Introduction

High-entropy alloys (HEAs) are a new generation of alloys that have been developed since 2004 [1; 2]. Despite intensive research over the past 20 years in the field of high-entropy materials, these alloys have not yet found widespread use in modern industry, although they continue to gain popularity in scientific studies each year [3; 4] due to their high physical, mechanical, and operational properties [5; 6]. High-entropy alloys are resistant to oxidation at high temperatures, which potentially broadens their technological applications, including replacing nickel-based alloys in turbine systems [7; 8]. In their review, O.N. Senkov et al. [9] explore two groups of HEAs for high-temperature applications. The first group includes HEAs based on 3d transition metals such as Co, Cr, Cu, Fe, Mn, Ni, and Ti. These alloys have a yield strength of over 1000 MPa at 600 °C. However, according to the authors, none of the HEAs presented possess properties superior to modern nickel-based heat-resistant alloys. The heat resistance of HEAs quickly decreases at temperatures exceeding ≈800 °C, similar to that of nickel-based heat-resistant alloys. Additionally, their ability to withstand high temperatures is limited by their melting points, which are only slightly different from those of commercial nickel-based heat-resistant alloys.

Refractory high-entropy alloys (RHEAs) represent the second group of HEAs, developed by O.N. Senkov and co-authors [10] for high-temperature applications. Since 2010, this category of alloys has attracted the interest of specialists due to their ability to maintain high static strength up to 1600 °C and potentially higher. The first RHEA was created using five refractory components (Mo, Nb, Ta, V, and W), but later alloys were made from elements of Group IV (Ti, Zr, and Hf), Group V (V, Nb, and Ta), and Group VI (Cr, Mo, and W) [10].

Refractory high-entropy alloys show promise for use in structures and products operating at high temperatures (above 1000 °C) and are considered as replacements for nickel-based heat-resistant alloys. In their recent review, W. Xiong et al. [11] demonstrated that HEAs exhibit excellent mechanical properties over a wide range of temperatures and increased resistance to high-temperature oxidation. Currently, there is a significant increase in research on RHEAs, which is also confirmed by the growing number of reviews on RHEAs developed for applications in nuclear engineering [12; 13].

Traditionally, gas-phase, liquid-phase, and solid-phase methods are used to produce HEAs [3]. Powder metallurgy methods (solid-phase methods) are considered the most rational for obtaining RHEAs for high-temperature applications [14]. Fig. 1 illustrates the process for the production of HEAs, enabling the creation of high-quality billets with geometries that meet consumer requirements. However, the analysis of recent reviews [11 – 13] in the field of HEAs for high-temperature applications indicates a lack of information on solid-phase powder metallurgy processes for HEAs since 2020.

Thus, it becomes relevant to assess the latest developments and trends in the field of HEAs for high-temperature applications. Therefore, this review examines the criteria for selecting chemical elements for the solid-phase powder metallurgy process, as well as consolidation methods, density, phase composition, mechanical properties, and future trends regarding HEAs.

Materials and methods

Using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) criteria [15], both Russian and international databases were analyzed: elibrary.ru, mdpi.com, Springer.com and sciencedirect.com.

The selected studies met the following criteria:

– mechanical properties at elevated temperatures;

– oxidation resistance;

– thermal stability.

Results and discussion

After screening for the specified criteria, thirty-nine studies related to powder metallurgy of high-entropy alloys (HEAs) for high-temperature applications were selected. The Table provides data on the studies that contain results meeting all of the aforementioned criteria.

Chemical composition

Recent studies have examined innovative oxide-dispersion strengthened (ODS) refractory high-entropy alloys (RHEAs). For example, in the study [32], 15 % Al2O3 was used to produce lightweight refractory alloys based on TaNbVTi. Zong L. et al. [33] used nanoscale ceramic particles of m-ZrO2 to strengthen the refractory high-entropy alloy NbMoTaW, and in the study [34], they applied similar reinforcement for the WMoNbTaV alloy. Similarly, nanoscale Y2O3 particles were used in the study [35]. A new NbTaTiV ODS RHEA containing 0.35 wt. % Al2O3 was investigated in the study [25].

Strengthening HEAs with nanoscale refractory oxides can only be achieved through powder metallurgy methods. The traditional chemical compositions of HEAs, presented in the Table, replicate their compositions obtained earlier using liquid-phase methods [2; 8; 10 – 12]. Therefore, the application of powder metallurgy methods expands the technological capabilities for producing HEAs with the widest range of chemical compositions [36 – 39].

Powder preparation

In the studies [40; 41], the approach of obtaining powder mixtures through simple mixing without additional milling was used. The most common method for producing powder is mechanical alloying in a planetary mill [42].

To expand the raw material base, in the study [43], a powder mixture was synthesized using a blend of titanium hydride and elemental powders. In the same study, Nb hydride powder and Ta hydride powder were used. The release of hydrogen during the decomposition of the hydrides helps to clean the surface of the metal powders from impurities.

For the agglomeration of fine powders, spray drying is applied. In the study [44], after spray drying, the HEA powder granules were processed in a plasma spheroidization unit (Tekna Nano-15). Induction thermal plasma (Fig. 2) was also used in the study [45] for spheroidizing WTaMoNbZr powder, which was originally irregularly shaped and obtained by grinding a hydrogenated ingot. The deoxidation during plasma processing contributed to refining the alloy.

In the study [46], pre-rolled plates with a known grain size were hydrogenated. The authors highlight the economic efficiency of the mechanical milling method and the clear correlation between the grain size of the plate and the resulting powders, which ranged from 6 to 102 µm.

Gas atomization (Fig. 3) is the primary method used for producing spherical powders. In the study [18], gas atomization was used to produce HEAs from 3d transition elements and refractory elements, while in the study [19], the same method was applied but exclusively for HEAs made from 3d transition elements. These powders have a homogeneous chemical composition and are suitable for various technological processes in powder metallurgy, as well as for additive manufacturing [47]. However, gas-atomized powders contain satellites, which limit their compactness. Therefore, for obtaining powders with a high degree of sphericity, the technology of centrifugal atomization of a rotating electrode is used.

In the study [48], both EIGA (Electrode Inert Gas Atomization, Fig. 4, а), and the PREP (Plasma Rotating Electrode Process, Fig. 4, b) were used to produce RHEA powders. The results demonstrated that the PREP method produced powders with high sphericity and no satellite particles, although the particle sizes were larger compared to those obtained with EIGA. The average particle sizes were 65.9 µm for PREP and 51.8 µm for EIGA.

In the study [26], self-propagating high-temperature synthesis (SHS) was used to obtain powders from mechanically activated powder, resulting in a change in the material’s phase composition. This approach expands the potential for obtaining new properties in known HEA chemical compositions.

A combination of magnesiothermy and SHS was applied in the study [49]. The authors used a powder mixture of WO3 , Nb2O5 , Ta2O3 and MoO3 in combination with pure magnesium powder for SHS. This approach contributes to the expansion of the raw material base in the production of RHEAs.

The expansion of synthesis methods allows for obtaining powders with various chemical compositions, morphologies, and sizes. This is crucial for the next technological cycle in powder metallurgy, namely consolidation (compaction) processes.

Consolidation process

The most widely used compaction method is spark plasma sintering (SPS). In the studies [50] and [51], the maximum temperature of 1900 °C was achieved under a pressure of 50 MPa. The maximum pressure for SPS, 80 MPa, was applied in the study [52]. A key limiting factor for the pressure is the use of graphite punches in SPS.

The main advantage of the SPS method is the controllable process speed, increased sample density, and the retention of metastable structures due to high cooling rates. However, SPS has limitations in producing complex-shaped and large-sized products.

Sintering by hot pressing (HP) is a widely used technology in powder metallurgy for producing products with minimal residual porosity. The main difference between HP and SPS is in the heating and cooling rates. Additionally, HP is preferable for manufacturing large parts in industry [17; 19].

Cold isostatic pressing (CIP) and pressureless sintering are common methods in powder metallurgy. In the study [53], the maximum sintering temperature using a mixture of H2 and Ar was 1400 °C. In the study [54], the same sintering atmosphere was used, but the maximum temperature reached 1450 °C. The data obtained on the sintering process can be adapted for high-throughput MIM (Metal Injection Moulding) technology [55; 56].

The method of hot extrusion is promising for producing long products with high mechanical properties. In the study [31], spherical powders in a stainless steel container were subjected to hot extrusion at a temperature of 1150 °C (extrusion ratio of 7:1). The production of long bars and wires by hot extrusion can be used both for making rod structures and for additive manufacturing processes, such as thermal spraying or wire arc additive manufacturing.

Among the reviewed studies on HEA powder metallurgy since 2020, no methods for producing billets by metal injection molding or hot isostatic pressing (HIP) were presented [57]. However, these methods enable the manufacturing of complex-shaped samples with high density and are promising for the production of parts from HEA powders. Thus, in the coming years, these methods are expected to be adapted for producing products for high-temperature applications.

Phase composition

In the reviewed studies (see Table), X-ray diffraction analysis of HEAs based on 3d transition metals primarily revealed a single-phase FCC solid solution, while for compositions based solely on refractory metals, a single-phase BCC solid solution was identified. However, for compositions containing both 3d transition metals and refractory metals, X-ray analysis detected the presence of two phases: FCC and BCC. Additionally, in some cases, the presence of carbide, oxide, sigma, and intermetallic phases was observed, which positively affect the high-temperature properties of the developed alloys.

Density

Density is a key factor for sintered samples, as it allows for assessing the effectiveness of the consolidation method.

Among the analyzed studies, the highest density was achieved for the RHEAs (W35Ta35Mo15Nb15)95Ni5 (14.55 g/cm3) [58] and equiatomic RHEA NbMoTaWRe (14.36 g/cm3) [49], due to the presence of W, Ta, Nb, Mo, and Re. The lowest density, 5.98 g/cm3, was obtained for the HEA TiAlV0.5CrMo [23]. Overall, chemical compositions containing Al have significantly lower densities. To further reduce the density, oxides are introduced into HEA compositions [32].

The density of powder samples is considered when calculating specific strength, which allows for comparing HEAs with different chemical compositions and densities.

It is important to note that density is also determined by the level of residual porosity, which is highest for pressureless sintering and lowest in the case of HP and SPS.

Mechanical properties at elevated temperatures

Only 20 % of the reviewed studies provide data on the properties of powder HEAs at elevated temperatures.

The authors of the study [16] found that the RHEA TaNbVTiAl0.2 exhibits exceptional specific strength both at room temperature (207.11 MPa·cm3/g) and at high temperatures (88.37 MPa·cm3/g at 900 °C and 16.03 MPa·cm3/g at 1200 °C), while maintaining acceptable ductility. Such RHEAs have the potential for use at temperatures exceeding 1200 °C. The high mechanical properties are determined by the homogeneous microstructure and solid solution strengthening.

In the study [17], a comparison of tensile test results at room temperature and at 900 °C showed that deformation increased 5.6 times, and the yield strength decreased fourfold. According to the authors, grain boundary strengthening was the dominant mechanism at elevated temperatures, where carbide particles made a significant contribution to increasing yield strength through dislocation and Orowan strengthening.

In the study [18], nanoindentation showed that increasing the temperature to 400 °C resulted in only a 10 % reduction in hardness.

In the study [21], the hot deformation characteristics of ultrafine-grained RHEA MoNbTaTiV were investigated using isothermal compression tests in the temperature range of 1100 to 1300 °C and strain rates from 0.0005 to 0.5 s\(^–\)1. It was found that at high temperature and low strain rate, the main deformation mechanism becomes grain boundary sliding, which is somewhat suppressed by grain growth and ultrafine precipitated phases distributed along the grain boundaries.

In the study [22], it was noted that the high strength of the NbMoTaWV alloy at elevated temperatures is primarily due to the presence of a secondary phase, which prevents grain boundary sliding. However, at elevated temperatures, the alloy became less ductile, likely due to the presence of the secondary phase, which leads to crack formation along the grain boundaries. At room temperature, the sintered NbMoTaWV demonstrated higher compressive strength and ductility compared to the corresponding cast HEA. The significant increase in strength is associated with the precipitation of the (Ta, V)O2 phase and grain boundary strengthening of the BCC matrix.

In the study [25], a new super-strong RHEA NbTaTiV, oxide-dispersion strengthened with 0.35 wt. % Al2O3 , was produced. The dual-phase material demonstrated a high yield strength (2075 MPa) and compressive ductility (15 %), maintaining high strength across a wide temperature range (25 – 1000 °C). The super-high strength of the dual-phase RHEA was mainly attributed to dispersion strengthening due to the high fraction of submicron Ti-(O, N) particles and solid solution strengthening. The alloy’s performance can be significantly improved through oxide strengthening, opening new prospects for developing high-performance RHEAs.

High-temperature tests conducted in all the published studies aimed to evaluate the static strength of materials at elevated temperatures (see Table), but for practical application, an assessment of the reliability of such materials will be required. Therefore, future studies should evaluate fracture toughness, creep resistance, durability, etc.

Oxidation resistance and thermal stability

In 15 % of the reviewed studies, data on thermal stability and/or oxidation resistance were provided.

A key feature of RHEAs is the high-temperature stability of the ultrafine-grained structure, obtained through mechanical alloying followed by SPS. The high recrystallization temperature of RHEAs ensures the retention of the nanostructures formed during the preparation of powder mixtures. Therefore, RHEAs exhibit higher thermal stability compared to HEAs based on 3d elements.

The introduction of active elements Al and Cr into RHEA compositions promotes the formation of oxide films, which enhance heat resistance [20; 30].

Conclusions and future prospects

This review has examined new and traditional approaches used in the production of high-entropy alloys (HEAs) for high-temperature applications. The primary goal of solid-state methods for producing HEAs from refractory elements is to create cost-effective components with precise geometries and properties that are difficult or impossible to achieve using gas-phase or liquid-phase methods.

Recent research in powder metallurgy shows the use of oxides and hydrides for powder production, significantly expanding the raw material base for HEA metallurgy.

Various approaches are used to produce powder mixtures, including mechanical alloying, SHS (self-propagating high-temperature synthesis), hydride formation, metallothermy, agglomeration, spheroidization, gas atomization, and plasma atomization of a centrifugally rotating electrode.

An analysis of powder sintering methods indicates that the most commonly used method is spark plasma sintering (SPS). However, this method has known limitations regarding the shape and size of products. Therefore, the study of free sintering processes is more important for mass production. In addition, to reduce the porosity of sintered powder samples, hot isostatic pressing (HIP), which is actively used in additive manufacturing for critical products, should be applied.

The production of long bars and wires from HEAs by hot extrusion of powders can be used for making rod structures as well as for additive manufacturing processes, such as thermal spraying or wire arc additive manufacturing.

An analysis of the chemical composition of high-entropy alloys shows that HEAs based on 3d transition elements are suitable for temperatures up to 1000 °C, while refractory HEAs (RHEAs) are used at higher temperatures. The addition of aluminum is aimed at reducing the density of RHEAs and increasing oxidation resistance.

One of the promising methods for improving strength at high temperatures is oxide dispersion strengthening. However, in some cases, nanoparticles chemically interact with the matrix, altering the chemical composition of the dispersed particles. Therefore, the selection of strengthening nanoscale powders requires prior analysis.

The high thermal stability of RHEAs and the retention of nanoscale grains at temperatures above 1000 °C are determined by the high recrystallisation temperature.

The results of this review confirm that HEAs have potential for use in high-temperature applications. The mechanical properties of sintered RHEA samples are superior to those of samples with similar chemical compositions obtained by liquid-phase methods. However, further research and development are required to improve the oxidation resistance and mechanical properties of powder RHEAs at the desired temperatures.

A key finding from the analysis is the identification of a limited range of methods for evaluating high-temperature properties (such as compressive strength, tensile strength, and nano-hardness). This restricts consumers’ ability to fully assess the feasibility of new alloys and production methods for practical applications. Therefore, it is essential to broaden the evaluation approaches to include a wider spectrum of performance characteristics, such as fracture toughness, heat resistance, wear resistance, fatigue strength, and overall durability.

Thus, future research should focus on:

– determining fatigue properties and the durability of powder products to ensure their reliability in real engineering applications;

– manufacturing large parts with complex shapes;

– reducing porosity without significantly increasing cost;

– developing low-temperature deformation methods;

– creating environmentally friendly and highly accurate production technologies.

When planning new research, it is important to focus on scalability, cost-effectiveness, and the practical application of powder synthesis and consolidation methods to enable their broader adoption in real-world engineering projects.