Recent Advances in High-Temperature Properties of High-Entropy Alloys

2.1. Tensile Strength and Ductility High-temperature short-term mechanical properties [26] refer to the yield strength and elongation exhibited by a material during short-term testing at elevated temperatures. As one of the key indicators of alloys’ high-temperature performance, strength and ductility are particularly critical for applications operating at moderate temperatures or for components subjected to very short service durations [27], such as those in rockets and missiles [28]. Generally, as the temperature increases, the yield strength of metallic materials decreases while their ductility increases. Figure 1 illustrates the variation in specific strength and yield strength of several alloy materials with temperature. It can be observed that HEAs exhibit remarkable strength retention at elevated temperatures. As shown in Figure 1, the AlCoCrFeNi alloy achieves a specific strength of 200 MPa·cm³/g within the temperature range of 800–900 °C, surpassing that of TiAl alloys at comparable temperatures. Furthermore, the VNbMoTaW alloy retains a specific strength of approximately 70 MPa·cm³/g even at 1500 °C. High-entropy alloys HEAs exhibit exceptional mechanical properties across a wide temperature spectrum from 0 to 1600 °C [29]. For instance, the MoNbTaVW HEA maintains a yield strength of 300–500 MPa at 1600 °C, whereas the yield strength of Inconel 718 drops sharply from ~1000 MPa at 500 °C to <200 MPa at 800 °C. These results highlight the superior mechanical performance of some HEAs in high-temperature environments compared to traditional alloy systems. In recent years, significant progress has been made in enhancing the high-temperature mechanical properties of HEAs. The core advantage of HEAs stems from their unique multi-principal element design, which enables synergistic strengthening mechanisms such as solid-solution strengthening, precipitation phase control, and grain boundary optimization [31]. These multiscale interactions effectively balance high-temperature strength and ductility in HEAs. Among these, solid-solution strengthening plays a foundational role, as the severe lattice distortions induced by multi-principal element mixing hinder dislocation motion and enhance strength across a broad temperature range. Alloys such as AlMo_0.5NbTa_0.5TiZr and AlNbTaTiZr [32] exemplify this approach, where the substitution of elements like Mo with Ta further improves oxidation resistance by promoting the formation of dense, adherent oxide films without compromising mechanical properties. In addition, as demonstrated in CoCrFeNiW_0.2 HEA [33], the incorporation of refractory W leads to pronounced solid-solution strengthening effects by impeding dislocation mobility and contributing to dynamic strain aging, as evidenced by the evolution of serrated flow behavior (A→B→C) under increasing temperature. Complementing this, precipitation strengthening is widely employed to enhance both strength and thermal stability. In systems like Al₁Ti₅Zr₄Nb₃Ta₃ and AlNbTi₃VZr_1.5 [34,35], the formation of nanoscale B2 and HCP precipitates provides strong barriers to dislocation glide and enables sustained work hardening during plastic deformation, even at elevated temperatures. These finely dispersed secondary phases are thermally stable and uniformly distributed, thus effectively delaying softening and maintaining load-bearing capacity under service conditions. Similarly, the Al_0.3CoCrFeNi alloy exhibits nanoscale B2 precipitates enriched in Al and Ni, which interact with dislocations and deformation twins, thereby influencing strain-hardening behavior across 300-700 °C. Notably, these precipitates form preferentially along grain boundaries, stabilizing microstructure against coarsening [36]. Interfacial strengthening is another pivotal mechanism that significantly contributes to the mechanical robustness of these alloys. Alloys such as TiNbTaWC_0.7 and NbMoTaWHf [37,38] incorporate coherent or semi-coherent phase boundaries, often with specific orientation relationships like {111}FCC//{110}BCC or faceted zig-zag interfaces that increase the resistance to dislocation penetration and enhance high-temperature structural integrity. These interfaces act as effective obstacles to deformation and play a crucial role in grain boundary stabilization. Moreover, transformation-induced strengthening mechanisms further expand the design space for achieving superior mechanical behavior. In the case of AlCoCrFeTi_0.5Ni_2.5 [35], the synergistic interplay between deformation-induced twinning in the FCC matrix and stress-assisted martensitic transformation in the BCC phase leads to enhanced strain hardening and delayed necking, resulting in both high strength and improved ductility at elevated temperatures. A novel, low-effective L1₂-strengthened single-crystal HEA (Co_37.8Ni_38.6Cr_5.2Mo_5.1Al_9.8Ta_2.6Ti_3.9) was successfully developed through directional solidification. [39] The high-entropy alloy with this composition was fabricated using directional solidification. As illustrated in Figure 2, after heat treatment, the L1₂ phase was uniformly distributed within the FCC matrix, with an average size of approximately 280 nm. These precipitates exhibited a highly coherent interface with the matrix (lattice mismatch of only 0.14%) and a volume fraction as high as 80%, which exceeds that of conventional single-crystal superalloys. [40,41]. Moreover, no detrimental topologically close-packed (TCP) phases were observed. At an intermediate temperature of 800 °C, the alloy exhibited a maximum yield strength of 897 MPa, an ultimate tensile strength of 1038 MPa, and an elongation of 37%. Moreover, at elevated temperatures (900 °C and 1000 °C), the HEA maintained excellent performance. Meanwhile, Lei Gao [42] et al. developed a novel Ni-Co-Cr-Al-based high-entropy superalloy (HESA) that exhibits outstanding high-temperature mechanical properties across a broad temperature range. This alloy features a face-centered cubic (fcc) matrix with low stacking fault energy (SFE). Their study demonstrated that the addition of Ti, Nb, Ta, Mo, and W to the Ni-Co-Cr-Al-based HESA effectively reduces the SFE of the fcc matrix and promotes the precipitation of nanoscale, near-spherical, multi-component particles with coherent interfaces (as shown in Figure 3). Remarkably, this HESA achieves an ultimate tensile strength (UTS) of ~1100 MPa at 750 °C, surpassing the performance of other contemporary materials. B.X. Cao [43] et al. developed an L1₂-strengthened, multi-component Co-rich high-entropy alloy (HEA) with exceptional microstructural stability. Their study revealed that high-density cuboidal γ′ precipitates were uniformly embedded within the alloy matrix. Remarkably, even after prolonged isothermal aging at 800–1100 °C, no brittle intermetallic phases formed at grain boundaries or within grains. As illustrated in Figure 4, atom probe tomography (APT) analysis demonstrated that Ta and Nb preferentially partitioned into the γ′ intermetallic phase, which is critical for stabilizing the L1₂-ordered crystal structure. Notably, this alloy exhibited an optimal combination of a high γ′ solvus temperature (1125 °C), low mass density (8.28 g/cm³), and excellent high-temperature tensile strength (755 MPa at 800 °C and 664 MPa at 900 °C), surpassing the performance of other Co-based HEAs [44,45] and several conventional superalloys. These findings provide a valuable paradigm for designing advanced Co-based L1₂-strengthened high-temperature alloys with extended operational temperature ranges. Chanho Lee [46] developed a refractory NbTaTiV high-entropy alloy (HEA) with a single body-centered cubic (BCC) structure using a combined experimental and theoretical approach. A typical dendritic microstructure was observed, which was attributed to elemental segregation during solidification in the as-cast state. However, after an appropriate homogenization treatment at 1200 °C for three days, structural inhomogeneity and chemical segregation were completely eliminated. Atomic probe tomography (APT) quantitatively confirmed the uniform distribution of elements. Importantly, the results demonstrated that this HEA exhibits both high yield strength and ductility at room temperature and elevated temperatures (up to 900 °C). Furthermore, first-principles calculations were conducted to analyze and quantify the effect of high configurational entropy on mechanical properties based on lattice distortion and atomic interactions within the NbTaTiV HEA. The study revealed that severe local lattice distortion is induced due to atomic interactions and atomic size mismatches in the homogenized NbTaTiV refractory HEA, significantly contributing to its superior mechanical performance. An Hf–Nb–Ti–V refractory high-entropy alloy (RHEA) was successfully fabricated by Yongyun Zhang [47] using directed energy deposition (DED), exhibiting excellent tensile properties across a wide temperature range. This performance was primarily attributed to the introduction of severe lattice distortion in the as-fabricated RHEA, which resulted from localized chemical fluctuations and significant atomic radius mismatches. These factors, combined with strong solute pinning effects, contributed to the exceptional yield strength of the DED-manufactured RHEA across a broad temperature spectrum. This alloy design strategy, which involves introducing substantial lattice distortion while maintaining a low-temperature sensitivity of the elastic modulus, opens new possibilities for the direct fabrication of RHEAs with outstanding high-temperature performance. Yuhao Jia [48] discovered that the addition of a small amount of boron significantly strengthens both FCC grain boundaries and FCC/B2 phase interfaces in the Ni₃₀Co₃₀Cr₁₀Fe₁₀Al₁₈W₂ eutectic high-entropy alloy (EHEA) at high temperatures. Even with a minor boron addition, the alloy achieved an impressive tensile yield strength exceeding 581 MPa and an elongation of 71% at 800 °C, as shown in Figure 5. Compared to its boron-free counterpart, this represents a 45% increase in yield strength and a 129% improvement in ductility. The enhanced phase boundary strengthening by boron facilitated the sustainable, dynamic recovery of both the FCC and B2 phases during high-temperature tensile deformation, ensuring excellent ductility. Benchmarking results further indicated that boron and boride-induced grain boundary strengthening contributed up to 166 MPa to the yield strength at 800 °C. These findings provide valuable insights into microalloying strategies for developing high-temperature eutectic with superior mechanical performance. Tiantian Wang [49] investigated the effect of Si addition on the AlMo_0.5NbTiV alloy. The incorporation of Si led to the formation of a hard and brittle M₅Si₃-type phase (with M = Ti and Nb) at the grain boundaries, gradually transforming the alloy from a single-phase body-centered cubic (BCC) structure into a dual-phase structure, as shown in Figure 6. With increasing Si content, the mechanical properties were significantly enhanced. For instance, the AlMo_0.5NbTiV Si_0.5 alloy exhibited optimal yield strengths of 1428 MPa and 390 MPa at 1073 K and 1273 K, respectively. Notably, the AlMo_0.5NbTiVSi_0.1 alloy showed the best compressive ductility at 1073 K, achieving a strain as high as 22.2%, which indicates that an appropriate Si addition can improve the alloy’s compressive ductility through a combination of grain refinement and a eutectic microstructure [50,51]. Specifically, Si segregation at the grain boundaries retards primary crystal growth, thereby inhibiting grain coarsening. Additionally, the eutectic structure composed of M₅Si₃ and BCC phases along the grain boundaries further restricts grain growth, resulting in overall grain refinement. The high-temperature strengthening mechanism of the alloy is attributed to the synergistic effects of grain refinement strengthening, second-phase strengthening, and solid-solution strengthening [52], with grain refinement playing the dominant role. 2.2. Creep Resistance High-temperature alloys often face cyclic loading in high-temperature environments, making their creep resistance crucial for engineering applications [53]. Creep [54] in metallic materials refers to the phenomenon where strain increases over time under constant stress. Unlike plastic deformation, creep occurs even when the stress is below the elastic limit. Therefore, evaluating the creep performance of new materials is vital for their practical engineering use. Over nearly two decades of development, HEAs have demonstrated numerous advantages in high-temperature environments, with growing attention being paid to their creep behavior [55]. Conventional high-temperature alloys, such as nickel-based alloys, improve creep resistance through the precipitation of strengthening phases, such as the γ′ phase. This strengthening mechanism performs well at high temperatures; however, it has a limitation: as the temperature increases further, the precipitate phases may coarsen or dissolve, leading to a decline in performance. Additionally, traditional high-temperature alloys tend to experience grain boundary weakening at elevated temperatures, where segregation and uneven distribution of precipitates at the grain boundaries can reduce their strength, thus affecting the alloy’s creep resistance. For HEAs, their slow element diffusion dynamics and lattice distortion effects effectively suppress dislocation motion and grain boundary sliding at high temperatures, theoretically improving their creep resistance. However, conventional high-temperature alloys still have competitive advantages in the medium-temperature range and mature application scenarios. At present, only some HEAs exhibit creep resistance superior to traditional high-temperature alloys, and the creep mechanisms of HEAs at high temperatures remain insufficiently understood [56]. However, some recent studies have also promoted the understanding of the creep behavior of high entropy alloys at high temperatures. In non-equiatomic FeMnCoCrAl, asymmetric atomic arrangements enhance solute drag and dislocation climb mechanisms under sustained stress [57]. Molecular dynamics simulations reveal that increasing W content in NbMoTaW enhances solute drag and grain stability, improving creep resistance at the atomic level [58]. In AlTiVNbZr_0.25, Nb₂Al precipitates formed at B2 phase interfaces enhance dislocation pinning, resulting in class-A creep at 1073 K [59]. Similarly, single-crystal high-entropy superalloys (HESAs) utilize spherical γ′ precipitates to retard dislocation movement via bypass and climb, ensuring resistance at intermediate temperatures [60]. LPBF-processed Ti-Ta-Nb-Mo-Zr RHEAs exhibit enhanced creep resistance via dislocation tangles and high-stress exponents under 923~1023 K [61]. In AlCoCrFeNi_2.1 EHEA, the dual-phase lamellar L1₂/B2 structure induces varied dislocation glide and shear behavior with temperature, resulting in temperature-dependent creep characteristics between 700–900 °C [62]. Te-Kang Tsao [63] introduced a novel high-entropy alloy and investigated its high-temperature tensile and creep behaviors. At 982 °C, its creep resistance is comparable to some nickel-based high-temperature alloys. Experimental results indicate that the alloy’s matrix has low stacking fault energy, the strengthening precipitates possess high anti-phase boundary energy, and the microstructure is thermally stable, all contributing to the enhancement of HESA’s properties. The creep curve of this HEA primarily exhibits three-stage creep behavior. The low creep rate of this high-entropy alloy is attributed to slow diffusion, low stacking fault energy in the γ matrix, and high creep activation energy. M. Zhang [64] conducted tensile creep tests on the CrMnFeCoNi high-entropy alloy at temperatures ranging from 1023 K to 1173 K, as shown in Figure 7. The stress exponent was calculated using the following formula: where $$\overset{\cdot}{\epsilon_{s s}}$$ is steady-state creep rate, $$\sigma$$ is applied stress, $$A$$ is a material constant, $$n$$ is the stress exponent, $$Q c$$ is the apparent activation energy of creep, $$R$$ is the ideal gas constant, and $$T$$ is the absolute temperature. Their study revealed that the stress exponent for the CrMnFeCoNi high-entropy alloy remained consistent at 3.7 ± 0.1 across all tested temperatures. The apparent activation energy for creep was determined to be approximately 230 kJ/mol, which decreased with increasing applied stress, indicating stress-assisted thermally activated behavior. The steady-state creep microstructure was characterized by the absence of sub-grain formation within the grains and a high dislocation density. Based on their findings, the creep rate of CrMnFeCoNi was primarily controlled by dislocation-dislocation interactions and dislocation-lattice interactions. This suggests that the creep behavior at high temperatures is influenced by the dynamics of dislocations in the material’s crystal structure. Xun Shen [56] prepared a bulk nanocrystalline VNbMoTaW refractory high-entropy alloy (RHEA) with an average grain size of 67 ± 17 nm via mechanical alloying (MA) followed by high-temperature, high-pressure sintering. Compression and creep tests were conducted to evaluate its high-temperature mechanical properties at 973 K and 1073 K. Although the creep performance of the nanocrystalline VNbMoTaW RHEA was slightly lower than that of its coarse-grained counterpart, it was significantly superior to previously reported coarse-grained and ultrafine-grained HEAs [59,65,66] (Figure 8e,f). Observations indicated that creep deformation was primarily controlled by grain boundary diffusion, particularly due to the slow diffusion rates of Mo and Nb, which dominate the high-temperature creep behavior. Moreover, STEM-EDS analyses (Figure 8a–d) revealed the segregation of V at the grain boundaries, further enhancing the alloy’s thermal stability and creep resistance. This work provides a fundamental understanding of the creep behavior and deformation mechanisms in nanocrystalline RHEAs, which will aid in the design of advanced, creep-resistant materials. Che-Jen Liu [67] et al. conducted a study on the creep deformation mechanisms of the high-entropy alloy HfNbTaTiZr, specifically below 1250 °C. They employed optical floating zone (OFZ) technology to prepare HfNbTaTiZr with an average grain size greater than 1 mm. The tensile creep tests were performed in a vacuum at temperatures ranging from 1100 °C to 1250 °C, under 5~30 MPa of progressively applied stress. The stress exponent and creep activation energy were determined to be 2.5–2.8 and 273 ± 15 kJ/mol, respectively. The experimental results are shown in Figure 9. The calculated stress exponent indicated a solute drag creep behavior, with the deformation controlled by a/2 <111> dislocations. To further investigate the influence of alloy composition on solute drag creep, they also simulated the intrinsic diffusion coefficients of all elements and compared the theoretical minimum creep strain rate with experimental values. Their analysis showed that the creep rate of HfNbTaTiZr was likely controlled by Ta, which exhibited the lowest intrinsic diffusion rate, thus contributing the most to solute drag and controlling the resistance to dislocation motion. Liu Yang [68] et al. synthesized a 27.3Ta-27.3Mo-27.3Ti-8Cr-10Al high-entropy alloy (HEA) via arc melting and fabricated bulk materials using high-temperature high-pressure sintering. The alloy’s creep behavior was evaluated through compression creep tests at 1000 °C under an applied stress of 125 MPa, revealing a minimum creep rate of ~2.5 × 10⁻⁹ s⁻¹ (Figure 10). Within the temperature range of 1000~1030 °C, the alloy demonstrated excellent creep resistance, primarily attributed to the precipitation strengthening effect of B2 phases and remarkable microstructural stability. At 1000 °C and 1030 °C, the B2 precipitates underwent directional coarsening, forming a raft-like structure akin to those observed in Ni-based superalloys. Notably, the creep resistance of this HEA was comparable to the single-crystal Ni-based superalloy CMSX-4 [69]and even surpassed it under specific conditions. These findings provide critical theoretical and experimental foundations for the development of novel high-temperature alloys.