Organizational Evolution during Performance Meritocracy of AlSi_0.5Cr_xCo_0.2Ni Lightweight High Entropy Alloys

Abstract

The Al-Si-Cr-Co-Ni High Entropy Alloy (HEA) with low density (about 5.4 g/cm³) and excellent performance had significant potential in the lightweight engineering material field. To further research and optimize the Al-Si-Cr-Co-Ni system HEA, the influences of element Cr on the microstructures and performances of lightweight AlSi_0.5Cr_xCo_0.2Ni (in mole ratio, x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs were investigated. The experiment results manifested that AlSi_0.5Cr_xCo_0.2Ni HEAs were composed of A2 (Cr-rich), B2 (Ni-Al), and Cr₃Si phases, indicating that the addition of Cr did not result in the formation of a new phase. However, ample Cr increased the Cr₃Si phase composition, further ensuring the high hardness (average HV 981.2) of HEAs. Electrochemical tests demonstrated that HEAs with elevated Cr₃Si and A2 phases afforded greater corrosion resistance, and the improvement in corrosion was more pronounced when x > 1.6. This work is crucial in the development of lightweight engineering HEAs, which are of tremendous practical utility in the fields of cutting tools, hard coating, etc.

1. Introduction

Traditional alloys (such as iron-based or aluminum-based alloys) are impacted by the inherent property of their single principal elements and cannot meet the requirements of contemporary industrial fields. Consequently, it is worthwhile to examine high entropy alloys (HEAs), a unique material found in the 21st century [1]. HEA is a collective designation for those alloys composed of N (5 ≤ N ≤ 13) elements, with the proportion of each element ranging from 5% to 35% (in mole ratio) [2]. In previous research, HEAs manifested plenty of distinctive characteristics, including high mixing entropy, cocktail effect, hysteresis diffusion, and lattice distortion, which made them exhibit exceptional properties and straightforward microstructures [3,4].

The simplicity of adjusting the composition to achieve the desired performance afforded HEA limitless technical opportunities for the future. Numerous investigations have therefore been undertaken on the various performances of HEAs, such as the great thermal stability [5,6], unique magnetic [7,8], and exceptional mechanical characteristics [9,10]. In various HEA systems, lightweight HEAs were not prevalent, but a rising number of academics demonstrated that they exhibited superior performances. Lightweight HEAs were referred as high entropy alloys whose densities were below 7 g/cm³ [11,12], the lower density and brilliant performances supplied great research significance to them. Mohsen et al. designed the MgAlZnCuMn lightweight HEA with a low melting point and high thermal stability, and they argued that the alloy could be used for light-weight impact-free high temperature or wear-resistant structural applications [13]. Wang et al. investigated the Zr_1.2V_0.8NbTi_xAl_y lightweight HEAs by varying Ti and Al contents, meanwhile revealing that the Ti_3.6Al_0.6 HEA had a higher specific yield strength than that of others [14]. Chen et al. developed the AlTiVNbMo lightweight refractory HEA coating on the surface of the TC4 titanium alloy [15]. In addition, the coating possessed a single Body Centered Cubic (BCC) phase with high hardness. Most studies of lightweight HEAs were focused on their mechanical properties, but fewer scholars investigated the corrosion resistance of lightweight HEAs. While research on lightweight HEAs is still in its infancy, it is crucial to build and improve lightweight HEAs with practical application potential in the realm of engineering materials. According to our previous research [16], AlCrCo_0.2Ni was a BCC HEA with low density (about 5.4 g/cm³) and outstanding corrosion resistance; the addition of doping element Si increased its hardness but diminished its corrosion resistance to some degree. The creation of Cr₃Si hindered the A2 phase from producing a homogenous passivation film, hence decreasing the corrosion resistance of AlSi_xCrCo_0.2Ni HEAs. Thus, further enhancing the performances of Al-Si-Cr-Co-Ni HEAs and investigating the changes of their microstructures were meaningful works for the development of lightweight HEAs.

Previous research has demonstrated that element Cr was critical to the corrosion resistance in many HEAs [17,18,19,20]. In an effort to enhance the corrosion resistance of the Al-Si-Cr-Co-Ni system while preserving the high hardness, AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs have been investigated to optimize the electrochemical corrosion properties in 3.5 wt.% NaCl solution and to examine the microstructures for the lightweight features. This work provides a benchmark for the development of lightweight high entropy alloys and is of major relevance in the field of engineering materials.

2. Experimental

2.1. Materials and Methods

Al, Si, Co, Cr, and Ni raw material particles (purity 99.99 wt.%) for AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs were prepared in accordance with the desired atom ratio, melted, and homogeneously mixed by vacuum arc melting in an argon atmosphere. The ingot was separated into two distinct types, one of which was processed into powder samples and the other into bulk samples (10 mm × 10 mm × 2.5 mm).

2.2. Characterization and Testing

Utilizing Cu Kα radiation (λ = 1.5406 Å) X-Ray Diffractometer (XRD, D8 ADVANCE, Bruker, Karlsruhe, Germany), powder samples were detected. Using a Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM/EDS, MIRA3 LKH, Tescan, Brno, Czech Republic), the microstructure of bulk materials was studied. An Electron Probe Micro-Analyzer (EPMA, JXA-8230, JEOL, Tokyo, Japan) was employed to obtain the precise elemental composition of HEAs. A Vickers hardness tester (TMVP-1, ShiDai, Beijing, China) with a 4.9 N load was employed to determine the hardness of some bulk samples. A three-electrode Autolab (PGSTAT302N, Metrohm, Herisau, Switzerland) was used to conduct electrochemical tests on the specimens. The reference and auxiliary electrodes were Ag/AgCl moreover platinum plates, respectively. Electrochemical Impedance Spectroscopy (EIS) was measured between 10⁵ and 10⁻² Hz. The scanning rate for the potentiodynamic polarization experiment was 1 mV/s, and the scanning range was −0.6 to 1.5 V. We used an X-ray electron spectrometer (XPS, K-Alpha +, Thermo Scientific, Waltham, MA, USA) to determine the valence state on the surface of pretreated samples. To improve the reliability of the experimental data, each electrochemical experiment was repeated three times under identical conditions.

3. Results and Discussion

3.1. Phase Analysis and Microstructures of AlSi_0.5Cr_xCo_0.2Ni HEAs

Figure 1a depicted the phase composition of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs, as determined by XRD investigation. To acquire the changing trend of the strongest diffraction peak in XRD patterns, a portion of the image was enlarged and displayed in Figure 1b. According to Figure 1a, there were several notable distinctive peaks at 27°, 39°, 44°, and 49°, etc., and these peaks may belong to the A2-BCC (Cr-rich), B2-BCC (Ni-Al), and Cr₃Si phases. Due to their greater Peierls-Nabarro force [21], the BCC phase alloys typically exhibited exceptional hardness and strength. Notably, the maxima peak of the three distinct phases had overlapped in certain regions (such as 44° and 65°). This conclusion regarding the structure of HEAs was on the verge of being supported by our previous research [16] and other existing study [22]. AlSi_0.5Cr_xCo_0.2Ni HEAs exhibited no obvious modifications in their XRD patterns as a result of the addition of Cr, nor did they form new phases. Nonetheless, it was evident from the magnified image in Figure 1b that the strength and location of the greatest diffraction peak varied. Due to the enlarged crystal lattice inducing by element Cr, which had the higher atomic radius, some peaks migrated toward lower 2θ values as the Cr content increased [23,24,25].

The changes in the crystallite size and atom percentage of phase composition of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys obtained by Rietveld refinements of XRD patterns are listed in

Table 1. Changes in crystallite size and atom percentage of phase composition of AlSi_0.5Cr_xCo_0.2Ni. (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys.

Cr Atomic Ratio	Crystallite Size (nm)	Atom Fraction of Phase Composition (%)
		B2 (Ni-Al)	A2 (Cr-rich)	Cr3Si
x = 1.0	38.5	51.78	30.01	18.21
x = 1.2	41.0	51.76	25.49	22.76
x = 1.4	44.5	41.32	31.68	27.00
x = 1.6	38.7	38.62	34.53	26.86
x = 1.8	45.2	41.00	30.84	28.17

. The percentage of Cr₃Si phase in the alloys tended toward a fixed value (about 27 at.%), which may be the saturation concentration of Cr₃Si in the AlSi_0.5Cr_xCo_0.2Ni HEAs. Based on our previous study [16], the primary contribution of Cr in Al-Si-Cr-Co-Ni HEA was to create an A2 phase with exceptional corrosion resistance, or to combine it with Si in a ratio of 3:1. After the Cr₃Si percentage neared 27%, it was possible to hypothesize that adding the Cr content boosted the corrosion resistance of the alloys. It may be an effective strategy for optimizing the characteristics of Al-Si-Cr-Co-Ni HEAs. According to Table 1, the crystallite size of alloy increased with the amount of Cr to a degree. When x = 1.8, the average crystallite size of HEAs was the greatest, which could have a detrimental effect on alloy properties.

Figure 2 illustrated the microstructures of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys after polishing and etching. The depictions in the upper right corner of each image are magnifications of the corresponding regions bounded by white lines.

Table 2. EDS analysis data of enlarged images and marked points in Figure 2.

Cr Atomic Ratio	Region	Atom Percent/%
		Al	Si	Cr	Co	Ni
x = 1.0	α	25.72 ± 0.19	15.93 ± 0.15	26.37 ± 0.15	5.74 ± 0.16	26.24 ± 0.26
	1	3.45 ± 0.26	24.29 ± 0.34	65.24 ± 0.56	2.05 ± 0.18	4.98 ± 0.43
	2	35.58 ± 0.47	12.28 ± 0.35	8.06 ± 0.26	7.68 ± 0.42	36.41 ± 0.71
x = 1.2	β	20.06 ± 0.17	16.91 ± 0.15	36.22 ± 0.18	4.78 ± 0.15	22.03 ± 0.25
	3	13.87 ± 0.36	19.84 ± 0.36	50.96 ± 0.51	2.44 ± 0.35	12.89 ± 0.54
	4	1 ± 0.12	23.81 ± 0.30	72.96 ± 0.59	0.76 ± 0.16	1.47 ± 0.17
x = 1.4	γ	20.41 ± 0.17	15.32 ± 0.15	36.85 ± 0.18	4.3 ± 0.15	23.12 ± 0.25
	5	2.87 ± 0.15	23.46 ± 0.32	69.71 ± 0.58	0.97 ± 0.17	2.99 ± 0.39
	6	41.76 ± 0.48	4.09 ± 0.17	5.07 ± 0.24	7.31 ± 0.42	41.77 ± 0.76
x = 1.6	δ	19.78 ± 0.17	13.72 ± 0.14	41.69 ± 0.20	3.83 ± 0.15	20.98 ± 0.25
	7	1.66 ± 0.13	22.42 ± 0.30	73.69 ± 0.60	0.88 ± 0.16	1.35 ± 0.19
	8	25.72 ± 0.43	13.89 ± 0.36	34.71 ± 0.43	3.83 ± 0.38	21.86 ± 0.62
	9	31.74 ± 0.43	5.47 ± 0.29	30.64 ± 0.41	5.31 ± 0.39	26.84 ± 0.66
x = 1.8	ε	19.84 ± 0.17	14.99 ± 0.15	39.72 ± 0.19	4.13 ± 0.15	21.32 ± 0.25
	10	9.14 ± 0.32	22.25 ± 0.37	58.49 ± 0.55	2.37 ± 0.19	7.74 ± 0.49
	11	1.23 ± 0.13	23.1 ± 0.31	74.13 ± 0.61	0.58 ± 0.16	0.96 ± 0.19
	12	42.59 ± 0.46	1.91 ± 0.15	6.94 ± 0.26	6.46 ± 0.42	42.09 ± 0.77

provided the EDS analysis data of enlarged images and the elemental composition of the points designated with red symbols, respectively. In accordance with Figure 2 and our previous study, all specimens in this work still had nanoscale BCC and Cr₃Si phases. With a raising in Cr concentration, no new phases were formed, whereas the dendrites became larger and the Cr₃Si phase increased. Due to its homogeneous distribution, the raised amount of Cr₃Si may enhance the strength of alloys [26,27,28]. Moreover, in some instances, grain size and corrosion resistance exhibited associations [29,30], but the underlying causes of this phenomenon needed further investigation. Point-scanning data listed in the table made it straightforward to identify different phases. According to the table, points 1, 4, 5, 7, and 11 had almost the same chemical composition, these bulging structures may represent the Cr₃Si phase; while points 6 and 12 may be the nanoscale B2 (Ni-Al) phases; points 8 and 10 were the A2 (Cr-rich) phase. Figure 3 displayed EDS elemental mapping pictures of AlSi_0.5Cr_xCo_0.2Ni high entropy alloys (x = 1.0, 1.4, and 1.8). In Figure 3, Al, Co, and Ni tended to be enriched together, whereas Cr and Si preferred to form the Cr₃Si phase, as suggested by previous research [22].

To further obtain the precise elemental compositions of HEAs, EPMA analysis was carried out and is revealed in Figure 4, and the relevant point scanning results were presented in

Table 3. Point scanning results of EPMA relevant to Figure 4.

Points	Al	Si	Cr	Co	Ni
1	26.07 ± 0.37	12.00 ± 0.78	29.90 ± 0.48	4.82 ± 0.57	27.22 ± 0.37
2	7.80 ± 0.68	29.40 ± 0.47	26.89 ± 0.50	9.78 ± 0.39	26.13 ± 0.37
3	32.60 ± 0.33	7.96 ± 0.99	16.32 ± 0.64	6.64 ± 0.47	36.48 ± 0.31
4	39.03 ± 0.31	3.45 ± 1.59	5.24 ± 1.15	7.59 ± 0.45	44.68 ± 0.29
5	24.74 ± 0.41	3.42 ± 1.59	23.36 ± 0.56	4.65 ± 0.60	43.83 ± 0.30
6	0.82 ± 2.59	22.55 ± 0.52	75.19 ± 0.31	0.58 ± 2.17	0.86 ± 2.52

. The results of EPMA were similar to EDS, which indicated that elements in alloys had segregations to varying degrees. Depending to the figure, when x = 1.0, different elements had uniform distribution. However, with the addition of element Cr, the changes of phase fraction led a more noticeable segregation. The distribution of elements Al, Ni and Co changed and resulted in separated regions that were rich and poor in different elements. This was due to the increase of element Cr, which promoted the expansion of A2 phase that can be observed by point 6 in Figure 4c. The homogenous and stable microstructure of AlSi_0.5Cr_xCo_0.2Ni HEAs may contribute to the exceptional overall performances. Thus, it was imperative to further homogenize the microstructure of Al-Si-Cr-Co-Ni HEA in future research.

3.2. Hardness Analysis of AlSi_0.5Cr_xCo_0.2Ni HEAs

Figure 5 illustrated the hardness test outcomes of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys. All the samples had a high hardness value of approximately 950~1000 HV, and the addition of Cr had no significant effect on the hardness of the alloys. The B2 (Ni-Al) phase had excellent mechanical properties and was one of the most promising engineering materials [31,32], whereas the Cr₃Si phase with a complicated microstructure further improved the hardness of the alloys. Thus, the Cr₃Si content of alloys was a critical component in determining their hardness. According to Table 1, the Cr₃Si phase fraction was generally proportionate to the Cr concentration. Excluding the effects of other variables, an abundance of Cr₃Si would significantly enhance the hardness of alloys. However, the expansion of crystallite size may diminish the hardness simultaneously. Thus, when Cr content was increased, the hardness values of the alloys remained approximately constant (average HV 981.2). It was evident from the preceding findings that adding Cr content had no negative effect on the hardness of alloys. AlSi_0.5Cr_xCo_0.2Ni HEAs have exceptional hardness, which made them extremely useful in the fields of cutting tools, hard coating, etc.

3.3. Corrosion Resistance of AlSi_0.5Cr_xCo_0.2Ni HEAs

Figure 6 depicted the potentiodynamic polarization curves of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs in 3.5 wt.% NaCl solution at room temperature. According to the graph, the addition of Cr to the alloys had no appreciable effect on their corrosion resistance, resulting in only minor changes to their electrochemical characteristics. Passivation was still present in all the curves, but it was most pronounced in the alloy with the highest Cr content. It was achieved by Cr’s contribution to a uniform passivation film [33].

Table 4. Corrosion potential and current density of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys.

Cr Mole Ratio	x = 1.0	x = 1.2	x = 1.4	x = 1.6	x = 1.8
Ecorr (VAg/AgCl)	−0.42	−0.45	−0.46	−0.46	−0.39
Icorr (A/cm2)	1.89 × 10−6	1.43 × 10−6	6.86 × 10−7	1.01 × 10−7	3.18 × 10−7

displayed the corrosion potential (E_corr) and the corrosion current density (I_corr) derived from the Tafel polarization curve extrapolation approach. The data demonstrated that the actual situation differed from what was anticipated. Specifically, no linear link existed between the Cr concentration and corrosion resistance. When 1.0 < x < 1.6, the corrosion potential of AlSi_0.5Cr_xCo_0.2Ni HEAs altered slightly, whereas the corrosion current density tended to decrease. Due to the relatively negative mixing enthalpy between Cr and Si [34], the post-added Cr contributed to forming a corrosion-resistant Cr₃Si phase. Despite the fact that Cr₃Si phase reduced the corrosion rate of the alloy, the Cr element in intermetallic compounds (Cr₃Si) could not generate a uniform passivation film. However, the excessive Cr (x = 1.8) increased the percentage of the A2 phase in the HEAs, greatly enhancing the corrosion resistance. The foregoing result demonstrated that increasing the Cr content of Al-Si-Cr-Co-Ni HEAs was an effective way for improving their overall performances. Especially when sufficient amounts of A2 and Cr₃Si phases were present, the corrosion resistance of HEAs would enhance significantly.

Electrochemical Impedance Spectroscopy of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs was shown in Figure 7. The circuit model that best fit the EIS test data was determined and depicted in Figure 7a, where R_s is the resistance of the solution; R_ct is the charge transfer resistance; C_dl is the double-layer capacitance; and the C_dl was replaced by a CPE (Constant Phase Element) [35] expressed by Equation (1).

$${\mathrm{Z}}_{\mathrm{CPE}}=Y_0^{-1}{\left(j\mathsf{\omega }\right)}^{-n}$$

(1)

where Y₀ is the proportionality factor; j is the imaginary unit; ω is the angular frequency; and n is the dimensionless index. Equivalent circuit elements fitting results for EIS data corresponding to AlSi_0.5Cr_xCo_0.2Ni high entropy alloys were listed in

Table 5. Equivalent circuit elements fitting results for EIS data corresponding to AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) high entropy alloys.

Cr Atomic Ratio	Rs (Ω·cm2)	Rct (Ω·cm2)	CPE
			Y0 (snΩ−1m−2)	n
x = 1.0	4.10	2.77 × 105	1.79 × 10−5	0.86
x = 1.2	3.97	3.99 × 105	1.91 × 10−5	0.89
x = 1.4	9.46	4.94 × 105	1.67 × 10−5	0.83
x = 1.6	8.03	1.29 × 106	1.36 × 10−5	0.83
x = 1.8	5.28	5.41 × 105	1.23 × 10−5	0.88

. The EIS results were consistent with the conclusion of potentiodynamic polarization curves, namely that as the Cr concentration raised, the corrosion resistance increased. Nonetheless, the AlSi_0.5Cr_1.6Co_0.2Ni HEA had a greater impedance because of its larger capacitive arc radius (Figure 7a), greater R_ct value, and the highest and the most expansive maximum phase angle (Figure 7b). This may be explained by the larger proportion of A2 phase in the AlSi_0.5Cr_1.6Co_0.2Ni HEA compared to other alloys; a high proportion of A2 phase resulted in a higher impedance and superior passivation for alloys. But the Cr₃Si phase was likewise the most important component in determining the corrosion resistance of alloys [36]. Thus, the most Cr₃Si content in the AlSi_0.5Cr_1.8Co_0.2Ni HEA dominated the corrosion resistance of the alloys, resulting in a more outstanding corrosion potential and corrosion current density for the AlSi_0.5Cr_1.8Co_0.2Ni alloy.

To gain a deeper understanding of the corrosion behavior of AlSi_0.5Cr_xCo_0.2Ni HEAs, corrosion microstructures of the alloys were acquired by SEM and are illustrated in Figure 8. As shown, the corrosion microstructures of AlSi_0.5Cr_xCo_0.2Ni HEAs followed the same trend as our previous research, with the petal like A2 (Cr-rich) phase remaining as the predominant phase in the corrosion microstructure. Figure 8e demonstrated that the volume of bulged Cr₃Si phase increased with the addition of Cr₃Si content. In addition, B2 (Ni-Al) phase was destroyed preferentially by chloride ions, making it harder to locate in images. Compared to Figure 8a and e, the AlSi_0.5Cr_1.0Co_0.2Ni HEA corrosion microstructure was more porous than that of the AlSi_0.5Cr_1.8Co_0.2Ni HEA. It can be extrapolated that, after corrosion, the denser microstructure of HEAs, which contained more Cr, will afford better mechanical properties for them.

XPS measurements have been conducted to evaluate the valence state on the surfaces of AlSi_0.5Cr_xCo_0.2Ni HEAs. Figure 9 and Figure 10 exhibited the fine spectroscopy of each element and their related semi-quantitative analysis, respectively. Less Co contributed to a worse fitting result than other factors. There existed a minor divergence in the binding energy of elements caused by other elements, which was observed [37] in Figure 9. The findings of the XPS study revealed the existence of an inadequate oxidation process. According to Figure 9, different sample fitting peaks of Cr³⁺ were more stable, but Cr⁰ changed obviously, it can be deduced that the post-added Cr crucial affected the corrosion behavior of HEAs. Meanwhile, the growth in element Cr reflected an increase in Cr³⁺ (Cr₂O₃), which improved the corrosion resistance of alloys [38]. The results of XPS confirmed the above expectations, the increment of Cr content enhanced the corrosion resistance of HEAs by improving the passivation behavior, which was an efficient method to optimize the corrosion resistance of Al-Si-Cr-Co-Ni HEA.

4. Conclusions

While ensuring the high hardness as the premise, to further enhance the corrosion resistance of the lightweight Al-Si-Cr-Co-Ni high entropy alloy, the structural evolution and performance characterization of AlSi_0.5Cr_xCo_0.2Ni (x = 1.0, 1.2, 1.4, 1.6, and 1.8) HEAs have been investigated. This work was indispensable to the development of lightweight engineering materials. The principal conclusions are summed up as follows:

(1)

AlSi_0.5Cr_xCo_0.2Ni HEAs were formed of A2 (Cr-rich), B2 (Ni-Al), and Cr₃Si phases; the saturation concentration of Cr₃Si was about 27 at.%, as determined by XRD. The addition of Cr could increase the crystallite size of HEAs.

(2)

With the addition of Cr content in the AlSi_0.5Cr_xCo_0.2Ni HEAs, the dendrites became larger and the Cr₃Si phase increased. Al, Co, and Ni tended to be enriched together, while Cr and Si were easy to form the Cr₃Si phase.

(3)

The hardness test of AlSi_0.5Cr_xCo_0.2Ni HEAs revealed that, with the increase of Cr content, the progressively more Cr₃Si phase and rising crystallite size maintained a high hardness value for HEAs (average HV 981.2).

(4)

The electrochemical investigation demonstrated that increasing the Cr content of Al-Si-Cr-Co-Ni HEAs was an effective way for improving their overall performances, with the premise being that the amount of post-added Cr must exceed a particular threshold.