Citation: Pešiˇ cka, J.; Kratochvíl, P.; Král, R.; Veselý, J.; Jaˇ ca, E.; Preisler, D.; Daniš, S.; Minárik, P.; ˇ Camek, L. Structure of Complex Concentrated Alloys Derived from Iron Aluminide Fe 3 Al. Materials 2023, 16, 5388. https://doi.org/10.3390/ma16155388 Academic Editor: In-Chul Choi Received: 15 June 2023 Revised: 24 July 2023 Accepted: 26 July 2023 Published: 31 July 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/). materials Article Structure of Complex Concentrated Alloys Derived from Iron Aluminide Fe 3 Al Josef Pešiˇ cka 1, * , Petr Kratochvíl 1 , Robert Král 1 , Jozef Veselý 1 , Eliška Jaˇ ca 1 , Dalibor Preisler 1 , Stanislav Daniš 2 , Peter Minárik 1 and Libor ˇ Camek 3 1 Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic; pekrat@met.mff.cuni.cz (P.K.) 2 Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic 3 Department of Foundry Engineering, Brno University of Technology, Technická 2896/2, 61669 Brno, Czech Republic * Correspondence: pesicka@met.mff.cuni.cz Abstract: The phase structure and composition of a series of four alloys based on Fe 3 Al was in- vestigated by means of scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The materials were composed of Fe and Al with a fixed ratio of 3:1 alloyed with V, Cr and Ni at 8, 12, 15 and 20 at. % each (composition formula: Fe 3(100-3x)/4 Al (100-3x)/4 V x Cr x Ni x ). For 8% alloying, the material is single-phase D0 3 . Furthermore, 12 and 15% alloying results in bcc–B2 phase separation on two length scales. Moreover, 20% alloying gives rise to the FeNiCrV σ phase supplemented by B2. These findings are discussed with respect to the results obtained via Calphad modeling using the TCHEA5 database and can serve in further improvement. Keywords: entropy alloys; microstructure; Calphad 1. Introduction For years, iron-aluminide-based alloys have been known as materials for the develop- ment of new structural materials with improved performance with wide applications in industry. They have excellent resistance to oxidation and sulfidation [1–3]. Their density is about two-thirds that of the steel density. The balance of the strength and plasticity of these materials is relatively good [4–10]. The input raw materials are relatively cheap due to their abundance in the Earth’s crust. Therefore, one may ask how far can these preferential properties (based on the B2 and DO 3 structures) be maintained or even improved via additional alloying. Similarly, the main drawbacks of these alloys (bad workability at room temperature and low high-temperature (HT) strength) have eventually been improved. The structural and mechanical properties of complex concentrated alloys (CCAs) are a widely studied subject [11–27]. Various single additives—Ti (0.5 to 4 at. %), Cr (0.5 to 8 at. %), Mo (0.5 to 4 at. %), and V (0.5 to 8 at. %)—effectively improved the yield stress of Fe 3 Al at 1073 K (800 ◦ C) [11]. The effect of higher concentrations of Cr and V (up to 25 at. %) on the structural and high-temperature mechanical properties was also studied [12]. A detailed study of the ordering in highly alloyed (Cr, V) iron aluminide FeAl using the ALCHEMI method [27] showed that Cr atoms preferentially occupy the positions of Fe atoms and V those of Al. With increasing concentrations of Cr and V, the B2 order decreases, leading to full disorder (A2 structure) in the equiatomic FeAlCrV. Adding the fifth element gets us to the realm of high-entropy alloys (HEAs). HEAs with body-centered cubic (bcc) structures possess excellent strength, but they often suf- fer from limited plasticity at room temperatures. Nevertheless, there are some promis- ing results: Wu et al. [15] studied structure development and mechanical properties of TiZrNbMo x V y alloy dependent on the content of Mo (x = 0–2) and V (y = 1 and 0.3). It is Materials 2023, 16, 5388. https://doi.org/10.3390/ma16155388 https://www.mdpi.com/journal/materials