OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 components of the existing HEAs [6, 19–21] or introducing additional elements as alloying agents, such as Ti, Zr, Si, V, C, Nb, and others [22–27]. Several studies have demonstrated the effect of Nb doping on the structure and properties of AlCoCrFeNi and related HEA systems [28–31]. It is well established that Nb and the HEA components exhibit negative mixing enthalpies. Furthermore, Nb possesses the largest atomic size in the system. These characteristics of Nb contribute to the formation, on one hand, of a stable solid solution with enhanced interatomic bonds, and on the other hand, of secondary phases that are essential for alloy strengthening. For instance, the work [28] showed that Nb doping of the AlCoCrFeNi HEA resulted in the formation of a eutectic structure that included the ordered Laves phase (CoCr)Nb. This leads to alterations in the microstructure and properties of the alloy, where the compressive yield strength and hardness increase, while ductility decreases. An optimal combination of mechanical properties is achieved in the hypoeutectic AlCoCrFeNiNb0.25 alloy, which was chosen for this investigation. Various heat treatment methods, including annealing and quenching, are employed to strengthen HEAs [20, 32–37]. In certain instances, heat treatment can enhance both strength and ductility of HEAs [5]. This unique effect, which is not typical for conventional alloys, necessitates thorough investigation and analysis. The purpose of this paper is to investigate the effect of heat treatment on the structure and properties of the AlCoCrFeNiNb0.25 high-entropy alloy (HEA). The heat treatment process involves heating to 900 °C, 1,000 °C, and 1,100 °C followed by air cooling. Methods The AlCoCrFeNiNb0.25 alloywith a near-equiatomic compositionwas produced by the arcmeltingmethod in a water-cooled copper crucible under an argon atmosphere. The alloy, whose chemical composition is detailed in Table 1, was made of components with more than 99.5 wt. % purity. To ensure the homogeneity of the chemical composition, the ingot was remelted at least five times. The dimensions of the resulting ingot were 70×35×12 mm. Prior to heat treatment, ingots were cut into fragments measuring 35×12×6 mm. After heat treatment, the central portions of the fragments were further cut into parallelepipeds measuring 10×4×4 mm. The cut samples were polished and used for compression testing. The remaining portions of the fragments were utilized for X-ray diffraction analysis, microstructural evaluation, and microhardness measurements. Ta b l e 1 Chemical composition of AlCoCrFeNiNb0.25 (at.% and wt.%) Element Al Co Cr Ni Fe Nb at.% 19.1 19.1 19.1 19.1 19.1 4.5 wt.% 9.8 21.5 18.9 21.4 20.4 8.0 Samples of the AlCoCrFeNiNb0.25 alloy were heat-treated as follows: heating to 900 °C, 1,000 °C, and 1,100 °C, holding for 1 h, and air cooling. For simplicity, the heat-treated samples were designated as T900, T1000, and T1100, respectively while the as-cast sample was designated T30 Thin sections were prepared from the samples, and their microstructure was analyzed using an Axio Observer A1m optical microscope and a Quanta 200 scanning electron microscope (SEM) equipped with an EDAX energy-dispersive X-ray spectroscopy (EDS) unit. The phase composition was determined using an XRD-6000 diffractometer with Cu-Kα radiation. The scanning angles ranged from 20° to 80° with a step of 0.02°. Microhardness measurements were performed with a PMT-3 hardness tester with a load of 100 g. Compression tests were performed using a universal testing machine (MTS SANS CMT5105) at a compression rate of 5×10−3 mm/s. At least three samples per treatment condition were measured.
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