechanisms at each deformation stage in as-cast CoCrFeNi and CoCrFeMnNi HEAs were revealed using the Convolutional Multiple Whole Profile (CMWP) software for peak-profile fitting. The possible mechanisms are reported. Keywords: high-entropy alloy; tensile property; in situ neutron-diffraction; deformation twins 1. Introduction High-entropy alloys (HEAs), an intriguing new class of equiatomic solid-solution alloys, have attracted interest for their promising engineering applications as structural ma- terials [1–4]. Comprehensive studies have been devoted to achieving excellent combinations of high strength and good ductility and determining the microstructural-evolution-driven deformation responses in single-phase, face-centered-cubic (FCC) HEAs under uniaxial loading [5–14]. The desired mechanical properties of a good strength–ductility balance in the HEAs may be tailored via tuning their elemental compositions [15,16]. The mechanical properties of FCC materials are strongly microstructure-sensitive in terms of the dislocation activities, whereby the interactions of dislocations with other defects and the dislocations strongly influence the accumulation of tensile damage under monotonic loading. Therefore, it is important to explore the real-time development of structural defects governing the deformation behavior during strain accumulation under tensile loading, which can be achieved via in situ neutron-diffraction measurements. The most popular FCC HEA, the Cantor alloy CoCrFeMnNi, not only exhibits good mechanical performance but also high fatigue resistance [5,10,11,17–20]. The as-cast mi- crostructure of the HEAs enhances fatigue resistance [21,22]. Specifically, the as-cast Crystals 2022, 12, 157. https://doi.org/10.3390/cryst12020157 https://www.mdpi.com/journal/crystals