The Design and Microstructure Evolution Mechanism of New Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2 Eutectic High-Entropy Alloys

Abstract

To expand the fundamental understanding of eutectic high-entropy alloys (EHEAs), three novel alloy systems—Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2—were rationally designed through synergistic phase diagram analysis and thermodynamic parameter calculations. Comprehensive microstructural characterization coupled with mechanical property evaluation revealed that these alloys possess FCC+BCC dual-phase architectures with atypical irregular eutectic morphologies. Notably, progressive microstructural evolution was observed, including amplified grain boundary density and the emergence of brittle nanoscale precipitates. Mechanical testing demonstrated superior compressive yield strengths in these alloys compared to conventional FCC+BCC EHEAs with ordered eutectic structures, albeit accompanied by reduced fracture strain. The Cr_1.3Ni₂TiAl alloy exhibited optimal ductility, with a maximum fracture strain of 15.6%, while V_0.3CoCr_1.2NiTi_1.1Al_0.2 achieved peak strength, with a compressive yield strength of 1389.5 MPa. Multiscale analysis suggests that the enhanced mechanical performance arises from the synergistic interplay between irregular eutectic configurations, expanded grain boundary area, and precipitation strengthening mechanisms.

1. Introduction

Eutectic high-entropy alloys (EHEAs) are produced by mixing four or more elements in specific ratios, enabling coupled growth and solidification of two (or more) phases in the liquid state, ultimately forming a eutectic microstructure [1]. EHEAs possess not only the high-entropy, lattice distortion, cocktail, and sluggish diffusion effects characteristic of high-entropy alloys [2,3] but also exhibit excellent casting properties and enhanced mechanical properties derived from their eutectic nature [4,5]. This dual-phase structure addresses the traditional challenge of achieving a balance between strength and ductility in single-phase high-entropy alloys, and their potential for manufacturing large components presents promising applications in engineering.

EHEAs can be classified into structural EHEAs and functional EHEAs based on their applications [5]. From the perspective of phase structure types, they can be divided into six categories: FCC+BCC [1,6,7], FCC+Laves [8,9], FCC+IMC [10,11], FCC+NMC [12], BCC+B2 [13,14], and BCC+Laves [15,16]. Among these, FCC+BCC EHEAs offer the best combination of mechanical properties, which is more conducive to promoting the application of EHEAs in the field of engineering structures [1,6,7]. Currently, FCC+BCC-type EHEAs primarily include systems such as Co-Cr-Fe-Ni-Al [1,4] and Co-Cr-Fe-Ni-Al-M (M: W [17], Mo [18], Ti [19]). The Co-Cr-Fe-Ni-Al system of EHEAs exhibits a microscopic morphology characterized by a regular eutectic structure, with mechanical properties primarily determined by the coordination relationship at the interface between the two phases [20]. The addition of trace amounts of large atomic radius elements (W, Mo, Ti) can effectively enhance the mechanical properties of EHEAs by increasing the degree of phase structure distortion [17,18,19,21,22]. Among these, the incorporation of W and Mo elements does not alter the regular eutectic morphology [17,18], whereas the addition of the Ti element induces a transformation from a regular eutectic to an irregular eutectic morphology [19,23,24]. The Ti element also tends to segregate at grain boundaries, contributing to the refinement of grain size [25]. Furthermore, the low density of Ti (4.507 g·cm⁻³) can facilitate the expansion and application of such EHEAs in the field of lightweight structures. Consequently, research on EHEAs containing the Ti element has garnered attention from some scholars in recent years.

C.-F. Lee et al. [19] investigated the influence of the Fe element on the microstructure and mechanical properties of Al_0.5CoCrFe_xNiTi_0.5 high-entropy alloys (HEAs). The study revealed that the Al_0.5CoCrFe_2.0NiTi_0.5 alloy system exhibits a three-phase eutectic structure consisting of FCC+BCC+B2. The synergistic effect of the eutectic structure endowed the sample with a compressive yield strength of 866 MPa and a true strain of 0.45. J. Wang et al. [24] investigated the inhibitory effect of the Ti element on crack propagation in CoCrFeNiMnTi_x HEAs during laser remelting. The segregation of the Ti element in the CoCrFeNiMnTi_0.6 alloy promoted the formation of a eutectic structure. The generation of the eutectic phase facilitated the transformation of large tensile residual stresses into compressive stresses, which is beneficial for preventing crack propagation. X. Chen et al. [22] introduced the Ti element into the AlCoCrFeNi_2.1 EHEA, resulting in a gradual transformation of the regular lamellar eutectic structure into an irregular petal-like eutectic structure. The lamellar L1₂ and B2 phases form a strict Kurdjumov–Sachs (K-S) orientation relationship (OR): {111}_FCC//{011}_B2, <011>_FCC//<111>_B2. In contrast, such an OR was not observed in the petal-like microstructure. The Ti_0.15 alloy demonstrated the most excellent combination of high ultimate tensile strength (UTS) (1253 MPa) and ductility (12.9%). J.-Q. Zheng et al. [26] observed similar phenomena in their study on the influence of the Ti element on Al₁₈Co₁₃Cr₁₀Fe₁₄Ni₄₅ EHEAs. S. Li et al. [27] investigated the effect of the Ti element on the microstructure and mechanical properties of AlCrFeNiTi_x EHEAs. Variations in Ti content resulted in four distinct morphologies of net-like eutectic microstructures, with the eutectic structure size gradually increasing. The AlCrFeNiTi_0.2 EHEA exhibited the lowest density (<7 g·cm⁻³), and its specific yield strength reached up to 237 MPa·cm⁻³·g⁻¹.

To summarize, current research primarily focuses on the influence of the Ti element on the microstructure and mechanical properties of EHEAs within fixed systems (such as Al, Co, Cr, Fe, Ni, Mn), yielding numerous valuable and significant findings. However, selecting alloy elements based on practical requirements, combined with rational theoretical design and calculations, to develop lightweight EHEAs containing the Ti element will enhance the tunability of these alloy systems. Our team has previously employed similar methods to develop six types of lightweight EHEAs (V-Co-Cr-Ni-Ti-Al system), demonstrating diversity in microstructure and mechanical properties [28]. Therefore, based on prior research, this study will further expand the database of V-Co-Cr-Ni-Ti-Al system EHEAs and investigate the coupling relationship between microstructure and mechanical properties under the synergistic effects of Ti and other elements.

2. Materials and Methods

2.1. Theoretical Calculations and Phase Diagram Simulations

In this study, V-Co-Cr-Ni-Ti-Al alloy systems were selected based on previous design concepts [28]. Combining the basic physical parameters of the alloying elements (

Table 1. The basic physical parameters for the elements [29].

ΔHmix (kJ·mol−1)	V	Co	Cr	Ni	Ti	Al
	-	−14	−2	−18	−2	−16
		-	−4	0	−28	−19
			-	−7	−2	−10
				-	−35	−22
					-	−30
						-
r (Å)	1.35	1.25	1.29	1.25	1.47	1.43
T (K)	2163	1768	2130	1728	1941	933
χ	1.63	1.88	1.66	1.91	1.54	1.61
VEC	5	9	6	10	4	3

[29]), the thermodynamic empirical parameters of the alloy system were calculated using Equations (1)–(6) [28,30]. By compiling research data on HEAs, the range of physical parameters satisfying the FCC+BCC dual-phase structure is summarized in

Table 2. Criteria for phase structure evaluation of HEAs.

Physical Parameters	Range	Structure
∆Smix (J·K−1·mol−1)	12 ≤ ∆Smix ≤ 17.5	Solid solution
∆δ (%) [31,32]	<6.5	Solid solution
∆Hmix (kJ·mol−1) [31,33]	−22 ≤ ∆Hmix ≤ 5	Solid solution
Ω [33]	≥1.1	Solid solution
Δχ (%) [34,35,36]	≤17.5	Solid solution
VEC [37,38]	VEC < 6.87	BCC Solid solution
	6.87 ≤ VEC < 8.0	FCC+BCC Solid solution
	VEC ≥ 8.0	FCC Solid solution

[31,32,33,34,35,36,37,38]. Based on the reported research findings on EHEAs since the concept was introduced by the scholar Y.-P. Lu in 2014 [1], and incorporating the theoretical studies on eutectic formation by Chanda et al., it was found that the thermophysical parameters required to form EHEAs satisfy the following: −18 kJ·mol⁻¹ < ΔH_mix < −6 kJ.mol⁻¹, Δδ > 3% and 6 < VEC < 8.5 [30,39,40].

$$\Delta {\mathrm{S}}_{\mathrm{m}\mathrm{i}\mathrm{x}}=-\mathrm{R}{\sum }_{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{c}}_{\mathrm{i}}\mathrm{l}\mathrm{n}{\mathrm{c}}_{\mathrm{i}}$$

(1)

$$\Delta {\mathrm{H}}_{\mathrm{m}\mathrm{i}\mathrm{x}}=\sum 4\Delta {\mathrm{H}}_{\mathrm{i}\mathrm{j}}^{\mathrm{m}\mathrm{i}\mathrm{x}}{\mathrm{c}}_{\mathrm{i}}{\mathrm{c}}_{\mathrm{j}}$$

(2)

$$\mathsf{\Omega }={\displaystyle \frac{{\mathrm{T}}_{\mathrm{m}}\Delta {\mathrm{S}}_{\mathrm{m}\mathrm{i}\mathrm{x}}}{\left|\Delta {\mathrm{H}}_{\mathrm{m}\mathrm{i}\mathrm{x}}\right|}}$$

(3)

$$\Delta \mathsf{\delta }=\sqrt{\sum {\mathrm{c}}_{\mathrm{i}}{(1-{\mathrm{r}}_{\mathrm{i}}/\overline{\mathrm{r}})}^2}$$

(4)

$$\Delta \mathsf{\chi }=\sqrt{\sum {\mathrm{c}}_{\mathrm{i}}{({\mathsf{\chi }}_{\mathrm{i}}-\overline{\mathsf{\chi }})}^2}$$

(5)

$$\Delta \mathrm{V}\mathrm{E}\mathrm{C}=\sum {\mathrm{c}}_{\mathrm{i}}{\mathrm{V}\mathrm{E}\mathrm{C}}_{\mathrm{i}}$$

(6)

where, R is the gas constant; c_i is the atomic fraction of the ith element; $\Delta {\mathrm{H}}_{\mathrm{i}\mathrm{j}}^{\mathrm{m}\mathrm{i}\mathrm{x}}$ is the mixing enthalpy between the ith and jth elements; T_m is the average melting point temperature; Δδ, r_i and $\overline{\mathrm{r}}$ are the atomic size difference, atomic size of the ith element, and the average atomic radius, respectively; χ_i and $\overline{\chi }$ are the valence electron concentrations of the ith element and the average valence electron concentration; and VEC_i is the electron concentration of the ith element [28].

Thermo-Calc software 2024b [41], combined with the TCHEA4 database, was used to perform thermodynamic calculations on the evolution of equilibrium phases and potential precipitates. Solidification calculations were conducted under the Scheil model assumption and the classic Scheil–Gulliver conditions (assuming infinite diffusion in the liquid, no diffusion in the solid, and thermodynamic equilibrium at the interface). Solidification simulations for solute trapping were carried out under Scheil–Gulliver conditions.

2.2. Raw Material Alloying Process

V-Co-Cr-Ni-Ti-Al EHEAs were prepared and screened using a GDHL-500A vacuum arc melter (Chengdu Jituo Instruments & Equipment Co., Ltd., Chengdu, China), which also verified the design scheme’s accuracy. High-purity elemental constituents (V, Co, Cr, Ni, Ti, Al ≥ 99.9 wt.%) were arc-melted in a water-cooled copper crucible under precisely controlled parameters: a current intensity of 250–400 A; an applied voltage of 30–45 V; a power input of 7.5–18 kW; and a chamber vacuum of <5 × 10⁻³ Pa. Ingots were remelted five times to ensure the chemical homogeneity of the alloys. To compensate for elemental burning loss during melting, the alloy formulation strategically incorporated compensation ratios: chromium was overcharged by 0.2 wt.% and aluminum by 0.1 wt.% relative to their theoretical stoichiometric ratios, while other alloying elements were maintained at nominal compositions. This protocol ensures precise alignment between the final alloy composition and the original design specifications.

2.3. Microstructural Characterization

Wire cutting was used to prepare samples with dimensions of 10 mm × 10 mm × 5 mm for phase structure and microstructure characterization, followed by grinding and polishing of the samples. The phases were identified using a D8 advanced X-ray diffractometer (XRD, BRUKER AXS GMBH, Billerica, MA, USA, 40 kV, 40 mA, Cu Kα radiation, 4°.min⁻¹ scanning speed). The metallographic specimens were etched with Kroll’s reagent—a precisely formulated solution comprising 2 vol% HF, 6 vol% HNO₃, and 92 vol% deionized H₂O—employing latex-free rubber droppers for controlled etchant application to ensure uniform surface reaction kinetics while preventing metallic contamination. After 40 s, it was rinsed thoroughly with pure water and then alcohol. The microstructures were characterized using a HITACHI-S4700 scanning electron microscope (SEM, HITACHI, Tokyo, Japan). Energy dispersed X-ray spectrometry (EDS) was used to measure the content and distribution of alloying elements in the EHEAs. A 10 mm × 10 mm × 0.5 mm thin sheet was polished with sandpaper to a thickness of 50–100 μm, and then ion thinning technology was used to obtain a sample with a thickness of 20 μm. An FEI TECNAI F30 transmission electron microscope (TEM, Royal Philips, Amsterdam, The Netherlands) was used to obtain magnified electron images, the distributions of elements and selected area diffraction patterns.

2.4. Mechanical Property Testing

A cylindrical sample with a diameter of 4 mm and a height of 6 mm was used for compression testing. The compression test was conducted at room temperature using an Instron 5969 universal testing machine, with a strain rate of 1 × 10⁻³ s⁻¹. The compressive properties were measured twice to calculate the average value. After compression, the fracture morphology was observed using SEM.

3. Result and Discussion

In the preliminary stage, the team first employed thermodynamic parameter calculations for initial screening to narrow down the range of alloy systems, followed by phase diagram calculations to determine the eutectic composition range, thereby achieving precise design and screening of EHEAs [28]. However, significant discrepancies exist among the criteria for solid solution HEAs (blue dashed line), the theoretical criteria for FCC+BCC EHEAs (red dashed line), and the literature data on FCC+BCC EHEAs (hollow points), as illustrated in Figure 1. Therefore, this study focused on the V-Co-Cr-Ni-Ti-Al alloy system and, after preliminary thermodynamic parameter calculations, appropriately expanded the screening range to avoid omissions.

Thermo-Calc software was utilized to calculate the solidification path diagrams for the alloy systems screened in the first step. Combined with the “lever rule” [42] in EHEAs, the EHEA systems were more precisely identified. Through preliminary screening, three groups of EHEAs were obtained: Cr_1.3Ni₂TiAl (1#), CoCr_1.5NiTi_1.5Al_0.2 (2#), and V_0.3CoCr_1.2NiTi_1.1Al_0.2 (3#), with the specific compositions of alloying elements listed in

Table 3. Element compositions of the three new EHEAs (wt.%).

		Co	Cr	Ni	Ti	V	Al
Cr1.3Ni2TiAl	Theoretical value	-	24.5	37.7	18.9	-	18.9
	Actual value	-	26.1	38.7	16.8	-	18.4
CoCr1.5NiTi1.5Al0.2	Theoretical value	19.2	28.8	19.2	28.8	-	3.8
	Actual value	20.7	29.2	20.3	26.1	-	3.7
V0.3CoCr1.2NiTi1.1Al0.2	Theoretical value	20.8	25.0	20.8	22.9	6.3	4.2
	Actual value	21.9	26.1	21.3	20.3	6.6	3.8

. The alloy element contents in the samples, as detected by SEM-EDS, were in good agreement with the theoretical calculations. The thermodynamic parameter calculation results for the three groups of EHEAs are summarized in Figure 1 (pink points), further illustrating the limitations of traditional HEA phase structure criteria and EHEA criteria.

Figure 2a–c shows the solidification path diagrams for the three new EHEAs based on the Scheil model. The initial solidification temperatures for the alloys Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2 were all above 1200 °C, and the final solidification temperatures were all above 1000 °C. The actual final solidification temperature was lower than the equilibrium solidification temperature due to the potential occurrence of undercooling during the actual solidification process, which led to the formation of the non-equilibrium phases in the alloy, reducing the final solidification temperature. The alloys Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2 exhibit solidification temperatures of 1232 °C, 1246 °C, and 1362 °C, respectively. The phase structure of the alloy Cr_1.3Ni₂TiAl consisted of FCC+BCC phase structures, while the alloys CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2 consisted of FCC+BCC+HCP phase structures. The XRD results of the three groups of EHEAs are shown in Figure 2d–f, and are largely consistent with the phase diagram calculations.

The microstructures of the three new EHEAs were shown in Figure 3. The eutectic structure of the alloy Cr_1.3Ni₂TiAl exhibited a feather-like appearance, while the eutectic structures of the alloys CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2 appeared petal-like, with the grain boundary regions gradually increasing in size. The microstructures of the three novel EHEAs were fully eutectic, validating the accuracy of the design in this study. The three alloy groups exhibited irregular eutectic microstructures, rendering the quantitative characterization of the eutectic structure dimensions challenging. However, qualitative observations revealed that the eutectic structures systematically underwent progressive coarsening with increasing multiplicity of the principal alloying elements. The elemental composition of the eutectic structures in the three groups of samples was examined using SEM-EDS, and the results are presented in

Table 4. Element compositions of the eutectic structures in the new EHEAs (wt.%).

	Co	Cr	Ni	Ti	V	Al
Point 1	-	94.04	2.24	2.56	-	1.14
Point 2	-	4.42	62.21	21.08	-	12.27
Point 3	3.81	91.80	1.13	3.21	-	0.02
Point 4	27.69	7.12	31.03	31.80	-	2.34
Point 5	9.78	72.79	3.05	2.61	11.48	0.28
Point 6	30.44	6.87	28.98	25.16	5.36	3.15

. In the microstructural morphology of the three EHEAs, the phase with higher potential in the eutectic structure was predominantly composed of the Cr element (Point 1, 3, and 5). In the Cr_1.3Ni₂TiAl sample, the low-potential phase was mainly composed of the Ni, Ti, and Al elements (Point 2). In both the CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2 samples, the low-potential phases were primarily composed of the Co, Ni, and Ti elements (Point 4 and Point 6).

Figure 3 effectively validated that the three new EHEAs were successfully designed and fabricated, with the main differences in their microstructures reflected in the eutectic structure and grain boundary size. Therefore, TEM characterization was used in this study to conduct an in-depth analysis of these two regions, as shown in Figure 4 and Figure 5. The eutectic structures of the three EHEAs primarily consisted of the FCC and BCC phases, with the BCC phase (Points A, D, G) enriched in the Cr element. The FCC phase (Point B) in the Cr_1.3Ni₂TiAl alloy exhibited significant enrichment with Ni, Ti, and Al, accompanied by the formation of nanoscale precipitates (10–20 nm in diameter) with lattice parameters and elemental partitionning behavior identical to those of the ordered FCC matrix phase_. The FCC phase (Point E) of CoCr_1.5NiTi_1.5Al_0.2 was enriched in the Ni, Ti, and Co elements. Although the lattice structure and elemental distribution of the nanoscale precipitates were also similar to the matrix phase, their size (50–60 nm in diameter) was larger compared to those in Cr_1.3Ni₂TiAl. The FCC phase (Point H) of V_0.3CoCr_1.2NiTi_1.1Al_0.2 was also rich in the Ni, Ti, and Co elements, but no nanoscale precipitates were observed.

The grain boundaries of the three new EHEAs were primarily composed of the FCC and BCC phases, with the FCC phase being significantly more abundant than the BCC phase, as shown in Figure 5. The FCC matrix phase of Cr_1.3Ni₂TiAl contained nanoscale precipitates (Point C), which, based on elemental distribution, could be identified as an ordered FCC phase, similar to the nanoscale precipitates in the FCC phase of the eutectic structure. In the FCC matrix phases of both CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2, two types of nanoscale phases precipitated: a spherical nanoscale phase (Points F, J) and a needle-like nanoscale phase (Points G, K). The spherical nanoscale phase was mainly composed of Cr, similar to the BCC matrix phase. The needle-like nanoscale phase was rich in Ni, Ti, and Co, and based on the elemental content of these three elements, this precipitate was identified as Ti(Ni,Co)₃ (a hexagonal close-packed structure), which aligns with the phase diagram calculations. The Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2 systems exhibited spherical nanoscale precipitates with distinct size distributions of 10–20 nm, 50–70 nm, and 100–150 nm, respectively. Needle-like phases in the CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2 systems displayed lengths of 100–200 nm and 200–1000 nm, respectively, with aspect ratios exceeding 10:1.

The compression performance test results for the three new EHEAs are shown in Figure 6a,b. The compressive yield strength of Cr_1.3Ni₂TiAl exceeded 1100 MPa, with a compressive strain close to 16%. As the number of principal elements increased (CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2), the yield strength of the samples gradually increased, but the compressive strain gradually decreased. The compressive yield strength and fracture strength of the three EHEAs (especially V_0.3CoCr_1.2NiTi_1.1Al_0.2) were superior to existing FCC+BCC EHEAs (Figure 6d), although their compressive strain was lower (Figure 6c). The fracture surfaces of the three EHEAs displayed cleavage patterns characteristic of brittle fractures. However, a eutectic fracture morphology was visible in the fracture surface of the sample of Cr_1.3Ni₂TiAl (Figure 6e), with this morphology reduced in the sample of CoCr_1.5NiTi_1.5Al_0.2 (Figure 6f), and entirely consisting of cleavage patterns in the sample of V_0.3CoCr_1.2NiTi_1.1Al_0.2 (Figure 6g).

In summary, the addition of Ti elements increases the sensitivity of the FCC phase-to-temperature gradient during the growth of EHEAs, resulting in an irregular growth morphology [43,44]. The BCC phase structure grows along with the FCC in a coupling growth mode, and finally forms the irregular FCC+BCC-type EHEAs. The irregular eutectic morphology will cause the atomic coordination relationship at the interface of the FCC and BCC phase to be disordered, which is different from most FCC+BCC-type EHEAs at present. The disordered atomic coordination relationship at the interface will increase the resistance of dislocation passing through the interface during the mechanical property testing process, thus improving the strength but decreasing the plasticity. With the increase in the number of principal alloying elements, the grain boundary area of the EHEAs increased significantly, accompanied by the precipitation of spherical nanoscale FCC phases and needle-like nanoscale Laves phases within the FCC matrix. Notably, the size of these nanoscale precipitates progressively enlarged. The combined effects of enhanced grain boundary density, coarsening of nanoscale precipitates, and increased content of brittle nanoscale precipitates deteriorated the ability of dislocations to pass through these barriers during plastic deformation. Consequently, this microstructural evolution led to a gradual enhancement in strength-related properties of EHEAs, while simultaneously causing a progressive reduction in ductility indicators.

4. Conclusions

(1) A phase-diagram-aided thermodynamic parameter calculation method successfully designed three FCC+BCC-type EHEAs: Cr_1.3Ni₂TiAl, CoCr_1.5NiTi_1.5Al_0.2, and V_0.3CoCr_1.2NiTi_1.1Al_0.2.

(2) In the eutectic microstructure of the Cr_1.3Ni₂TiAl and CoCr_1.5NiTi_1.5Al_0.2 EHEAs, ordered FCC nano-precipitates were formed within the FCC phase, whereas no such nano-precipitates were observed in a similar region of the V_0.3CoCr_1.2NiTi_1.1Al_0.2 EHEA. Additionally, ordered FCC nano-precipitates were also formed in the grain boundary regions of the FCC phase in the Cr_1.3Ni₂TiAl EHEA, while BCC- and HCP-structured nano-precipitates were observed in the corresponding regions of the CoCr_1.5NiTi_1.5Al_0.2 and V_0.3CoCr_1.2NiTi_1.1Al_0.2 EHEAs. Furthermore, both the grain boundary regions and the size of the nano-precipitates increased with the number of principal elements in the EHEAs.

(3) The compressive strength of the three EHEAs gradually increased, while the compressive strain decreased. The eutectic structure with an irregular morphology, the gradually increasing grain boundary size, and the increasing content of the brittle nano-precipitated phase in the FCC base phase at the grain boundary were the main reasons for the mechanical properties.