Citation: Shahmir, H.; Saeedpour, P.; Mehranpour, M.S.; Shams, S.A.A.; Lee, C.S. Hetero-Deformation Induced Hardening in a CoCrFeNiMn High-Entropy Alloy. Crystals 2023, 13, 844. https:// doi.org/10.3390/cryst13050844 Academic Editor: Chongde Cao Received: 24 April 2023 Revised: 13 May 2023 Accepted: 14 May 2023 Published: 19 May 2023 Copyright: © 2023 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 Hetero-Deformation Induced Hardening in a CoCrFeNiMn High-Entropy Alloy Hamed Shahmir 1, *, Parham Saeedpour 1 , Mohammad Sajad Mehranpour 2 , Seyed Amir Arsalan Shams 3 and Chong Soo Lee 3 1 Department of Materials Engineering, Tarbiat Modares University, Tehran 14115-143, Iran 2 School of Metallurgy and Materials, College of Engineering, University of Tehran, Tehran 11155-4563, Iran 3 Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 37673, Republic of Korea * Correspondence: shahmir@modares.ac.ir; Tel.: +98-2182883305 Abstract: One of the most important issues in materials science is to overcome the strength–ductility trade-off in engineering alloys. The formation of heterogeneous and complex microstructures is a useful approach to achieving this purpose. In this investigation, a CoCrFeNiMn high-entropy alloy was processed via cold rolling followed by post-deformation annealing over a temperature range of 650–750 ◦ C, which led to a wide range of grain sizes. Annealing at 650 ◦ C led to the formation of a heterogeneous structure containing recrystallized areas with ultrafine and fine grains and non- recrystallized areas with an average size of ~75 μm. The processed material showed strength–ductility synergy with very high strengths of over ~1 GPa and uniform elongations of over 12%. Different deformation mechanisms such as dislocation slip, deformation twinning and hetero-deformation- induced hardening were responsible for achieving this mechanical property. Increasing the annealing temperature up to 700 ◦ C facilitated the acquisition of bimodal grain size distributions of ~1.5 and ~6 μm, and the heterogeneous structure was eliminated via annealing at higher temperatures, which led to a significant decrease in strength. Keywords: hetero-deformation induced hardening; heterogeneous microstructure; bimodal grain size; microstructure engineering; high-entropy alloy 1. Introduction The CoCrFeNiMn high-entropy alloy (HEA), known as the Cantor alloy, with a face- centered cubic (fcc) single phase structure, has gained attention due to its high ductility, cryo- genic fracture toughness and corrosion resistance [1–5]. However, the strength–ductility trade-off of this HEA is an important and challenging dilemma, similar to other conven- tional alloys. The low yield stress and ultimate tensile strength of this alloy in fully annealed conditions, which are 300 and 530 MPa, respectively [6,7], have led to many attempts to improve the alloy’s strength via grain refinement using severe plastic deformation or thermomechanical treatment, together with precipitation hardening [8–16]. Thermomechanical treatment, including cold rolling, to activate strain hardening followed by post-deformation annealing to obtain a favorable microstructure is a pro- cess that is easy to develop for industrial applications. However, the degree of plastic deformation, ranging from small to heavy/severe plastic deformation, the processing temperature (cryo- to warm rolling), annealing temperature (500–1000 ◦ C) and time (a few to a hundred minutes) are control parameters for microstructure engineering, used to obtain desirable mechanical properties [17,18]. The alloy strength increased slightly after applying the thermomechanical procedure due to the formation of fine or ultrafine grains. The fabrication of nanograin structures using high-pressure torsion led to a significant increase in the strength (>1.5 GPa) at the expense of a dramatic decrease in the ductility (elongation of <4%) [19]. Post-deformation annealing at an appropriate temperature for Crystals 2023, 13, 844. https://doi.org/10.3390/cryst13050844 https://www.mdpi.com/journal/crystals