Annealing Treatment of Al₂CoCrFeNi High-Entropy Alloys: Synergistic Effect of Microstructure Modulation on Mechanical and Thermoelectric Properties

Abstract

This study synthesized Al₂CoCrFeNi high-entropy alloy (HEA) using spark plasma sintering (SPS) followed by annealing treatment. The effects of heat treatment on the microstructure, mechanical properties, wear resistance, and thermoelectric properties were systematically investigated. The annealed alloy exhibited a microhardness increase from 538.5 HV to 550.9 HV and a significant improvement in ultimate compressive strength from 1540.74 MPa to 2563.67 MPa, attributed to grain homogenization and reduced dislocation density. Wear resistance tests revealed a decrease in wear rate from 7.15 × 10⁻⁵ mm³/(N·m) to 4.74 × 10⁻⁵ mm³/(N·m), with wear morphology analysis confirming enhanced resistance to plastic deformation. Thermoelectric characterization demonstrated that thermal diffusivity increased from 2.98 mm²/s to 3.11 mm²/s at room temperature, while the absolute Seebeck coefficient reached 8.0 μV/K at 200 °C, indicating improved electron transport efficiency due to lattice ordering. This combination of high hardness, high thermal conductivity, and excellent wear resistance presents unique application value in extreme tribological fields involving thermal management and simultaneous surface wear resistance and heat dissipation.

1. Introduction

High-entropy alloys (HEAs) have attracted considerable attention owing to their innovative compositional design and exceptional multifunctional properties [1,2,3]. Distinguished from conventional alloys, HEAs consist of multiple principal elements in equimolar or near-equimolar ratios, resulting in distinct microstructural and property characteristics [4]. Extensive research has demonstrated that HEAs possess superior mechanical [5], high-temperature [6], wear-resistant [7], and corrosion-resistant [8] properties, positioning them as promising candidates for advanced engineering applications. The enhancement of high-entropy alloy performance through appropriate heat treatment represents the most commonly employed and effective strategy. For instance, Sha et al. [9] demonstrated that AlCoCrFeNiTi_0.5 high-entropy alloy coatings subjected to 900 °C annealing achieved a microhardness of 9890 MPa, representing a 73.5% improvement compared to as-cast coatings, accompanied by a 92.5% reduction in wear loss. Bhattacharje et al. [10] developed a nanolamellar AlCoCrFeNi_2.1 eutectic high-entropy alloy through cryorolling and subsequent annealing processes. The hierarchical microstructure induced concurrent improvements in strength (yield strength: 1437 MPa; ultimate tensile strength: 1562 MPa) and ductility (failure elongation: 14%), thereby achieving a strength–ductility trade-off. Kush et al. [11] fabricated Ni₂CuCrFeAl_x high-entropy alloys through powder metallurgy followed by annealing heat treatment, demonstrating that the thermal quality factor was significantly enhanced with increasing temperature. The reduced Fermi level and wide bandgap facilitated the transition of charge carriers from p-type to n-type, thereby modifying the electrical conductivity characteristics.

Al₂CoCrFeNi HEA, a typical multi-principal element alloy, is renowned for its excellent mechanical properties and corrosion resistance [12,13]. Yan et al. [12] observed that annealing at 1000 °C improved the microhardness (482 HV to 516 HV) and elongation (7.55% to 12.06%) of Al₂CoCrFeNi HEA. Thermoelectric performance also exhibited temperature-dependent enhancement, with electrical conductivity peaking at 1000 °C. However, studies on its wear resistance and thermoelectric properties remain limited [14]. Therefore, it remains necessary to further investigate achieving a desirable combination of mechanical and thermoelectric properties in alloys through appropriate heat treatment.

In this work, Al₂CoCrFeNi HEA was synthesized via spark plasma sintering (SPS) to achieve fine-grained, high-density bulk materials. The synergistic effects of annealing on microstructure, mechanical properties, wear resistance, and thermoelectric performance were systematically analyzed. This study provides critical insights into the application of Al₂CoCrFeNi HEA under extreme operational conditions.

2. Experimental Materials and Methods

Al₂CoCrFeNi HEA was synthesized using ball milling and SPS (Figure 1). Elemental powders (Al, Co, Cr, Fe, Ni) were mixed in a high-energy ball mill (QM-QX-2L, Changsha Mickey, Changsha, China) under an argon atmosphere (ball-to-powder ratio: 5:1, rotational speed: 240 rpm, duration: 6 h). Ethanol (10 mL) was added to prevent agglomeration. The dried powder was compacted in a graphite mold (Φ20 mm × 40 mm) under 10 MPa pre-pressure and sintered in an SPS system (LABOX-325, Sinter Land) under vacuum (<10 Pa). The specific sintering process flow is detailed in Figure 1. The sintered ingots were air-cooled, machined into Φ20 mm × 10 mm samples, and polished for characterization.

Phase composition was analyzed via X-ray diffraction (XRD, D8 ADVANCE Davinci, Billerica, MA, USA) with Cu-Kα radiation (40 kV, 2θ = 20°–90°, step size: 0.02°). Microstructure and elemental distributions were examined using SEM (TESCAN MIRA3 LMU, Brno, Czech Republic) coupled with EDS. The element distribution testing results are presented in

Table 1. Composition of Al₂CoCrFeNi HEA.

	Al	Co	Cr	Fe	Ni
Atomic mass	26.982	58.933	51.996	55.845	58.693
At%	33.2	16.7	16.7	16.7	16.7
Mass%	19.22	21.11	18.63	20.01	21.03

. Mechanical properties were evaluated via microhardness testing (FM-700, Kawasaki, Japan, 5 kg load,) and compression tests (WDW-200 universal tester, Jinan, China). Wear resistance was assessed using a ball-on-disk tribometer (MPX-3G, Hengxu, Shanghai, China) under 10 N load (Al₂O₃ counterface, 0.1 m/s sliding velocity). Thermoelectric properties, including thermal diffusivity (Netzsch LFA 457, Selb, Germany) and Seebeck coefficient (Netzsch SBA 458, Burlington, MA, USA), were measured under argon atmosphere.

Subsequent mechanical property evaluations were systematically performed. Compression tests were carried out using a WDW-200 universal testing machine. Microhardness measurements were obtained using an FM-700 microhardness tester under a 5 kg load, with a minimum of seven indentations recorded per specimen. The average value was calculated, and data variability is presented. Dry sliding wear behavior at room temperature was evaluated using a high-temperature ball-on-disk wear testing machine (MPX-3G, Hengxu, Jinan, China) under 10 N normal load for 30 min. The tribological configuration employed an Al₂O₃ counterface ball (6.25 mm diameter) with 0.1 m/s sliding velocity. Specimens for wear analysis were mechanically ground, followed by final polishing with 1 μm diamond paste. Finally, thermoelectric characterization involved precise measurements of thermal diffusivity (D) using laser flash analysis (Netzsch LFA 457, Selb, Germany), along with the simultaneous determination of electrical conductivity and Seebeck coefficient using a Seebeck analyzer (Netzsch SBA 458, Burlington, MA, USA). The thermal conductivity and thermoelectric figure of merit are calculated using formulas.

3. Results and Discussion

3.1. Phase and Microstructure

Figure 2 presents the X-ray diffraction pattern of Al₂CoCrFeNi HEA samples, and XRD analysis revealed that the as-cast Al₂CoCrFeNi HEA primarily consists of the body-centered cubic (BCC) phase and the B2 (ordered BCC) phase. The BCC phase is typically associated with high strength and hardness in high-entropy alloys, while the B2 phase exhibits good thermal stability due to its ordered structure [15]. After annealing at 900 °C, the diffraction peaks of the BCC and B2 phases remained, and the lattice parameters were consistent, indicating that no significant phase transformation occurred after annealing. This observation aligns with the findings of Zhang et al. [16], who reported that the high-entropy alloys prepared by SPS retained the structural stability of the BCC and B2 phases after annealing.

Figure 3 illustrates the electron backscatter diffraction (EBSD) patterns of the as-cast and the annealed Al₂CoCrFeNi high-entropy alloys (HEAs). The grain orientation difference distribution curves (Figure 3c,d) reveal that the annealed Al₂CoCrFeNi HEA is predominantly characterized by a grain orientation angle of 60°, indicating the presence of a large number of twin boundaries in the alloy. The presence of twin boundaries has been shown to enhance plasticity and fracture toughness in metallic materials [17]. The average grain size of the as-cast alloy was 3.2 μm (compared to 3.1 μm before annealing) (Figure 3e,f), demonstrating negligible grain coarsening following 900 °C annealing. Notably, despite the conventional understanding that grain growth typically compromises material strength, the annealed alloy exhibits enhanced hardness and compressive strength relative to its as-cast counterpart (Figure 4a,b). This anomalous strengthening behavior may be attributed to reduced lattice distortion and effective internal stress relief during thermal processing [18].

3.2. Mechanical Properties

To investigate the influence of the spark plasma sintering process on mechanical properties, microhardness and compression tests were performed on Al₂CoCrFeNi HEA. Figure 4a shows the microhardness of the alloy in both the as-cast and annealed conditions. The microhardness of the as-cast alloy is 538.5 HV, while the annealed one increased this value to 550.9 HV. This enhancement in hardness can be primarily attributed to the reduction in lattice distortion and dislocation density after annealing, which enhances the overall strength of alloy [19].

Figure 4b illustrates the compressive stress–strain curve of Al₂CoCrFeNi HEA. The ultimate compressive strength of the as-cast alloy is 1540.7 MPa, whereas that of the annealed alloy increases significantly to 2563.6 MPa. This substantial strength improvement may be attributed to the homogenization of the grains and the release of internal stresses during the annealing process, which lead to reduced lattice distortion and a marked enhancement in plasticity. Furthermore, as evidenced by the electron backscatter diffraction (EBSD) analysis (Figure 3c,f), the presence of abundant twin boundaries within the heat-treated alloy contributes to its enhanced plastic deformation capability.

To analyze the strengthening mechanisms of the alloy, Figure 5 presents the microscopic morphology of the compression fracture surface in the annealed alloy state. Distinct features observable on the fracture surface include microvoids, dimples, and fragmented granular particles, accompanied by localized spalling. These characteristics reflect strengthening mechanisms arising from impeded dislocation motion during deformation and obstruction by grain boundaries associated with grain refinement. Furthermore, the substantial increase in compressive strength accompanied by only a marginal hardness enhancement may be attributed to the presence of the B2 phase. Characterized by long-range chemical ordering, the B2 phase requires dislocations to overcome considerable energy barriers when shearing through the ordered lattice, thereby significantly enhancing compressive strength. Conversely, during hardness testing, elevated stress gradients may promote dislocation cross-slip, which diminishes the obstruction to dislocation motion and consequently compromises the strengthening effect [20,21].

Wear morphology is closely related to the hardness of the metal and is essential for understanding the wear mechanism. Figure 6 presents the wear morphology of the annealed Al₂CoCrFeNi HEA, revealing distinct plowing grooves with localized fragmentation and spalling of the alloy, indicating that the predominant wear mechanisms are abrasive wear [22].

Wear resistance of an alloy is primarily evaluated by its wear rate (W), calculated as

W = V/(F − Ls)

V is the wear mark volume (mm³), F is the load (N), and Ls is the total friction distance (mm), calculated as

Ls = L − υ − t

L is the perimeter of the wear mark (mm), υ is the rotational speed (r/min), and t is the total wear time (min).

The wear rate of the as-cast alloy was calculated as 7.15 × 10⁻⁵ mm³/(N·m), while that of the annealed alloy decreased to 4.74 × 10⁻⁵ mm³/(N·m). This result suggests that the annealing treatment improved the wear resistance of the alloy (Figure 7c).

Figure 7a,b show the 3D wear morphology of the as-cast and annealed alloys. The as-cast alloy has a wear scar width of approximately 424 μm, whereas that of the annealed alloy is reduced to 312 μm, thus demonstrating enhanced wear resistance in the annealed specimen. This phenomenon may be attributed to the increased hardness and strength of the annealed alloy, which effectively resists plastic deformation and material loss during wear [23].

3.3. Thermoelectric Properties

Thermoelectric properties are crucial for assessing the ability of a material to generate electrical energy across a temperature gradient. Figure 8 shows the variation in thermal diffusivity and thermal conductivity with temperature for Al₂CoCrFeNi HEA. Through data fitting, the half-rise time (the time required for the surface temperature to rise by 50%) was determined for the specimen. This parameter enables calculation of thermal diffusivity using the following equation:

α = 0.1388d²/t_0.5

where α represents the thermal diffusion coefficient of the material (m²/s), d represents the measured sample thickness (m), and t_0.5 represents the half-heating time of the sample (s).

The thermal conductivity of a material can be calculated from its characteristic parameters—density, specific heat capacity, and thermal diffusivity—calculated as follows:

λ = α·ρ·Cp

where λ represents the thermal conductivity (W·m⁻¹K⁻¹), α represents the thermal diffusion coefficient of the material (m²/s), ρ represents the sample density obtained by the test (kg/m³), and Cp represents the specific heat capacity of the sample (J·kg⁻¹K⁻¹).

Thermal diffusivity increases with temperature, with the annealed alloy exhibiting significantly higher diffusivity than the as-cast alloy. The thermal diffusivity at room temperature of the as-cast is 2.98 mm²/s, while after annealing at 900 °C, it increases to 3.11 mm²/s. As the temperature increases to 150 °C, the thermal diffusivities before and after annealing are 3.28 mm²/s and 3.37 mm²/s, respectively. The thermal conductivity exhibits a modest increase with rising temperature, with the annealed alloy demonstrating consistently higher values than the as-cast alloy. When temperature ascends from room temperature to 150 °C, the thermal conductivity of annealed alloy rises from 10.73 W·m⁻¹·K⁻¹ to 11.64 W·m⁻¹·K⁻¹, whereas the as-cast alloy achieves only 50% of this value. This phenomenon is attributed to reduced lattice distortion during the annealing process, which lowers the interfacial potential barrier and enhances heat conduction [24].

Figure 9a,b show the variation in conductivity and Seebeck coefficient with temperature for Al₂CoCrFeNi HEA, respectively. The conductivity of the alloy decreases with temperature, likely due to enhanced electron–phonon scattering at higher temperatures, which hinders electron movement [25]. The conductivity of the annealed alloy is 7025.84 S/cm, slightly lower than that of the as-cast alloy, but still achieving 94% of its conductivity. The negative Seebeck coefficient indicates that electron transport from the hot end to the cold end is the dominant mode of electron diffusion. The absolute value of the Seebeck coefficient for the annealed alloy increases with temperature, reaching 8.0 μV/K at 200 °C, indicating improved thermoelectric properties. This phenomenon is likely related to the more ordered lattice structure of the annealed alloy and more efficient electron transport [26].

The thermoelectric figure of merit (ZT) serves as the quantitative metric for assessing a material’s efficiency potential in converting thermal energy into electrical energy. It constitutes the core evaluation criterion in thermoelectric materials research, calculated as follows:

ZT = S²·θ·T/k

where S represents the Seebeck coefficient (V/K) of the material, θ is the measured conductivity (S/m), k is the thermal conductivity of the material (W·m⁻¹·K⁻¹), and T is the measured temperature (K).

Figure 10 presents the thermoelectric figure of merit (ZT) of Al₂CoCrFeNi HEA at varying temperatures. Although the annealed alloy exhibits generally lower ZT values than the as-cast alloy, it demonstrates an increasing trend with rising temperature, reaching 1.1 × 10⁻³ at 150 °C. It has been reported that the optimized ZT for Al₂CoCrFeNi can attain 0.015 at approximately 500 °C [27], despite underperforming conventional thermoelectric materials such as PbTe [28] and Bi₂Te₃ [29], and it demonstrates potential as a viable high-entropy thermoelectric material for practical applications.

4. Conclusions

This investigation fabricated Al₂CoCrFeNi high-entropy alloys through ball milling and spark plasma sintering (SPS) systems, with subsequent annealing treatments implemented to regulate microstructural evolution for enhanced mechanical and thermoelectric properties. The summary and conclusions from this study follow.

(1) After annealing at 900 °C, the alloy maintains the structural stability of BCC and B2 phases, indicating that no significant phase transformation occurred after annealing.

(2) The reduction in lattice distortion, release of internal stresses, and homogenization collectively contributed to excellent hardness (550.9 HV) and compressive strength (2563.7 MPa). The enhanced hardness effectively reduces the wear rate, thus improving the wear resistance of the material.

(3) Additionally, the thermoelectric properties of the alloys improved after annealing. Both thermal diffusivity and thermal conductivity increased with temperature, reaching 3.34 mm²/s and 11.64 W·m⁻¹·K⁻¹ at 150 °C, and the ordered lattice structure facilitated more efficient electron transport. The temperature-dependent enhancement of the thermoelectric figure of merit (ZT) demonstrates substantial promise for Al₂CoCrFeNi high-entropy alloy as an emerging thermoelectric material.

(4) In conclusion, this study provides a theoretical foundation for the application of Al₂CoCrFeNi high-entropy alloys under extreme operational conditions involving elevated temperatures and pressures. Appropriate heat treatment imparts the Al₂CoCrFeNi high-entropy alloy with high hardness, elevated thermal conductivity, and favorable wear resistance. This synergistic property profile demonstrates unique applicability in extreme tribological environments necessitating integrated thermal management with concurrent surface wear resistance and heat dissipation, as exemplified by aerospace turbine blades and high-power laser heat spreaders. While the diminished thermoelectric figure of merit (ZT) restricts its implementation in waste heat recovery technologies, it concurrently signifies enhanced thermal stability, indicating structural viability for thermal shock-resistant applications.