Introduction

High-entropy alloys (HEAs) were firstly proposed by Yeh and Cantor in 2004 (Ref 1, 2). They had been a new hot topic in metal material field because of their excellent mechanical properties (Ref 3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23). It was an important breakthrough in the traditional metallic materials field; it also opened a new path to study and develop new metal material with outstanding mechanical properties.

For traditional alloy materials, one or two elements were selected as the major component based on the primary property requirement, and other alloy elements were added in the matrix for conferring the secondary properties without altering the primary properties of the matrix (Ref 2). Some similar methods were also used in HEAs field, and some studies had revealed that the addition of alloying elements had positive effect on the microstructure and mechanical properties. For example, Stepanov et al. (Ref 24) had studied the effect of V element on microstructure and mechanical properties of the CoCrFeMnNiVx HEAs and they found that the sigma phase was generated in the matrix, the volume fraction of sigma phase increased with increasing V content, and the strength increased with increasing volume fraction of sigma phase. Wang et al. (Ref 25) investigated the microstructure and mechanical properties of CoCrCuFeNiTix HEAs. They found that the compression strength increased and the magnetic conversion changed from paramagnetism to superparamagnetism as Ti content increased. The correlation between the microstructure and mechanical properties of the (FeCoNiCrMn)100−xAlx HEAs also was presented by He et al. (Ref 26). They found that the phase structure transformed from the FCC to BCC structure and the mechanical properties increased in strength but reduced in ductility with increasing Al content (Ref 26). Li et al. concluded the effect of the Mn, Ti and V addition on microstructure and mechanical properties of AlCrFeCoNiCu HEA, and they found that these three elements had positive effect on phase evolution and performance optimization. The V element showed the best strengthening effect and the lowest decreases in the ultimate strain on AlCrFeCoNiCu HEA (Ref 27). The effect of V element on microstructure and mechanical properties of AlCoCrFeNiVx was reported by Dong et al. (Ref 28). They found that the Vickers hardness increased with an increase in the V content and the fine nanoscale spinodal decomposition microstructure had a major contribution to the high fracture strength of the AlCoCrFeNiV0.2 HEA. Wang et al. reported the effect of Al addition on the microstructure and mechanical properties of AlxCoCrFeNi HEAs. The research showed that the hardness was increased by generating BCC phase, and the volume fraction of BCC phase increased as the Al content increased (Ref 29). Table 1 summarizes the alloy effect of some reported alloy elements on the phase structure and mechanical properties of some HEAs. These reports (24,25,26,27,28,29,30,31,32,33,34,35,36,37,38) focus on the effect of some alloy elements on phase precipitation, microstructure and mechanical properties of some HEAs. They indicated that some elements had positive effect in improving the strength of HEAs. CoCrCuFeNi high-entropy alloys (HEAs) possessed face-centered cubic (FCC) crystal structure, which was relatively weak in strength and had outstanding ductility (Ref 25). V element had positive effect in improving the strength of some HEAs (Ref 24, 28), so it was possible that V had potential in strengthening this CoCrCuFeNi HEA. However, the effect of V addition on the microstructure and mechanical properties of CoCrCuFeNi HEA had not been reported based on the previous reports (Ref 5, 24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39) and this gap should be filled.

Table 1 Effect of some reported alloy elements on the phase structure and mechanical properties of some HEAs
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In this paper, the CoCrCuFeNi HEA (all elements in equal atomic proportions) with a single FCC phase solid solution was selected as matrix. The alloying effect of V element on the phase evolution, microstructure and mechanical properties of the (CoCrCuFeNi)100−xVx (x = 0-16, atomic ratio, hereafter in at.%) HEAs was studied systematically. Furthermore, the relationship between the microstructure and mechanical properties was investigated and the strengthening mechanism of V element on CoCrCuFeNi HEA was also revealed.

Experimental

The ingots of (CoCrCuFeNi)100−xVx (x = 0-16 at.%) HEAs were prepared by arc melting in a copper mold under high-purity argon atmosphere. The metal powders (Co, Cr, Cu, Fe, Ni and V) with purity more than 99% were selected to be the raw material. For the chemical composition uniformity, each ingot was smelted seven times. We used electric discharging machining to cut the ingots into several shapes for detecting. The x-ray diffraction (XRD) using Cu Kα radiation (MXP21VAHF) scanning from 20° to 100° in 2θ at a scanning rate of 4°/min identified the phase structures of the HEAs. The specimens were observed by scanning electron microscopy (SEM) using a Zeiss Supra 55 instrument operated at 15 kV and equipped with an energy-dispersive spectrometer (EDS) to analyze the chemical composition distribution. The SEM samples were grounded, polished and electro-polished in the corrosive liquid of 90% acetic acid and 10% perchloric acid mixture at room temperature and an applied voltage of 27 V for 15 s. The diameter and the height of the compression samples were, respectively, 4 and 6 mm. Compression tests were carried out on an AG–X Plus 250-kN electronic universal material testing machine at a strain rate of 0.5 × 10−3 m/min at room temperature; for each specimen, at least 3 samples were measured to acquire an accurate value.

Results and Discussion

The XRD patterns showed that the sigma phase precipitated in the (CoCrCuFeNi)100−xVx HEAs with increasing V content. The duplex-phase structure had better solid-solution strengthening effect than single-phase structure (Ref 40). As shown in Fig. 1, only patterns related FCC phase structure was detected when V content was 0-12%. The sigma phase precipitated until V content reached 16%. It implied that V addition had positive effect on phase precipitation from FCC phase to sigma phase in the (CoCrCuFeNi)100−xVx HEAs. As shown in Fig. 2, the lattice constant showed a rising tendency when V content increased from 0 to 12%, while the lattice constant decreased when V content increased from 12 to 16%. The main factor was that some atoms were dissolved in this matrix when V content increased from 0 to 12%, which contributed to the increases in lattice constant. When V content reached 12%, the sigma phase was precipitated, which released the lattice distortion energy, so the lattice constant was decreased (Ref 16,17,18, 36).

Fig. 1
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XRD patterns of (CoCrCuFeNi)100−xVx (x = 0, 4, 8, 12 and 16) HEAs

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Fig. 2
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Lattice constant of (CoCrCuFeNi)100−xVx (x = 0, 4, 8, 12 and 16) HEAs attained by XRD

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To analyze the microstructure, the polished samples were observed by SEM and the elements distribution maps were also attained by EDS. Figure 3 shows the microstructures of the (CoCrCuFeNi)100−xVx HEAs. When V content was 0%, as shown in Fig. 3(a), it showed that the matrix possessed a typical dendritic structure, and Cu was found precipitated as second phase in the interdendritic region. The Cu-rich phase changed to the spherical morphology from strip shape when V content increased from 4 to 8% as shown in Fig. 3(b) and (c). When V content reached 12%, the sigma phase was found in (CoCrCuFeNi)88V12 HEA (see Fig. 3d). However, the sigma phase was not tested by XRD for its thin volume fraction. The volume fraction of sigma phase increased with V content increase as shown in Fig. 3(e). Figure 3(f) shows the image of (CoCrCuFeNi)84V16 HEA with high magnification; it shows the sigma and Cu-rich phases clearly.

Fig. 3
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SEM images of the as-cast (CoCrCuFeNi)100−xVx alloys. (a–e represent the SEM images of CoCrCuFeNi (CoCrCuFeNi)96V4, (CoCrCuFeNi)92V8, (CoCrCuFeNi)88V12, (CoCrCuFeNi)84V16 HEAs, and (f) represents the image of (CoCrCuFeNi)84V16 HEA with high magnification), (g) represents the high-magnification image of the interdendritic region in CoCrCuFeNi HEA

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Figure 4 shows the elements distribution images of (CoCrCuFeNi)84V16 HEA by EDS. Table 2 shows the compositions (at.%) of various phases in as-cast (CoCrCuFeNi)100−xVx (x = 0, 4, 8, 12 and 16) HEAs by EDS. They showed that sigma phase was V-rich region and the spheroidal particles were present Cu-rich region, which indicated that V was added in CoCrCuFeNi HEA, which had positive effect in sigma phase precipitation.

Fig. 4
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Elements distribution of (CoCrCuFeNi)84V16 HEA by EDS analysis

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Table 2 Compositions (at.%) of various phases in the as-cast (CoCrCuFeNi)100−xVx (x = 0, 4, 8, 12 and 16) HEAs
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Figure 5 shows the engineering stress–strain curve of (CoCrCuFeNi)100−xVx HEAs measured by compressive text. As shown in Fig. 5, the yield stress of the alloy gradually increased from 300 to 613 MPa with V content increasing from 0 to 16%, while the compression fracture strain decreased from 50 to 28%. When V content was 0, 4, 8 and 12%, respectively, the compressive specimens had no fracture when the strain reached 50%.

Fig. 5
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Compressive engineering stress–strain curves of (CoCrCuFeNi)100−xVx (x = 0-16) HEAs

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Figure 6 shows the effect of sigma phase volume fraction (attained by Grid Statistic Method) on the yield stress and the fracture strain of (CoCrCuFeNi)100−xVx HEAs. It showed that the volume fraction of sigma phase increased gradually from 0 to 12% as the V content increased, the yield stress increased from 300 to 613 MPa, and the compression fracture strain decreased from 50 to 28%. It implied that V addition had positive effect on promoting sigma phase precipitation and improving the strength of CoCrCuFeNi HEA. According to the formula: δ = δfccVfcc + δsigmaVsigma (Ref 42), the plasticity and strength of HEAs could be roughly estimated, where the δfcc and δsigma represent the strength of the FCC phase and sigma phase, respectively, and the Vfcc and Vsigma represent the volume fraction of the two phases, respectively. The volume fraction of the sigma phase (attained by Grid Statistic Method) gradually increased with V content increase in (CoCrCuFeNi)100−xVx HEAs system. Therefore, V addition was beneficial in improving the strength of CoCrCuFeNi HEA.

Fig. 6
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Effect of the volume fraction of the sigma phase (attained by Grid Statistic Method) on the yield stress and the fracture strain of (CoCrCuFeNi)100−xVx HEAs

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In order to reveal the effects of sigma phase on the deformation and fracture process, the microstructural features of the deformation and the fracture-compressed samples were observed by SEM. Figure 7(a), (b) and (c) shows the microstructural features of the deformation (CoCrCuFeNi)92V8 and (CoCrCuFeNi)88V12 HEAs, and Fig. 7(d) shows the compression fracture features of the (CoCrCuFeNi)84V16 HEA. As shown in Fig. 7(a), in the process of compression deformation, the Cu-rich phases were pressed and deformed. However, the sigma phases were not deformed (see Fig. 7b and c). It was indicated that the sigma phases play a key role in improving the strength of this CoCrCuFeNi HEA. As shown in Fig. 7(d), it showed the compression fracture features of the (CoCrCuFeNi)84V16 HEA. The EDS result (Fig. 7d) showed the crack occurs on the V-rich phase (sigma phase). In the process of compression deformation, the Cu-rich phases and matrix phase were deformed; however, the sigma phases do not have enough deformability, breakdown and formation of the cracks. These cracks as a starting point lead to the fracture of the compression specimen.

Fig. 7
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Microstructural features of the deformation HEAs. (a) The microstructural features of the deformation (CoCrCuFeNi)92V8 HEA (backscatter signal), (b) the microstructural features of the deformation (CoCrCuFeNi)88V12 HEA (backscatter signal), (c) the high-magnification microstructural features of the deformation (CoCrCuFeNi)88V12 HEA (backscatter signal), (d) compression fracture features of the deformation (CoCrCuFeNi)84V16 HEA (secondary electron signal)

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For describing the process of sigma phase precipitation more clearly, an atomic diffusion model was established. As shown in Fig. 8, when V content was 0% in (CoCrCuFeNi)100−xVx HEAs, the matrix of the HEA formed FCC phase structure, as shown in Fig. 8(a). When V content was low, V atoms could be dissolved in the matrix, as shown in Fig. 8(b), which caused a large lattice distortion and improved the effect of solid-solution strength. As the V element further increased, more and more atoms were dissolved in the matrix and replaced the matrix atoms. When V content exceeded the limits of it could be dissolved in the matrix, a part of V atoms would be separated out from FCC matrix and formed sigma phase with some matrix atoms as shown in Fig. 8(c).

Fig. 8
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Atomic diffusion model for sigma phase formation in (CoCrCuFeNi)100−xVx HEAs

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Valence electron concentration (VEC) and electronegativity (χ) were important physical parameters for phase formation in HEA field based on these papers (Ref 6, 41,42,43,44,45,46,47,48,49,50). Figure 9 shows the VEC and electronegativity (χ) of (CoCrCuFeNi)100−xVx HEAs. VEC could be attained by this formula: \({\text{VEC}} = \sum\nolimits_{n = 1}^{n} {c_{i} \left( {\text{VEC}} \right)_{i} }\), where the VEC of the ith component element is presented by \(\left( {\text{VEC}} \right)_{i}\) and the atomic percentage of the ith component element is represented by ci (Ref 5, 38, 45,46,47). As shown in Fig. 9(a), the VEC of (CoCrCuFeNi)100−xVx HEAs decreased with increasing V content. The electronegativity (χ) could be calculated by this formula: \(\chi = \sum\nolimits_{n = 1}^{n} {c_{i} (\chi )_{i} }\), where χ of the ith component element is represented by (χ)i and the atomic percentage of the ith component element is represented by \(c_{i}\) (Ref 6, 39). As shown in Fig. 9(b), the value of electronegativity (χ) also decreased with increasing V content. The results indicated that the decreases in VEC and electronegativity (χ) played a positive role in promoting sigma phase formation.

Fig. 9
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(a) VEC of (CoCrCuFeNi)100−xVx HEAs. (b) χ of (CoCrCuFeNi)100−xVx HEAs

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Conclusions

The microstructure and mechanical properties of (CoCrCuFeNi)100−xVx HEAs had been studied in this paper. Based on the experiment and subsequent analysis results, several conclusions could be attained as follows:

  1. 1.

    V addition played a positive role in sigma phase precipitation. The volume fraction of sigma phase increased from 0 to 12% with increasing V content from 0% to 16 at.%.

  2. 2.

    The compressive yield stress of (CoCrCuFeNi)100−xVx HEAs increased from 300 to 613 MPa with V content increasing from 0 to 16%; the compression fracture strain decreased from 50 to 28%.

  3. 3.

    The increases in the sigma phase volume fraction played a key role in increasing the yield stress of (CoCrCuFeNi)100−xVx HEAs.