Effects of Y Additions on the Microstructure and Mechanical Properties of CoCr_1.7Ni Medium-Entropy Alloys

Abstract

In order to improve the room temperature yield strength of X and enhance its engineering applicability, a series of CoCr_1.7NiY_x (x = 0, 0.01, 0.02, 0.03, 0.04, and 0.1 at.%) medium-entropy alloys were synthesized to investigate the effect of Y addition on the microstructures and mechanical properties of the CoCr_1.7Ni-based alloy. The X-ray diffraction results show that the alloys exhibit face-centered cubic (FCC) + body-centered cubic (BCC) + hexagonal close packing (HCP) triphasic structure when the Y is adopted, whereas the CoCr_1.7Ni-based alloy has a FCC+BCC biphasic structure. The volume fraction of BCC and HCP phase increased with increasing Y content, which led to alloy grain refinement. As a result, the microhardness and strength of alloys were both enhanced. The addition of Y resulted in dispersion strengthening and solid solution strengthening of CoCr_1.7Ni alloy, the appearance of HCP, and an increase in BCC, which improved the room temperature yield strength and hardness of CoCr_1.7Ni alloy. In particular, for CoCr_1.7NiY_0.1 alloy, its microhardness and yield strength, respectively, increased by 98.18% and 260.59% as compared with those of CoCr_1.7Ni alloy.

1. Introduction

In recent years, medium-entropy alloys (MEAs) and high-entropy alloys (HEAs) have become a quite popular topic; they contain at least three principal metallic elements rather than only one, as in traditional alloys [1,2,3]. Due to the thermodynamic high-entropy effect, structural lattice distortion effect, kinetic hysteresis diffusion effect, and property “cocktail” effect [4], MEAs and HEAs have excellent properties, such as cryogenic strength and toughness, thermal phase stability, wear resistance, corrosion resistance, and so on [5,6,7,8]. CoCrNi MEAs with single face-centered cubic (FCC) structures have unique low-temperature mechanical properties and outperform most HEAs in terms of strength and plasticity [9,10,11]. Despite this, CoCrNi MEAs struggle to meet the demands of practical engineering applications because of low room temperature yield strength. Therefore, it is of scientific significance to improve the room temperature yield strength of CoCrNi MEAs.

To improve the mechanical properties of MEAs, most researchers generally use the method of changing element contents and adding elements. Our previous work investigated the effects of Cr content on the microstructure and mechanical properties of CoCr_xNi MEAs [12]. The yield strength is enhanced with increasing Cr content due to the precipitated hard phase of Cr-rich BCC precipitated from the FCC phase matrix. In particular, for CoCr_1.7Ni MEA, it has excellent yield strength, and meanwhile maintains relatively good plasticity. Hong et al. [13] add 1.0 at.% Y into CoCrNi MEAs to improve the strength of the alloy, and the yield strength increases to 782.6 ± 22.8 MPa. Lee et al. [14] added 7.5 at.% Al into CoCrNi MEAs to precipitate the NiAl-rich B2 phase from the FCC phase matrix, resulting in significant grain refinement and yield strength increases to 680 MPa. Increasing the Al, Ti, and Ta elements not only promotes precipitation, but also improves the thermal stability of the nanoscale γ-phase and enhances the high-temperature yield strength (700 °C) of (CoCrNi)₉₅Al₂Ti₂Ta MEA up to 620 MPa [15]. Furthermore, elements such as Hf [16], Nb [17,18], Si [19], and Mo [20,21,22] were also used to enhance the mechanical properties of CoCrNi MEAs.

Although the positive effect of the above elements on the mechanical properties of HEAs/MEAs has been confirmed, they can only improve the yield strength to a small extent (most of the improvement rates are limited to 100%); CoCrNi alloys have to face the dilemma of substandard yield strength. The Y element, with a hexagonal close-packed (HCP) structure, is perfectly suited to solve this problem. With increasing Y content, two hexagonal structure phases (CaCu₅ type and Ni₃Y type) precipitated from the FCC phase matrix substantially boosted the yield strength of CoCrFeNiY_x HEAs (from 202 MPa to 1440 MPa, with an improvement rate of 712.87%) [23]. The Y has more outstanding properties for improving the yield strength of HEAs/MEAs than other elements. Y induced the FCC phase precipitatation from the BCC phase matrix, leading to excellent ultimate tensile strength and plasticity of CoCr₃Fe₅NiY_xAl_y HEAs [24]. Furthermore, the ability of Y to improve high-temperature oxidation resistance [25,26] and corrosion resistance [27] has been confirmed.

Therefore, combined with our previous work [12], to further improve the room temperature yield strength of CoCr_1.7Ni MEA and make it more suitable for practical engineering applications, a small amount (~0.1 at.%) of Y was added to CoCr_1.7Ni MEA. The effects of Y content on the crystal structure, microstructure, and room temperature mechanical properties of CoCr_1.7NiY_x MEAs were studied, and the mechanism will be discussed.

2. Material and Methods

2.1. Preparation of CoCr_1.7NiY_x MEAs

CoCr_1.7NiY_x MEAs (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.1) were prepared by a vacuum electric arc melting furnace (WK-II). The raw materials adopted Co, Cr, Ni, and Y metal particles with a purity higher than 99.9%. To ensure all metals were melted completely, the raw materials were placed in the copper crucible in descending order of melting point. The ingot was repeatedly turned over six times under the protection of high-purity argon gas to ensure the uniformity of the alloy composition.

2.2. Composition and Structure Characterization

The crystal structure of the alloys was analyzed using an X-ray diffractometer (XRD, D8ADVANCE-A25, Bruker, Billerica, MA, USA) with Cu-Kα ray radiation operated in the scan 2θ range of 20–100° at a scan rate of 5°/min. Microstructures were characterized by scanning electron microscopy (SEM, FEI Nova Nano SEM450, IOCB Prague, Praha, Czech Republic) equipped with electron backscatter diffraction (EBSD) and an energy-dispersive X-ray spectrometer (EDS) detector, and the volume fraction of the precipitated phase was calculated. All samples for XRD and SEM were sliced to 10 × 10 × 3 mm by wire electrical emancipation machining (WEDM), sanded to 2000 # SiC sandpaper, and polished with diamond polishing paste.

A transmission electron microscope (TEM, Talos F200X, Thermo Fisher Scientific, Waltham, MA, USA) with an EDS detector was used to further observe the microstructure and confirm the elemental distribution. Specimens of 10 × 10 × 0.3 mm were sliced by an EDM wire cutter and polished to 100 μm by SiC sandpaper, after which they were punched into φ3 mm discs and pre-thinned, and finally thinned by a pit thinner and an ion thinner. Finally, the TEM results were analyzed and processed using Digital Micrograph software (Gatan Digital Micrograph basic).

2.3. Mechanical Property Measurements

To evaluate the mechanical properties of the alloys at room temperature, microhardness tests and compression tests were carried out at room temperature. The microhardness tests were performed with a micro Vickers hardness tester (HVS-1000A, Jinan Hensgrand Instrument Co., Ltd., Jinan, China) under a load of 0.5 kg for 15 s. For each sample, 20 points were measured and averaged to ensure the accuracy of the measurement. The samples for microhardness tests were sliced into 10 × 10 × 0.3 mm by WEDM. Compression tests were performed using a universal electronic compression tester with a strain rate of 1 × 10⁻³ s⁻¹. The samples for compression tests were sliced into φ 4 × 6 mm by WEDM.

3. Results and Discussion

3.1. Microstructure and Phase Identification

Figure 1a shows the XRD patterns of CoCr_1.7NiY_x MEAs. To facilitate the observation of the HCP diffraction peaks, the XRD pattern of Y-0.1 alloy was enlarged, as shown in Figure 1b. It can be found that Y-0 alloy without the addition of Y has a biphasic FCC + BCC structure. No new phase precipitation was seen in the XRD pattern after the addition of a small amount of Y; the Y-0, Y-0.01, Y-0.02, Y-0.03 and Y-0.04 MEAs were still FCC + BCC structures. The crystal structure of the alloys transformed from FCC + BCC biphasic structure to FCC + BCC + HCP triphasic structure at x = 0.1; a new third phase was recognized as a hexagonal structure (YNi₅, CaCu₅ type), and the space group of the HCP phase was identified as P6/mm. The peak intensity of the HCP precipitates rose together with the Y content, indicating an increasing volume fraction of precipitates. This indicated that the addition of Y induced the formation of the HCP phase in CoCr_1.7NiY_x MEAs.

The SEM images in Figure 2a–f show the microstructures of CoCr_1.7NiY_x MEAs. In the FCC phase matrix of Y-0 MEA, a small number of BCC phases were scattered (Figure 2a). After adding Y, precipitates appeared in Y-0.01 (Figure 2b) with random distributions at the grain boundaries and inside the grains. Although the atomic radius of the Y element (r Y = 1.80 Å) is larger than those of Co, Cr, and Ni (r Co = 1.25 Å, r Cr = 1.28 Å, r Ni = 1.24 Å), the dissolution of Y in the CoCrNi FCC matrix would cause lattice distortion. Combined with the XRD phase analysis of Y-0.1 MEA, it is inferred that the non-spot precipitates were YNi₅ with an HCP phase structure. This diffraction peak was not reflected in the XRD patterns of Y-0.01, Y-0.02, Y-0.03, and Y-0.04 MEAs, probably due to the small volume fraction of the HCP phase and the limited detection accuracy of the XRD equipment. The volume proportion of both precipitates rose with the rise in Y content, according to SEM analysis.

Figure 3 displays a succession of EBSD maps for Y-0.01, Y-0.02, Y-0.03, Y-0.04, and Y-0.1. The alloy possessed equiaxed grains, but the grain sizes were not consistent. Additionally, the photos showed deformation twinning. Twins typically arise when the energy of the stacking fault is low [28]. The emergence of twin boundaries restricts the dislocations’ motion, which lowers SFE. As a result, the twins in the metal would promote the high yield strength of the alloys. The fraction of twin borders only marginally rose with the rise in Y-doping content, according to the results.

Since no HCP phase diffraction peaks were found in the XRD patterns of Y-0.01, Y-0.02, Y-0.03, and Y-0.04 MEAs, the elemental distribution of Y-0.03 MEA was qualitatively characterized by EBSD and TEM-EDS to further confirm the presence of HCP precipitated phase, and the analytical results are shown in Figure 4a–e and Figure 5a–e, respectively. The variation of precipitation phase volume fraction with Y content is shown in Figure 4f, which increases with the increase in Y content, and the precipitation phase volume fraction gradually increases.

The EBSD images of Y-0.01, Y-0.02, Y-0.03, Y-0.04 and Y-0.1 are presented in Figure 4a–e, with the grain boundaries and precipitates highlighted with black lines and blue, respectively, for a clear view of the grain boundaries and precipitates. It was evident from the illustration that the precipitates (in blue) were dispersed at random along the grain borders and within the grains. Figure 4f depicts the volume fraction of precipitates as a function curve of Y content and it rose as Y content increased, peaking at about 9.97% for Y-0.1. Additionally, a small amount of Cr-rich structure (in yellow) was visible both inside and at the grain borders. The grain size reduces as the Y content increases, as illustrated in Figure 2a–f. This occurrence was brought about by the inhibition of grain formation, which was brought about by the Y element in the solid solution matrix as well as the presence of precipitates [14].

TEM pattern observations were carried out to further investigate the precipitates. A micro-scaled precipitate was seen in the TEM bright-field image of Y-0.03, shown in Figure 5a. The corresponding selected area electron diffraction (SAED) images for the matrix, the precipitate and the Cr-rich organization are shown in Figure 5f–h, respectively. The main diffraction spots along the [$\overline{1}$ 1 2] crystalline band axis (Figure 5f) confirmed the FCC phase structure of the matrix. The main diffraction spots along the [0 0 1] zone axis SAED of the matrix (Figure 5g) confirmed the BCC phase for Cr-rich organization, while the SAED pattern of the precipitate taken along the [$\overline{2}$ 4 $\overline{2}$ 3] zone axis (Figure 5h) confirmed the HCP phase for the precipitate. From the TEM-EDS images in Figure 5b–e, it could be found that the precipitated HCP phase consisted of a large amount of Ni and Y elements, and a small amount of Co element. The Cr-rich organization of the BCC phase structure was distributed at the edge of the HCP phase. In contrast, the Co, Cr, and Ni elements were more uniformly distributed in the FCC phase matrix. Moreover, the lattice constants of the three different phase structures were consistent with the PDF cards, and the results of the TEM analysis of Y-0.03 MEA were also compatible with the XRD and BSE analysis results, which further confirmed that CoCr_1.7NiY_x MEAs are in FCC + BCC + HCP triphasic structure, and the precipitates of two kinds are Cr-rich organization in BCC phase structure and YNi₅ in HCP phase structure, respectively.

3.2. Mechanical Properties at Room Temperature

Figure 6a shows the experimental result of the microhardness of CoCr_1.7NiY_x MEAs at room temperature.

Table 1. The increase in (compared to Y-0) and the averages of microhardness of CoCr_1.7NiY_x MEAs at room temperature.

	Y-0	Y-0.01	Y-0.02	Y-0.03	Y-0.04	Y-0.1
Average microhardness (HV)	172.75	175.47	186.69	216.76	226.66	342.35
Rate of increase (%)	-	1.57	8.07	25.47	31.20	98.17

shows the increase in (compared to Y-0) and the averages of microhardness of CoCr_1.7NiY_x MEAs at room temperature. Compared with Y-0 MEA, the microhardness of Y-0.01 and Y-0.02 MEAs increased slightly (less than 10.00%), the microhardness of Y-0.03 and Y-0.04 MEAs was improved significantly (25–35%), while the microhardness of Y-0.1 MEA increased to 342.358 HV (an increase up to 98.17%). As shown in Figure 6a, the maximum, minimum and range values of the hardness data measured for each alloy also maintained an increasing trend. Since the test sites of microhardness were randomly selected, the hardness measured was relatively low when the test site was on the FCC matrix and relatively high when the test site was on the HCP precipitate. The maximum value of each set of data increased with the increase in Y content, indicating that the increase in Y content raised the volume fraction of the HCP precipitated phases. In this process, the range value of each dataset also increased with the increase in Y content, indicating that the increase in hardness of the HCP precipitated phase is larger than that of the FCC matrix hardness. The above results show that the increase in Y content would increase the microhardness of CoCr_1.7NiY_x significantly through solid solution strengthening and precipitation strengthening, and the contribution of precipitation strengthening to hardness is higher than that of solid solution strengthening in this system. Therefore, we believed that the improvement of the mechanical properties of CoCr_1.7Ni HEA by adding Y was mainly attributed to precipitation strengthening.

Figure 6b shows the compressive stress–strain curves of CoCr_1.7NiY_x MEAs at room temperature. The measured yield strength, ultimate compressive strength and compression are listed in

Table 2. The measured yield strength, ultimate compressive strength, and compression of CoCr_1.7NiY_x MEAs.

	Yield Strength (MPa)	UCS (MPa)	Compression (%)
Y-0	236	-	-
Y-0.01	266	1732	47.5
Y-0.02	307	1485	40.6
Y-0.03	398	1522	39.0
Y-0.04	455	1473	35.9
Y-0.1	851	1170	15.7

. The compressive properties of CoCr_1.7NiY_x MEAs at room temperature were closely related to the amount of Y added, and their yield strength would increase significantly with the increase in Y content. The yield strength of Y-0.01 MEA was increased to 266 MPa, which was 12.71% higher than Y-0 MEA, while the plasticity of the alloys deteriorated. The ultimate compressive strength was 1732 MPa, and the compression decreased to 47.5%. The yield strengths of Y-0.02, Y-0.03, Y-0.04, and Y-0.1 MEAs were further improved with the increase in Y content. In particular, the yield strength of Y-0.1 MEA was up to 851 MPa, which was 260.59% higher than Y-0 MEA. However, the ultimate compressive strength decreased to 1170 MPa, and the plasticity of this alloy decreased much further, with the compression rate falling to 15.7%. From this, it can be seen that with the increase in the amount of Y added, the yield strength increases, but the ultimate yield strength decreases, especially for Y-0.1 MEAs. Moreover, with the increase in the amount of Y added, the deformation of the sample after failure and cracking decreases continuously after reaching the ultimate yield strength, indicating that the plasticity of the alloy decreases with the increase in the amount of Y added.

3.3. Strengthening Mechanism

Since the phase structure has an essential influence on the alloys, it is an important characteristic that affects the microstructure and mechanical properties. Therefore, in this section, the effects of the Y content variation on the mechanical properties of CoCr_1.7Ni MEAs and the corresponding strengthening mechanism are illustrated by analyzing the mechanism of phase structure formation of CoCr_1.7NiY_x MEAs.

Based on the measurement of the hardness of the matrix grains (Figure 6a), both solid solution strengthening and grain boundary strengthening contributed sporadically to the increase in hardness that resulted from the addition of Y to the alloy. It is clear from Figure 6a that the presence of HCP precipitates, whose hardness was significantly higher than that of the FCC matrix, contributed significantly to the hardening. A minor strength improvement of Y-0.01 was seen in the compression test (Figure 6b) compared to the CoCr_1.7Ni MEA, but the alloy’s strength rose dramatically in the presence of precipitates. Therefore, it was evident from both the hardness measurements and the compression tests that precipitation had a significant role in the strengthening and hardening of our system. However, an increase in the volume percent of the HCP phase would lead to a decrease in ductility since the FCC matrix includes more slip systems than HCP precipitates. Therefore, the increased Y content resulted in a significant increase in the strength of CoCr_1.7NiY_x MEAs, but simultaneously, it also led to a deterioration in the plasticity of the alloys. The presence of deformation twins in Figure 3 does help to strengthen, but the inclusion of Y has essentially little impact on the boundary fraction of deformation twins, it should be highlighted. Therefore, we think that precipitation strengthening was mostly responsible for the increase in the mechanical characteristics of CoCr1.7Ni HEA caused by the addition of Y.

The relationship between the microhardness and yield strength of CoCr_1.7NiY_x MEAs and the integral number of HCP precipitates are shown in Figure 7, and a good linear fit is obtained. Compared to the study by Hong et al. [13], we added less Y content but obtained better mechanical properties. Under the same Y content, the microhardness and yield strength were almost twice that of them. Most importantly, we did not conduct subsequent rolling and heat treatment.

The fitting equation for the relationship between microhardness and volume fraction of HCP precipitates is as follows:

$$\mathrm{H}\mathrm{V}=172.7+64.8V_{HCP}-36V_{HCP}^{1.5}+3V_{HCP}^{2.5}-0.27V_{HCP}^3$$

(1)

The fitting equation for the relationship between yield strength and HCP precipitation phase volume fraction is as follows:

$$\mathrm{F}=236.2+137.5V_{HCP}^{0.5}-16.7V_{HCP}-3.4V_{HCP}^{2.5}+1.4V_{HCP}^3$$

(2)

where HV and F are microhardness and yield strength, respectively; V_HCP is the volume fraction of HCP precipitates.

From the above two equations, it can be seen that the relationship between microhardness, yield strength, and volume fraction of HCP precipitates is positively correlated. The addition of Y increases the content of volume fraction of HCP precipitates, thus increasing microhardness and yield strength.

From the crystal structure and microstructure analysis in Section 3.1, it was clear that the volume fraction of the BCC and HCP phase structures increased gradually with the increase in Y content. For multi-component alloys, the Hume–Rothery law is usually used to analyze the mechanism of phase structure formation of the alloys [29,30,31,32,33]. And the main influencing factors include the valence electron concentration ($VEC$), mixing enthalpy (${∆H}_{mix}$), mixing entropy (${∆S}_{mix}$), melting point ($T_m$), relative coefficient of enthalpy–entropy ($\Omega$), atomic size difference ($\delta$), and Gibbs free energy ($∆G_{mix}$). The specific calculation method is shown in Equations (3)–(9).

$$VEC=\sum _{i=1}^n{c}_i{\left(VEC\right)}_i$$

(3)

$${∆H}_{mix}=\sum _{i=1}^{n-1}\sum _{j=i+1}^{n}4{∆H}_{mix}^{ij}{c}_i{c}_j$$

(4)

$${∆S}_{mix}=-R\sum _{i=1}^n{c}_i\ln c_i$$

(5)

$$T_m=\sum _{i=1}^n{c}_i{\left(T_m\right)}_i$$

(6)

$$\Omega ={\displaystyle \frac{T_m{∆S}_{mix}}{\left|{∆H}_{mix}\right|}}$$

(7)

$$\delta =\sqrt{{\sum }_{i=1}^n{c}_i{\left(1-{\displaystyle \frac{r_i}{r}}\right)}^2}$$

(8)

$$∆G_{mix}={∆H}_{mix}-T_m{∆S}_{mix}$$

(9)

In the equation, $c_i$ represents the molar percentage of the ith element, ${\left(VEC\right)}_i$ represents the number of valence electrons of the ith element, ${∆H}_{mix}^{ij}$ represents the mixing enthalpy of the ij two-element alloy system, $c_i$ and $c_j$ represent the molar percentages of the ith and jth elements, R represents the gas constant (8.314 J/mol·K), ${\left(T_m\right)}_i$ represents the melting point of the ith element, $r$ represents the average atomic radius of the alloys, and $r_i$ represents the atomic radius of the ith element. The relevant parameters of CoCr_1.7NiY_x MEAs (

Table 3. Mixing enthalpies, $∆H$ (kJ/mol), for different binary components [34].

Element (Atomic Radius (Å), Melting Point (°C), Valence Electrons)	Co	Cr	Ni	Y
Co (1.26, 1495, 9)	-	−4	0	−22
Cr (1.27, 1907, 6)	-	-	−7	11
Ni (1.24, 1453, 10)	-	-	-	−32
Y (1.80, 1522, 3)	-	-	-	-

) were brought into Equations (3)–(8) to obtain the $VEC$, ${∆H}_{mix}$, ${∆S}_{mix}$, $T_m$, $\Omega$, $\delta$, and $∆G_{mix}$ of CoCr_1.7NiY_x MEAs in

Table 4. The valence electron concentration ($VEC$), mixing enthalpy (${∆H}_{mix}$), mixing entropy (${∆S}_{mix}$), melting point ($T_m$), relative coefficient of enthalpy–entropy ($\Omega$), atomic size difference ($\delta$), and Gibbs free energy ($∆G_{mix}$) of CoCr1.7NiYx MEAs.

	${∆\mathit{H}}_{\mathit{m}\mathit{i}\mathit{x}}$ (kJ/mol)	${∆\mathit{S}}_{\mathit{m}\mathit{i}\mathit{x}}$ (J/mol·k)	$\mathit{\Omega }$	$\mathit{\delta }$/%	$\mathit{V}\mathit{E}\mathit{C}$	${\mathit{T}}_{\mathit{m}}$(K)	$∆{\mathit{G}}_{\mathit{m}\mathit{i}\mathit{x}}$
Y-0	−5.46	8.85	2709	0.98	7.89	1672.94	−14,812
Y-0.01	−5.54	8.98	2713	2.43	7.88	1672.53	−15,028
Y-0.02	−5.61	9.08	2707	3.28	7.87	1672.13	−15,190
Y-0.03	−5.68	9.17	2698	3.95	7.85	1671.73	−15,333
Y-0.04	−5.75	9.25	2687	4.50	7.84	1671.33	−15,462
Y-0.1	−6.16	9.63	2609	6.87	7.76	1668.97	−16,077

.

According to the method proposed by Guo et al. [35] to forecast FCC and BCC phase formation in HEAs by valence electron concentration ($\mathrm{V}\mathrm{E}\mathrm{C}$), when $\mathrm{V}\mathrm{E}\mathrm{C}$ ≥ 8, only the single FCC solid solution structure is formed in the alloys. When $\mathrm{V}\mathrm{E}\mathrm{C}$ ≤ 6.87, only the single BCC solid solution structure is formed in the alloys, and when 6.87 < $\mathrm{V}\mathrm{E}\mathrm{C}$ < 8, both FCC solid solution structure and BCC solid solution structure will be present in the alloys. From Table 4, it can be found that the $\mathrm{V}\mathrm{E}\mathrm{C}$ values of CoCr_1.7NiY_x MEAs were all between 6.87 and 8, which was consistent with the coexistence of FCC and BCC phase structures. Combined with the analysis results of SEM and TEM, it was further confirmed that both FCC and BCC phase structures existed in CoCr_1.7NiY_x MEAs. And with the addition of Y element, a three-phase structure of FCC, BCC, and HCP was formed in CoCr_1.7NiY_x MEAs, further improving the mechanical properties, especially the microhardness and yield strength of CoCr_1.7NiY_x MEAs.

As shown in Table 4, with the increase in Y content, the mixing enthalpy of the alloys kept decreasing, while the mixing entropy kept increasing. The enthalpy–entropy ratio of the alloys kept decreasing, while the atomic size difference kept increasing. In conjunction with [36], a higher enthalpy–entropy ratio promotes the formation of solid solution, while a lower enthalpy–entropy ratio facilitates the formation of intermetallic compounds, such as YNi₅. The mixing enthalpy between the elements (Table 3) can also reflect the binding force between them to some extent [36]. In the Co-Cr-Ni-Y system, the mixing enthalpy between Y and Ni (−32 kJ/mol) is the smallest. Therefore, it can be concluded that Y and Ni are more likely to aggregate and form a Y-Ni enriched phase than other atoms. And the lattice distortion in the alloys is more severe with the atomic size difference increasing. Table 4 shows the Gibbs free energy change trends of CoCr_1.7NiY_x MEAs. It decreases with the increase in Y content, which indicates that the system becomes more and more stable due to the addition of Y.

The above analysis shows that the increase in Y content would promote the enrichment of Y and Ni, which led to the increasing volume fraction of YNi₅ HCP precipitates. According to the TEM-EDS patterns of Y-0.3 (Figure 5b–e), the HCP phase contains only a small amount of Co in addition to Y and Ni. This indicated that YNi₅ replaces the original Cr position and thus promotes the enrichment of Cr to precipitate the BCC phase. Therefore, the volume fraction of the BCC phase also increased with the increase in Y content.

Y has a certain degree of solid solution in CoCrNi MEAs, which means that Y elements with a larger atomic radius (1.80 Å) would displace Co, Cr, and Ni elements, resulting in changes of lattice constants, which in turn causes lattice distortion and solid solution strengthening. With the increase in Y content, the atomic size difference of the alloys was increased, which led to the increasing lattice distortion effect of the matrix of the FCC phase structure. Therefore, the alloys’ matrix increased in hardness and strength under the influence of solid solution strengthening and dispersion strengthening. With the increase in Y content, the effects of solid solution strengthening and dispersion strengthening are more significant; the minimum values of hardness measured by Y-0.03, Y-0.04, and Y-0.1 MEAs showed a significant increase compared to Y-0 MEA (Figure 6a), which also indicated that the solid solution strengthening and dispersion strengthening increases the hardness of CoCr_1.7NiY_x MEAs matrix. However, the solid solution strengthening and dispersion strengthening effects were smaller for small amounts of Y (Y-0.01 and Y-0.02 MEAS), which is due to the fact that solid solution strengthening is not only related to lattice mismatch but also parameters such as shear modulus mismatch [37,38]. And when the Y content is low, the sediment content is also called high, and the dispersion strengthening effect is not significant.

It can be found from Figure 2 that the number of precipitated object integrals in the HCP and BCC phase structures increased with increasing Y content, which led to the significant refinement of the grains of CoCr_1.7NiY_x MEAs. When plastic deformation occurred in fine grains by external forces, it could be dispersed in more grains, and the plastic deformation was more uniform than coarse grains, and the stress concentration was more minor. Fine grains have more curved grain boundaries and larger surfaces, a feature that is not conducive to continuous crack expansion. As a result, the strength of the alloys is continuously increased by the effect of fine grain strengthening. In Figure 2, the Y-0.03 MEA shows a significant grain refinement compared to Y-0.02 MEA, combined with the increase in yield strength of Y-0.03 MEA compared to Y-0.02 MEA of up to 29.64% in Table 2. And the ultimate compressive strength increased instead of decreasing, which indicates that the fine grain strengthening contributed to the strength increase of CoCr_1.7NiY_x MEAs.

The addition of Y promoted the increase in the volume fraction of both precipitates; the interaction between precipitates and dislocations blocked the dislocation motion and improved the deformation resistance of the alloys, so the strength and hardness of the alloys increased significantly with the increase in the volume fraction of precipitates. However, the plasticity of the alloys deteriorated continuously with the increase in the volume fraction of precipitates because the slip system of the HCP phase (one slip plane × three slip directions) is less than that of the FCC phase (four slip planes × three slip directions). In Y-0.1 MEA, the precipitates were uniformly distributed in the matrix as fine diffuse particles (Figure 2f). Compared with Y-0 MEA, its hardness increased up to 98.18%, and the yield strength increased up to 260.59%, indicating that the precipitation strengthening contributed significantly to the hardness and strength improvement of CoCr_1.7NiY_x MEAs. The range value of hardness measured in Figure 7 increased with the increase in Y content, indicating that the hardness enhancement rate of the precipitated phase was much higher than that of the matrix, which also confirmed that the contribution of precipitation strengthening to the hardness of CoCr_1.7NiY_x MEAs was significantly higher than that of solid solution strengthening.

4. Conclusions

In this study, the effects of Y addition on microstructures and room temperature mechanical properties of CoCr_1.7NiY_x MEAs were investigated, and the following conclusions were summarized.

• Without the addition of Y, the Y-0 alloy matrix has an FCC phase structure. The volume fraction of Cr-rich BCC precipitates was lower. The YNi₅ HCP precipitate formed in Y-0.01 MEA. The volume fraction of BCC and HCP phase precipitates increased with Y content, which led to the precipitation and dispersion strengthening of CoCr_1.7NiY_x MEAs.
• Most of the added Y elements were enriched with Ni to form HCP phase YNi₅ precipitates, and the remaining Y elements were solidly soluble in the FCC phase matrix. As the relatively large atomic radius of Y element replaces the position of Co, Cr and Ni elements in the matrix, lattice distortion is caused. At the same time, the degree of lattice distortion increased with the increase in Y addition, resulting in solid solution strengthening effect on CoCr_1.7NiY_x MEAs.
• The addition of element Y significantly improved the strength and hardness of the CoCr_1.7NiY_x MEAs, and the degree of improvement increased with the increase in Y content, and the highest level of improvement is achieved when the Y content is 0.1. The microhardness and yield strength of Y-0.1 MEA reached 342 HV and 851 MPa, respectively; compared with Y-0 MEA, the increase reached 98.18% and 260.59%, respectively. However, the plasticity of CoCr_1.7NiY_x MEAs deteriorated due to the less slip system of the HCP phase.