Vol. 27 No. 3 2025 3 EDITORIAL COUNCIL EDITORIAL BOARD EDITOR-IN-CHIEF: Anatoliy A. Bataev, D.Sc. (Engineering), Professor, Rector, Novosibirsk State Technical University, Novosibirsk, Russian Federation DEPUTIES EDITOR-IN-CHIEF: Vladimir V. Ivancivsky, D.Sc. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Vadim Y. Skeeba, Ph.D. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Editor of the English translation: Elena A. Lozhkina, Ph.D. (Engineering), Department of Material Science in Mechanical Engineering, Novosibirsk State Technical University, Novosibirsk, Russian Federation The journal is issued since 1999 Publication frequency – 4 numbers a year Data on the journal are published in «Ulrich's Periodical Directory» Journal “Obrabotka Metallov” (“Metal Working and Material Science”) has been Indexed in Clarivate Analytics Services. Novosibirsk State Technical University, Prospekt K. Marksa, 20, Novosibirsk, 630073, Russia Tel.: +7 (383) 346-17-75 http://journals.nstu.ru/obrabotka_metallov E-mail: metal_working@mail.ru; metal_working@corp.nstu.ru Journal “Obrabotka Metallov – Metal Working and Material Science” is indexed in the world's largest abstracting bibliographic and scientometric databases Web of Science and Scopus. Journal “Obrabotka Metallov” (“Metal Working & Material Science”) has entered into an electronic licensing relationship with EBSCO Publishing, the world's leading aggregator of full text journals, magazines and eBooks. The full text of JOURNAL can be found in the EBSCOhost™ databases.
OBRABOTKAMETALLOV Vol. 27 No. 3 2025 4 EDITORIAL COUNCIL EDITORIAL COUNCIL CHAIRMAN: Nikolai V. Pustovoy, D.Sc. (Engineering), Professor, President, Novosibirsk State Technical University, Novosibirsk, Russian Federation MEMBERS: The Federative Republic of Brazil: Alberto Moreira Jorge Junior, Dr.-Ing., Full Professor; Federal University of São Carlos, São Carlos The Federal Republic of Germany: Moniko Greif, Dr.-Ing., Professor, Hochschule RheinMain University of Applied Sciences, Russelsheim Florian Nürnberger, Dr.-Ing., Chief Engineer and Head of the Department “Technology of Materials”, Leibniz Universität Hannover, Garbsen; Thomas Hassel, Dr.-Ing., Head of Underwater Technology Center Hanover, Leibniz Universität Hannover, Garbsen The Spain: Andrey L. Chuvilin, Ph.D. (Physics and Mathematics), Ikerbasque Research Professor, Head of Electron Microscopy Laboratory “CIC nanoGUNE”, San Sebastian The Republic of Belarus: Fyodor I. Panteleenko, D.Sc. (Engineering), Professor, First Vice-Rector, Corresponding Member of National Academy of Sciences of Belarus, Belarusian National Technical University, Minsk The Russian Federation: Vladimir G. Atapin, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Victor P. Balkov, Deputy general director, Research and Development Tooling Institute “VNIIINSTRUMENT”, Moscow; Vladimir A. Bataev, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Vladimir G. Burov, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Aleksandr N. Korotkov, D.Sc. (Engineering), Professor, Kuzbass State Technical University, Kemerovo; Dmitry V. Lobanov, D.Sc. (Engineering), Associate Professor, I.N. Ulianov Chuvash State University, Cheboksary; Aleksey V. Makarov, D.Sc. (Engineering), Corresponding Member of RAS, Head of division, Head of laboratory (Laboratory of Mechanical Properties) M.N. Miheev Institute of Metal Physics, Russian Academy of Sciences (Ural Branch), Yekaterinburg; Aleksandr G. Ovcharenko, D.Sc. (Engineering), Professor, Biysk Technological Institute, Biysk; Yuriy N. Saraev, D.Sc. (Engineering), Professor, V.P. Larionov Institute of the Physical-Technical Problems of the North of the Siberian Branch of the RAS, Yakutsk; Alexander S. Yanyushkin, D.Sc. (Engineering), Professor, I.N. Ulianov Chuvash State University, Cheboksary
Vol. 27 No. 3 2025 5 CONTENTS OBRABOTKAMETALLOV TECHNOLOGY Kondratiev V.V., Gozbenko V.E., Kononenko R.V., Konstantinova M.V., Guseva E.A. Determination of the main parameters of resistance spot welding of Al-5 Mg aluminum alloy..................................................................................... 6 Gvindjiliya V.E., Fominov E.V., Marchenko A.A., Lavrenova T.V., Debeeva S.A. Infl uence of cutting speed on pulse changes in the temperature of the front cutter surface during turning of heat-resistant steel 0.17 C-Cr-Ni-0.6 Mo-V................................................................................................................................................................ 23 Karelin R.D., Komarov V.S., Cherkasov V.V., OsokinA.A., Sergienko K.V., Yusupov V.S., Andreev V.A. Production of rods and sheets from TiNiHf alloy with high-temperature shape memory eff ect by longitudinal rolling and rotary forging methods.................................................................................................................................................................... 37 EQUIPMENT. INSTRUMENTS Zakovorotny V.L., Gvindjiliya V.E., Kislov K.V. Information properties of vibroacoustic emission in diagnostic systems for cutting tool wear................................................................................................................................................ 50 Zhukov A.S., Ardashev D.V., Batuev V.V., Kulygin V.L., Schuleshko E.I. Modal analysis of various grinding wheel types for the evaluation of their integral elastic parameters...................................................................................... 71 Nishandar S.V., Pise A.T., Bagade P.M. Numerical and experimental investigation of heat transfer augmentation in roughened pipes................................................................................................................................................................ 87 Nosenko V.A., Rivas Perez D.E., Alexandrov A.A., Sarazov A.V. The eff ect of the grinding method on the grain shape coeffi cient of black silicon carbide....................................................................................................................................... 108 MATERIAL SCIENCE Karlina Yu.I., Konyukhov V.Yu., Oparina T.A. Investigation of the process of surface decarburization of steel 20 after cementation and heat treatment.................................................................................................................................. 122 Kovalevskaya Z.G., Liu Y. Eff ect of heat treatment on the structure and properties of high-entropy alloy AlCoCrFeNiNb0.25............................................................................................................................................................. 137 Sirota V.V., Prokhorenkov D.S., Churikov A.S., Podgorny D.S., Alfi mova N.I., Konnov A.V. Corrosion properties of coatings produced from self-fl uxing powders by the detonation spraying method............................................................ 151 Filippov A.V., Shamarin N.N., Tarasov S.Yu., Semenchyuk N.A. The infl uence of structural state on the mechanical and tribological properties of Cu-Al-Si-Mn bronze............................................................................................................. 166 Waheed F., Qayoom A., Shirazi M.F. Fabrication, characterization and performance evaluation of zinc oxide doped nanographite material as a humidity sensor......................................................................................................................... 183 Dolgova S.V., Malikov A.G., Golyshev A.A., Nikulina A.A. Features of the structure of gradient layers «steel - Inconel - steel», obtained by laser direct metal deposition.................................................................................................. 205 Burkov A.A., Dvornik M.A., Kulik M.A., Bytsura A.Yu. The infl uence of tungsten carbide particle size on the characteristics of metalloceramic WC/Fe-Ni-Al coatings.................................................................................................... 221 Patil S., Chinchanikar S. Investigation on the mechanical properties of stir-cast Al7075-T6-based nanocomposites with microstructural and fractographic surface analysis...................................................................................................... 236 EDITORIALMATERIALS 252 FOUNDERS MATERIALS 263 CONTENTS
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Effect of heat treatment on the structure and properties of high-entropy alloy AlCoCrFeNiNb0.25 Zhanna Kovalevskaya а, *, Yuanxun Liu b National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk, 634050, Russian Federation a https://orcid.org/0000-0003-3040-8851, kovalevskaya@tpu.ru; b https://orcid.org/0009-0002-8501-2643, yuansyun1@tpu.ru Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2025 vol. 27 no. 3 pp. 137–150 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2025-27.3-137-150 ART I CLE I NFO Article history: Received: 10 April 2025 Revised: 24 April 2025 Accepted: 13 June 2025 Available online: 15 September 2025 Keywords: High-Entropy Alloy AlCoCrFeNiNb0.25 Heat Treatment Microstructure Microhardness Compression Tests ABSTRACT Introduction. Currently, one of the most studied high-entropy alloys (HEAs) is the CoCrFeNi system with the addition of a fifth component. An example of such an alloy is AlCoCrFeNi alloyed with additional elements. Nb alloying promotes the formation of a solid solution and a secondary Laves phase in the alloy, and leads to the formation of eutectics between these phases. The optimal combination of mechanical properties achieved in the hypoeutectic alloy AlCoCrFeNiNb0.25 was the basis for the choice of this alloy for further studies under heat treatment conditions. Purpose of the work. To investigate the effect of heat treatment, including heating to temperatures of 900°C, 1,000°C and 1,100°C with subsequent cooling in air, on the structure and properties of AlCoCrFeNiNb0.25. The methods of investigation were optical metallography, X-ray diffraction analysis, microhardness measurement, and compression tests. Results and Discussion. AlCoCrFeNiNb0.25 alloy retains the solid solution structure based on the BCC phase not only in the cast state, but also after heat treatment. Irrespective of heat treatment parameters, the alloy retains the hypoeutectic structure consisting of solid solution dendrites and eutectic with the Laves phase in the interdendritic space. Heat treatment leads to changes in the phase composition of the alloy and refinement of structural components. When heated to 900°C, along with the existing solid solution and Laves phase, σ-phase is released in the structure, which increases the microhardness of the alloy, but does not provide improvement of strength properties due to its low plasticity. The strength properties of the alloy are significantly improved by heat treatment with heating up to 1,000°C and 1,100°C. Heating up to 1,100°C is accompanied by an increase in residual strain. The main reasons for this effect may be transformations occurring both in the solid solution of the BCC phase (dissolution of the B2 phase, rearrangement of the substructure, increase in the lattice parameter) and in the eutectic (increase in the proportion of the Laves phase, refinement of eutectic cells). For citation: Kovalevskaya Z.G., Liu Y. Effect of heat treatment on the structure and properties of high-entropy alloy AlCoCrFeNiNb0.25. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science, 2025, vol. 27, no. 3, pp. 137–150. DOI: 10.17212/1994-6309-2025-27.3-137-150. (In Russian). ______ * Corresponding author Kovalevskaya Zhanna G., D.Sc. (Engineering), Professor National Research Tomsk Polytechnic University, 30 Lenin ave., 634050, Tomsk, Russian Federation Tel.: +7 3822 706-351, e-mail: kovalevskaya@tpu.ru Introduction For over two decades, the global community of materials scientists has been developing and investigating a novel class of metal alloys, known as high-entropy alloys (HEAs) [1–4]. Unlike conventional metal alloys with a single principal component, HEAs are composed of multiple principal components in equiatomic or near-equiatomic concentrations [3]. Due to the high mixing entropy, HEAs typically exhibit disordered solid solutions. This phase configuration endows them with enhanced strengthening capabilities and favorable ductility characteristics, making HEAs promising candidates for structural materials [4–6]. One of the most extensively studied systems is the CoCrFeNi alloy, which is often modified by the addition of a fifth element, such as Cu, Mo, Mn, or Al [7–11]. For instance, the thoroughly studied AlCoCrFeNi alloy demonstrates excellent synergy of its constituents, as well as the ability to control its phase composition and structure by heat treatment. Consequently, the resulting alloy achieves an advantageous combination of strength and ductility properties [12–19]. In the search for optimal HEA compositions for the manufacture of machine components, contemporary researchers are advancing in two primary directions: either reducing/increasing the content of one of the
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.
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Results and Discussion Fig. 1, a shows X-ray diffraction (XRD) patterns of the AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment. In the as-cast state, the alloy consists of a primary BCC phase, which represents a disordered solid solution of all components present in the system. The disorder of the solid solution in the primary phase is attributed to the redistribution of components with various atomic radii in the BCC lattice and their segregation into two phases with different parameters. The solid solution disorder manifests in the XRD pattern as splitting of the main BCC peak into two distinct peaks (Fig. 1, b). Furthermore, the XRD pattern exhibits peaks corresponding to the crystal lattice of the Nb-rich Laves phase, which can be identified as (CoCr)Nb with a hexagonal structure, as well as the [001] reflection peak of the B2 phase representing AlNi with a BCC lattice [29, 32]. b Fig. 1. XRD patterns of AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment (a) with enlarged image in the 2θ range of 41–48° (b) а
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 The existence of the Laves phase is characteristic of AlFeNiCoCrNb alloys where the Nb concentration corresponds to a molar ratio of 0.25 or higher. In this case, Nb not only dissolves in the primary BCC phase but also promotes the formation of the secondary – Laves – phase, which forms a eutectic mixture with the BCC phase [29]. According to the findings of [32], during cooling of the AlFeNiCoCr alloy, the primary crystallized BCC phase may incoherently separate into a mixture of an unordered BCC phase enriched in Cr-Fe and an ordered B2 phase enriched in Al-Ni, which is evidenced by the presence of a peak corresponding to the B2 phase in the XRD pattern. Subsequent heat treatment of the alloy leads to the following changes in the XRD patterns. As the heating temperature increases, the intensity of the B2-phase peak decreases, while the intensity of the peak corresponding to the Laves phase slightly increases. In addition, changes occur in the lattice of the primary BCC phase. Fig. 1, b presents an enlarged view of the (110) peak of the BCC phase. It is evident that, with an increase in the heating temperature, the peak shifts towards smaller angles, indicating an increase in the lattice parameter of the BCC solid solution, which suggests changes in the composition of the solid solution. After heat treatment at 900 °C, peaks of a new phase emerge, which is identified as the tetragonal σ phase composed of Cr and Fe. The σ phase is absent at higher heating temperatures. The phenomenon of the σ-phase precipitation and dissolution in the BCC phase within a similar temperature range was also observed previously [29]. The XRD analysis of the AlCoCrFeNiNb0.25 alloy throughout the entire heating temperature range shows that the primary phase remains an unordered BCC solid solution. However, upon heating of the AlFeNiCoCr alloy without Nb, part of the material transforms into an FCC solid solution [29]. Thus, the addition of Nb helps stabilize the BCC phase and maintain a predominantly single-phase structure in the high-entropy alloy. Fig. 2 depicts the microstructure of the AlCoCrFeNiNb0.25 alloy both in the as-cast state and after heat treatment. The alloy consistently exhibits a dendritic morphology with hypoeutectic characteristics. The microstructure comprises primary dendritic and interdendritic eutectic regions. Dendritic regions consist of a BCC phase, while the eutectic structure is a mixture of the BCC and Laves phases. In the as-cast state, dendritic segregation results in compositional heterogeneity: dendritic cores (BCC phase) are enriched in Ni and Al, whereas the dendritic periphery and eutectic regions are enriched in Cr and Fe. Nb partially dissolves in the BCC phase, but most of it enters the composition of the Laves phase [29]. Results of the elemental analysis in different zones of the as-cast alloy are detailed in Table 2. The same pattern of formation of the dendritic structure in the alloy was reported in [15, 29]. The dendritic structure that exhibits a dark contrast after etching is surrounded by lighter layers of the Laves phase, which represents a secondary phase. The secondary Laves phase forms along the solid solution boundaries during dendritic growth and is attributed to the reduced solubility of niobium in the solid solution of the principal components during cooling. The enrichment of the peripheral regions of dendrites with niobium and chromium creates conditions for the formation of the secondary Laves phase based on these components. Fig. 2, b shows a defective eutectic structure. Grains of the Laves phase are divided into fragments with random crystallographic orientations. Since the eutectic, which includes the Laves phase, forms in the interdendritic space, it is structurally impossible to distinguish between the secondary Laves phase and the Laves phase present in the eutectic. Heat treatment at 900–1,100 °C does not alter the dendritic structure of the alloy (Fig. 2). The dendrite width along the secondary axes ranges from 11 to 15 μm. Increasing the heating temperature from 900 °C to 1,100 °C leads to changes in the structure of the eutectic, this can be observed in high-magnification metallographic images (Figs. 2, f, h). At the heating temperature of 900 °C, no noticeable changes in the eutectic structure occur. However, at 1,000 °C, Laves phase fragments begin to align, which is typical of the eutectic, and the formed lines alternate with the solid solution. At 1,100 °C, eutectic grains are well defined in the interdendritic space (Fig. 2, h).The XRD analysis confirms the eutectic transformation: the peak intensity of the Laves phase increases with temperature. This may arise from either the coalescence of
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Fig. 2. Microstructure of AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment: T30 (a, b); T900 (c, d); T1000 (e, f); T1100 (g, h) a b c d e f g h
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Ta b l e 2 Chemical compositions (at.%) in the AlCoCrFeNi alloy in the as-cast state Т30 Al Cr Fe Co Ni Nb Dendrite core 17.76 16.46 18.61 20.30 24.56 2.40 Dendrite periphery 14.85 21.61 20.75 20.14 21.06 1.59 Solid solution in eutectics 12.09 23.86 22.05 19.78 19.28 2.96 Laves phase 3.80 18.36 21.43 23.66 11.56 21.17 secondary-phase grains or from its increased fraction in interdendritic zones via solid solution precipitation (Fig. 1, a). The XRD analysis confirms that an ordered σ phase forms in the alloy at 900°C. In [32], it was reported that the σ phase precipitated from the disordered solid solution where it was enriched in Cr and Fe in the form of dispersed particles. Our SEM studies reveal that σ-phase particles are distributed throughout the dendrite volume (Fig. 3, b). After heat treatment at 1,000 °C, σ-phase particles are still observed, but in smaller amounts (Fig. 3, c). At 1,100 °C, they completely disappear (Fig. 3, d). The SEM analysis reveals that heat treatment significantly alters the structure of the solid solution within dendrites (Fig. 3). During alloy solidification, the cooling process leads to the spinodal decomposition of the disordered solid solution into a Fe- and Cr-enriched disordered solid solution and an ordered B2 phase enriched in Ni and Al [32]. This decomposition results in the formation of the so-called basket weave a b c d Fig. 3. Microstructure of AlFeNiCoCrNb0.25 alloy in the as-cast state and after heat treatment, obtained using SEM: T30 (a); T900 (b); T1000 (c); T1100 (d)
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 structure, which was described in detail in [15, 32, 37]. In the investigated as-cast alloy, a characteristic basket weave or banded structure forms in the peripheral regions of dendrites, which is attributed to the spinodal decomposition of the solid solution. No banded structure is observed in the center of dendrites (Fig. 3, a). When the alloy is heated to 900 °C and subsequently cooled, structural heterogeneity in dendrites and the basket weave structure become more pronounced (Fig. 3, b). Furthermore, as mentioned above, σ-phase particles precipitate from the solid solution. According to X-ray structural analysis, after heat treatment, the proportion of the ordered B2 phase decreases, suggesting that the observed contrast within the basket weave structure is due not to the spinodal decomposition of the solid solution into two phases but rather to the heterogeneous segregation of atomic components within the disordered solid solution, as described in [15]. At 1,000 °C, the basket weave structure increases in size and occupies the entire volume of dendrites (Fig. 3, c). Further heating to 1,100 °C causes coarsening (Fig. 3, d). The average microhardness and the microhardness of the structural components of the alloy are presented in Table 3. In all heat treatment conditions, the interdendritic regions demonstrate significantly higher microhardness compared to the dendritic cores. Ta b l e 3 Microhardness of AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment Measurement area Т30 HV Т900 HV Т1000 HV Т1100 HV Dendrites 614 ± 44 582 ± 37 489 ± 53 520 ± 35 Eutectic 640 ± 47 902 ± 66 620 ± 45 636 ± 46 Average value 625 ± 28 730 ± 47 545 ± 52 572 ± 56 The maximum microhardness in dendrites of the as-cast alloy is attributed to the unique structure of the solid solution of the alloy components, which forms during crystallization and cooling. The spinodal decomposition of the disordered solid solution coupled with the precipitation of the ordered B2 phase strengthens the alloy. However, heating of the alloy during heat treatment leads to a partial loss of the order characteristic of the B2 phase, resulting in a decrease in the microhardness of dendrites. Nevertheless, the precipitation of σ-phase particles upon heating to 900 °C allows the microhardness to remain at a high level. When the heating temperature increases to 1,000 °C, the strengthening effect of the σ-phase particles disappears. During heat treatment at 1,100 °C, coalescence of the basket weave structure occurs, leading to the formation of more distinct phase boundaries, possibly due to the increased heterogeneous segregation of atomic components within the solid solution. This, in turn, slightly increases the microhardness in the dendritic zones of the alloy. In the interdendritic space of the as-cast alloy, the microhardness of the eutectic is only slightly higher than that of the solid solution in dendrites. This means that strengthening due to spinodal decomposition is comparable in magnitude to that due to the Laves phase (a solid intermetallic compound) in the eutectic. During heating at 900 °C, as mentioned above, σ-phase particles precipitate from the solid solution and concentrate in the Cr-rich interdendritic space. This significantly increases the microhardness of the eutectic due to strengthening of the solid solution with σ-phase particles in its composition. Dispersed σ-phase particles precipitate in the solid solution of all structural components, significantly enhancing the microhardness in dendrites. Heating to 1,000 °C and 1,100 °C leads to the dissolution of σ-phase particles in the primary phase [11], which contributes to a decrease in the microhardness in the interdendritic space to the values of the initial structure. This is due to the removal of the effect of strengthening due to σ-phase particles. When evaluating the integral microhardness of all structural components of the alloy, the overall trend in the variation of microhardness with heating temperature is retained (Table 3).
OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 27 No. 3 2025 Fig. 4 illustrates compressive stress-strain responses of the as-cast and heat-treated alloys. The offset yield strength, compressive strength, and residual strain are presented in Table 4. The as-cast alloy exhibits good strength and plasticity characteristics. Structural transformations in the alloy during heating to 900 °C have little effect on the strength properties of the material but significantly reduce its plasticity. This reduction is attributed to the precipitation of the brittle σ phase in the structure. The authors of [32] also pointed out the effect of decreased plasticity within a similar temperature range of heat treatment. During heat treatment at 1,000 °C and 1,100 °C, a significant increase is observed in the strength characteristics of the alloy. At 1,100 °C, the residual strain also increases. Based on the results of XRD analysis and optical microscopy, it can be suggested that the main reason for this effect is transformations occurring both in the solid solution of the BCC phase (B2-phase dissolution, substructure rearrangement, and an increase in the lattice parameter) and in the eutectic (increase in the proportion of the Laves phase and refinement of eutectic cells). The simultaneous increase in plasticity is likely due to the relief of internal stresses, a reduction in the number of crystalline defects, and coalescence of structural components in dendrites and the eutectic. However, a more precise analysis of this unique effect on the properties of the AlCoCrFeNiNb0.25 alloy requires further research. Fig. 4. Compressive stress-strain curves of the AlFeNiCoCrNb0.25 alloy in the as-cast state and after heat treatment Ta b l e 4 Offset yield strength, compressive strength and residual strain of AlCoCrFeNiNb0.25 alloy in the as-cast state and after heat treatment σ0.2 (MPa) σu(MPa) ɛ (%) Т30 1356 1962 7.7 Т900 1605 1894 2.8 Т1000 1502 2438 9.8 Т1100 1369 2494 16.4 Conclusions Doping of the AlCoCrFeNi high entropy alloy with niobium in a molar ratio of 0.25 led to the stabilization of the solid solution based on the body-centered cubic (BCC) phase both in the as-cast state and after heat treatment involving heating to 900 °C, 1,000 °C, and 1,100 °C followed by air cooling. The resulting structure of the alloy, regardless of the heat treatment modes, consisted of dendrites of the solid solution and a eutectic with the Laves phase in the interdendritic space. Heat treatment altered the phase composition of the alloy and improved its structural components. Upon heating to 900 °C, alongside the already formed solid solution and Laves phase, the σ phase precipitated in the structure, which increased the microhardness of the alloy. However, this did not improve the strength properties due to the low plastic characteristics of the σ phase. The strength characteristics of the alloy significantly increased during heat treatment at 1,000 °C and 1,100 °C. At 1,100 °C, the residual strain also rose. The main reasons for this effect may include
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