Effect of W Contents and Annealing Temperatures on the Microstructure and Mechanical Properties of CoFeNi Medium Entropy Alloys

Abstract

In this work, the W element, with a larger atomic radius compared to Co, Fe, and Ni, was added to modify the microstructure and enhance the yield strength of CoFeNi medium entropy alloy (MEA). A detailed study was conducted to clarify the effects of W additions and annealing temperatures on the microstructure evolution and mechanical properties of CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs. CoFeNiW_0.1 retained a single FCC structure without the formation of precipitates in the FCC phase, indicating that W, with a larger atomic radius, can completely dissolve in CoFeNiW_0.1. For CoFeNiW_0.3 MEA, coarse particles with an average diameter of ~2 μm appeared after homogenizing. Nevertheless, when the alloy was annealed at 800 °C and 900 °C, fine particles formed, with the average diameters of approximately 144 nm and 225 nm, respectively. After annealing at 800 °C, the CoFeNiW_0.3 with a partially recrystallized microstructure exhibited better comprehensive mechanical properties.

1. Introductions

High entropy alloys (HEAs) and medium entropy alloys (MEAs), also known as multi-principal elements alloys, largely broaden the realm of alloy design and offer more choices for fabricating high-performance structural materials [1]. Yeh et al. pointed out that larger mixing entropy enabled HEAs and MEAs to exhibit a single-phase structure [2]. HEAs and MEAs with a single face-centered cubic (FCC) structure were widely studied due to their lower yield strength and excellent work hardening capacities [3,4,5]. CoFeNi with a single FCC structure can show a yield strength of ~180 MPa and an ultimate tensile strength of 442 MPa with an outstanding elongation of ~50% [6], indicating it is necessary to enhance the yield strength of this MEA.

L1₂-type coherent phase can significantly strengthen FCC-structured MEAs while preserving good ductility, which has become a research hotspot recently. CoFeNi MEAs strengthened by coherent L1₂ phase were reported in Ti/V, Ti, V, or Ti/Al-added CoFeNi alloy systems [6,7,8,9]. Wang et al. investigated L1₂-type (Ni, Co, Fe)₃(Ti, V) nanoparticles reinforced CoFeNi MEA. The results suggested that (FeCoNi)₈₄Ti₈V₈ can show a higher yield strength of ~950 MPa and an ultimate tensile strength of ~1394 MPa, while maintaining good ductility of ~25% [6]. L1₂-type Ni₃Ti-strengthened CoFeNi was fabricated by alloying Ti, (FeCoNi)₉₄Ti₆ MEA exhibited an outstanding combination of mechanical properties, featuring a yield strength of 893 MPa, an ultimate tensile strength of 1263 MPa, and an elongation of 24% [7]. L1₂ phase, coherent with the disordered FCC phase, was found in (FeCoNi)₈₀V₂₀ MEA. This MEA displayed a yield strength of 600 MPa and had excellent work hardening capacity, leading to an ultimate tensile strength of 1100 MPa [8]. Yang et al. reported the coherent strengthening by the L1₂-type Ni₃Al in (FeCoNi)₈₆Al₇Ti₇ led to exceptional mechanical properties, including a yield strength of 1 GPa, an ultimate tensile strength of 1.5 GPa, and a tensile elongation of ~50% [9]. The aforementioned investigations have indicated L1₂-type coherent phase is effective in improving strength while maintaining good ductility. Nevertheless, the requirements for composition modulation are more stringent to avoid the detrimental brittle L2₁ phase [10].

Furthermore, Prof. C.T. Liu et al. previously demonstrated that the addition of refractory elements with larger atomic radius into the FCC lattice would induce lattice distortion. As a result of this, the lattice distortion energy can be produced, which would destabilize the FCC matrix and promote the appearance of a second phase [11]. W element with a larger atomic radius than transition elements, such as Co, Fe, and Ni, has been added to modify the microstructure and mechanical properties [12,13]. Chang et al. reported that doping 3 at.% W into CoCrNi led to an enhancement of yield strength of approximately 33% while maintaining good ductility [12]. By adding more W, μ phase appeared in CoCrNi MEA, causing the yield strength to increase from 427 MPa to 1356 MPa, with a good elongation of ~10%. The improvement of yield strength was attributed to enhanced grain refinement strengthening and precipitation strengthening [13]. Adding W into MEAs can display better mechanical properties, which can be an effective method to modify FCC-structured MEAs.

In this work, W was selected to be incorporated into FCC-type CoFeNi with varying contents. Meanwhile, a detail investigation was conducted to elucidate the effects of annealing temperatures on the microstructure evolution and mechanical properties of cold-rolled CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs. Our study could provide a better understanding of W-added MEAs and pave the way to achieve strength-ductility balance.

2. Experiment Methods

CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs were prepared by melting Co, Fe, Ni, and W pure metals with the purity of ≥99.9 wt.% using the arc-melting technique under Ti-gettered Ar atmosphere. The ingots were melted at least five times to ensure chemical uniformity, and then were cast into a Cu mold with the dimensions of ~60 mm × 11 mm × 5 mm. The obtained specimens were sealed in quartz tubes and homogenized at 1150 °C for 22 h, subsequently followed by water quenching, which was cold-rolled to a thickness reduction of ~70%. The samples taken from cold-rolled CoFeNiW_x MEAs were sealed in quartz tubes and subsequently heat-treated at 700 °C, 800 °C, and 900 °C for 1 h, respectively, followed by water quenching.

The crystal structures of annealed CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs were analyzed by an Empyrean X-ray diffractometer (XRD, PANalytical B.V., Almelo, The Netherlands) at a range of 30°~100° with a step size of 0.039391°. The microstructure was characterized by a scanning electron microscope (SEM, Zeiss SUPRA 55, Oberkochen, Germany) under backscattered electron (BSE) mode, and a transmission electron microscope (TEM, JEOL JEM-2100F, Tokyo, Japan). The electron backscattered diffraction (EBSD) samples were prepared by mechanical polishing followed by ion etching (Gatan Ilion II 697, Pleasanton, CA, USA). In addition, the EBSD measurements were carried out by an EBSD detector (Oxford NordlysMax², Oxford, UK) equipped on the SUPRA55 SEM. TEM samples were firstly ground to ~50 μm, then punched into disks with a diameter of 3 mm and finally thinned by ion milling (Gatan 695, Pleasanton, CA, USA). Dog-bone-shaped samples of ~1.5 mm × 12.5 mm × 3.2 mm were obtained by an electro-discharge cutting machine (Guangdong Datie Numerical Control Machinery Co., Ltd., DK350, Guangzhou, China). The obtained tensile samples were mechanical ground by SiC paper to remove oxide layers and tested at a strain rate of 1 × 10⁻³ s⁻¹. An extensometer with a gauge length of 10 mm was used during the elastic deformation process.

3. Results

3.1. Crystal Structures

Figure 1 depicts the XRD patterns of cold-rolled CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs after annealing at 700 °C, 800 °C, and 900 °C for 1 h. The XRD patterns of as-cast CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs were also given in the Supplementary Material (as shown in Figure S1). As-cast CoFeNi and CoFeNiW_0.1 exhibited a single FCC phase. After cold-rolling and annealing, a single FCC solid solution phase formed in CoFeNiW_x (x = 0, 0.1) without observing obvious additional peaks in the XRD patterns. For x = 0.3, only FCC phase peaks were observed from the XRD pattern of as-cast CoFeNiW_0.3 MEA (as depicted in Figure S1). After cold-rolling and annealing at 900 °C, these minor peaks appeared in CoFeNiW_0.3 MEA, probably corresponding to Fe₇W₆-type μ phase (JCPDS card #20-0538) in additional to FCC phase peaks. Nevertheless, no obvious minor peaks can be found in CoFeNiW_0.3 MEA after annealing at 700 °C and 800 °C, possibly because of the lower volume fraction of the intermetallic phase in the alloy. Moreover, the lattice constants (a) for CoFeNiW_x alloys (as shown in Figure 1d) were calculated to be 3.5820 Å, 3.5911 Å, and 3.6140 Å, respectively, which increased with W additions due to the enhanced lattice distortions caused by W dissolutions into the FCC phase.

3.2. Microstructure Analysis

The microstructure under BSE mode of as-cast and annealed CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs is depicted in Figure S2 and Figure 2. From Figure S2, a single FCC solid solution phase formed with no distinct chemical segregations in all as-cast MEAs. After cold-rolling and annealing at 700 °C, 800 °C, and 900 °C, CoFeNi and CoFeNiW_0.1 still retained a single FCC structure without forming precipitates in the FCC matrix. W with a larger atomic radius can completely dissolve in CoFeNiW_0.1 MEA. Unlike as-cast CoFeNiW_0.3 with a single FCC structure, coarse particles (denoted by red arrows) with an average diameter of ~2 μm appeared in cold-rolled CoFeNiW_0.3 after annealing at 700 °C, 800 °C, and 900 °C. Fine particles (denoted by blue arrows) with the average diameters of ~144 nm and ~225 nm were found in CoFeNiW_0.3 after annealing at 800 °C and 900 °C. The chemical compositions of different regions in cold-rolled CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs after annealing are shown in

Table 1. Chemical compositions (in at.%) of CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs by SEM-EDS.

Alloys	Regions	Co	Fe	Ni	W
W0-700 °C	FCC	33.77	33.56	32.67	0
W0-800 °C	FCC	33.75	33.60	32.64	0
W0-900 °C	FCC	33.37	33.60	33.03	0
W0.1-700 °C	FCC	32.53	32.52	31.44	3.50
W0.1-800 °C	FCC	32.50	32.45	31.21	3.83
W0.1-900 °C	FCC	32.41	32.63	31.09	3.87
W0.3-700 °C	FCC	29.98	30.28	30.19	9.55
	Coarse particle	25.60	19.22	11.75	43.43
W0.3-800 °C	FCC	30.59	30.14	29.55	9.73
	Coarse particle	25.79	18.79	11.22	44.19
	Fine particle	29.90	26.11	20.34	23.64
W0.3-900 °C	FCC1	28.40	30.78	25.93	14.89
	FCC2	29.87	30.41	27.01	12.71
	Coarse particle	22.60	19.65	11.26	46.49
	Fine particle	24.86	22.51	13.62	39.01

. For CoFeNiW_0.3 MEA annealed at 900 °C, W-rich fine particles precipitated from the FCC matrix, resulting in darker contrasts near these fine particles. This phenomenon is possibly due to a larger amount of W consumed by the precipitated phase. However, no obvious fine particles can be observed in CoFeNiW_0.3 annealed at 700 °C, which could be due to the sluggish diffusion process in this MEA. Coarse particles and fine particles were both W-rich phases. According to the XRD results, these particles should belong to the μ phase, but needed to be confirmed further by TEM. The BSE images of homogenized and cold-rolled CoFeNiW_0.3 are shown in Figure S3. It can be inferred that W-rich coarse particles formed after homogenizing. Fine W-rich particles precipitated from the CoFeNi matrix after annealing at 800 °C and 900 °C.

The EBSD inverse pole figure (IPF) images of cold-rolled CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs after annealing at 700 °C, 800 °C, and 900 °C are given in Figure 3. For cold-rolled CoFeNi MEA annealed at 700 °C, 800 °C, and 900 °C, its average grain sizes were measured to be 16.9 μm, 17.1 μm, and 20 μm, respectively. The average grain sizes of CoFeNiW_0.1 MEA after annealing were significantly smaller compared to CoFeNi, which were measured to be 8.2 μm, 12.3 μm, and 13.5 μm, respectively. This is because W as solute atoms can pin the migration of grain boundaries and effectively hinder the grain growth. The additions of W significantly affected the recrystallization process of CoFeNiW_0.3 by solute-drag effect and Zener pinning effect. Specifically, upon annealing at 700 °C, CoFeNiW_0.3 exhibited elongated grains with a distinct non-recrystallized microstructure. When CoFeNiW_0.3 MEA was annealed at 800 °C, it displayed partial recrystallization, featuring the coexistence of non-recrystallized and recrystallized regions. The fractions of non-recrystallized and recrystallized regions were estimated to be 51.6% and 48.4%, respectively. With increasing annealing temperature being further elevated to 900 °C, fully recrystallized grains can be observed in CoFeNiW_0.3.

TEM characterizations were further carried out to investigate the phase structure in CoFeNiW_0.3 MEA. The bright-field images and corresponding selected area electron diffraction (SAED) patterns are shown in Figure 4. As can be seen in Figure 4a, the coarser particles belonged to the Fe₇W₆-type μ phase. From Figure 4b,c, fine precipitates formed during annealing were also confirmed to be μ-phase. In contrast to the reported crystal structures of CrFeNiW_0.1 MEA, only μ phase appeared in CoFeNiW_x MEAs due to the absence of the Cr element to avoid the formation of σ phase [14].

3.3. Mechanical Properties

The engineering stress-strain curves of cold-rolled CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs annealing at 700 °C, 800 °C, and 900 °C for 1 h were depicted in Figure 5a–c. In addition, their mechanical properties were summarized in

Table 2. The mechanical properties of CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs.

MEAs	YS (MPa)	UTS (MPa)	EL (%)
W0-700 °C	232	491	47.1
W0-800 °C	205	493	51.7
W0-900 °C	192	485	52.9
W0.1-700 °C	373	619	32.5
W0.1-800 °C	306	601	45.5
W0.1-900 °C	242	554	44.8
W0.3-700 °C	1063	1207	5.5
W0.3-800 °C	620	898	23
W0.3-900 °C	484	845	37

. With increasing annealing temperatures, the yield strength (YS) values of CoFeNiW_x (x = 0, 0.1, and 0.3) decreased, while the elongation (EL) of CoFeNiW_x (x = 0 and 0.3) increased. In contrast to EL variations in CoFeNi and CoFeNiW_0.3, the EL of CoFeNiW_0.1 annealed at 800 °C was similar to CoFeNiW_0.1 annealed at 900 °C. It was possibly because the finer grains in CoFeNiW_0.1 annealed at 800 °C enabled more stress concentrations distributed among more grains to avoid its failure upon tensile stress [15]. From Table 2, it is obvious that CoFeNiW_0.3 annealed at 800 °C can exhibit better comprehensive mechanical properties among CoFeNiW_x MEAs. In Figure 5d, compared to other reported HEAs and MEAs, CoFeNiW_0.3 annealed at 800 °C can display superior YS and EL.

4. Discussions

4.1. Phase Formations

In this study, a minor amount of W can completely dissolve in CoFeNi MEA to form an FCC-structured solid solution, while the μ phase is formed in CoFeNi by adding more W element. From previous studies, there are several solid solutions formation criteria for HEAs or MEAs, such as ${∆H}_{mix}$, $\mathsf{\delta }$, $\mathsf{\Lambda }$, $\mathsf{\gamma },$ and $\mathsf{\Omega }$ [24,25,26,27]. The calculated results are shown in

Table 3. The calculated parameters of phase formations in CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs.

MEAs	${∆\mathit{H}}_{\mathit{mix}}$ (kJ/mol)	$\mathsf{\delta }$ (%)	${∆\mathit{S}}_{\mathit{mix}}$ (J/mol/K)	$\mathsf{\Lambda }$ (J/mol/K)	$\mathsf{\gamma }$	$\mathsf{\Omega }$
CoFeNi	−1.333	0.653	9.134	21.407	1.017	12.118
CoFeNiW0.1	−1.415	2.073	10.024	2.332	1.134	12.970
CoFeNiW0.3	−1.543	3.246	10.836	1.028	1.133	13.654

.

$${∆H}_{mix}=\sum _{i=1,i\ne j}^n{4∆H}_{mix}^{AB}{c}_i{c}_j$$

(1)

$$\mathsf{\delta }=\sqrt{\sum _{i=1}^n{c_i(1-r_i/\overline{r})}^2}$$

(2)

$$\overline{r}=\sum _{i=1}^n{c}_i{r}_i$$

(3)

$${∆S}_{mix}=-R\sum _{i=1}^n\left(c_i\mathrm{l}\mathrm{n}{c}_i\right)$$

(4)

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

(5)

$$\mathsf{\Lambda }={\displaystyle \frac{{∆S}_{mix}}{{\delta }^2}}$$

(6)

$$\mathsf{\gamma }=(1-\sqrt{{\displaystyle \frac{{\left(r_s+\overline{r}\right)}^2-{\overline{r}}^2}{{\left(r_s+\overline{r}\right)}^2}}})/(1-\sqrt{{\displaystyle \frac{{\left(r_L+\overline{r}\right)}^2-{\overline{r}}^2}{{\left(r_L+\overline{r}\right)}^2}}})$$

(7)

$$\mathsf{\Omega }={\displaystyle \frac{T_m∆S_{mix}}{\left|∆H_{mix}\right|}}$$

(8)

where ${∆H}_{mix}$ and $\mathsf{\delta }$ are mixing enthalpy and atomic size differences, respectively; $c_i$ and $c_j$ are the atomic percent of the ith element and jth element, respectively; $\overline{r}$ is the average atomic radius, $r_i$ as the atomic radius of the ith element can be obtained from Refs. [28,29]; ${∆S}_{mix}$ is the mixing entropy, R is gas constant (8.314 J/mol/K); $T_m$ is the average melting point, ${\left(T_m\right)}_i$ is melting point of ith element, as cited from literature [28,29,30]; $\mathsf{\Lambda }$ is a geometrical parameter; $\mathsf{\gamma }$ is the atomic packing parameter, $r_s$ and $r_L$ are the radii of the smallest and largest atoms, respectively; $\mathsf{\Omega }$ is a parameter of comparing the relative magnitudes between the entropy effect and the enthalpy effect.

For forming solid solutions, the calculated parameters (${∆H}_{mix}$, $\mathsf{\delta }$, $\mathsf{\Lambda }$, $\mathsf{\gamma },$ and $\mathsf{\Omega }$) should meet the following requirements [24,25,26,27]: (1) −15 < ΔH_mix < 5 kJ/mol, δ ≤ 6.6%; (2) $\mathsf{\Lambda }>0$.96; (3) $\mathsf{\gamma }<1$.175; (4)$\mathsf{\Omega }\ge 1$.1, δ ≤ 6.6%. In CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs, the values of ${∆H}_{mix}$, $\mathsf{\delta }$, $\mathsf{\Lambda }$, $\mathsf{\gamma }$ and $\mathsf{\Omega }$ all satisfied criteria abovementioned to form solid solution phases. However, based on experimental investigations, a single FCC solid solution phase formed in as-cast CoFeNiW_x alloys, whereas μ appeared in CoFeNiW_0.3 after homogenizing and annealing. Incorporating alloying elements with a larger atomic radius than other elements in the alloy system into an FCC lattice would induce lattice distortion [11]. The resulting lattice distortion energy destabilized the FCC matrix, promoting the appearance of a second phase. Nevertheless, there was not enough time for as-cast CoFeNiW_0.3 to nucleate and precipitate due to the higher cooling rate during the solidification process, which enabled the as-cast CoFeNiW_0.3 MEA to attain a saturated state. After homogenizing and annealing, the solubility of W with the highest melting point reduced with decreasing temperatures, and thus, W-rich μ phase tended to form in the alloy due to the clustering of W atoms.

4.2. Strengthening Mechanisms

CoFeNiW_0.3 annealed at 800 °C can exhibit better comprehensive mechanical properties. Herein, the yield strength increment of CoFeNiW_0.3 MEA annealed at 800 °C was discussed in detail to clarify its primary strengthening mechanism. The strengthening mechanisms for the MEA can be attributed to solid solution strengthening, grain refinement strengthening, dislocation strengthening, as well as precipitation strengthening.

$$\mathsf{\sigma }={\sigma }_0+{∆\sigma }_{SS}+f_{RS}·{∆\sigma }_{GB}+∆{\sigma }_D+{∆\sigma }_P$$

(9)

where ${\sigma }_0$ is the friction stress, approximately equivalent to the yield strength of as-cast CoFeNi (~112 MPa, as shown in Figure S4); $f_{RS}$ is the fraction of recrystallized regions (48.4%).

The contribution from solid solution strengthening can be calculated from the following equation [14]:

$${∆\sigma }_{SS}=M·{\displaystyle \frac{G·{{\epsilon }_s}^{3/2}·c^{1/2}}{700}}$$

(10)

where M is the Taylor factor (3.06) [14]; G (78 GPa) is the shear modulus of CoFeNi, which can be obtained by the rule of mixture [16], and C is the atomic concentration of W dissolving in the FCC matrix. As given in Table 1, the W concentration in the FCC matrix was slightly larger than its nominal chemical composition, possibly due to the limited accuracy of the EDS technique. Therefore, the content of W dissolved in FCC of CoFeNiW_0.3 was estimated to be ~0.0625 because CoFeNiW_0.2 can show a single FCC phase (as depicted in Figure S5) after thermal-mechanical processing. The interaction parameter ${\epsilon }_s$ is expressed as [14]:

$${\epsilon }_s=\left|{\displaystyle \frac{{\epsilon }_G}{1+0.5{\epsilon }_G}}-3·{\epsilon }_a\right|$$

(11)

where ${\epsilon }_G$ is elastic mismatch,${\epsilon }_a$ is atomic size mismatch. They are expressed as [14]:

$${\epsilon }_G={\displaystyle \frac{1}{G}}{\displaystyle \frac{\partial G}{\partial c}}$$

(12)

$${\epsilon }_a={\displaystyle \frac{1}{a}}{\displaystyle \frac{\partial a}{\partial c}}$$

(13)

where a is the lattice constant of as-cast CoFeNi (0.35783 nm). ${\epsilon }_G$ is usually neglected. Therefore, the strength contribution from solid solution strengthening was ~28.2 MPa.

The ${∆\sigma }_{GB}$ can be calculated by Equation (14) [31]:

$${∆\sigma }_{GB}=k{d}^{-1/2}$$

(14)

where $k$ is the Hall–Petch coefficient (~363 MPa·μm^1/2) [32]; $d$ is the average grain size of W0.3-800 °C (~3.5 μm). The value of ${∆\sigma }_{GB}$ was estimated to be ~194 MPa.

The contribution value from dislocation strengthening was normally estimated using the following formula [14]:

$${∆\sigma }_D=M\alpha Gb{\rho }^{1/2}$$

(15)

where $\alpha$ as a constant is 0.2 [14]; $b$ is Burger’s vector (~0.256 nm); $\rho$ as the dislocation density (4.36 × 10¹⁴ m²) was calculated by Equation (16) [33]:

$$\mathsf{\rho }={\displaystyle \frac{2\theta }{\mu b}}$$

(16)

where $\theta$ is the local misorientation angle obtained from EBSD (0.0173); $\mu$ is the step size (310 nm). Combining Equations (15) and (16), the calculated ${∆\sigma }_D$ was about ~255.2 MPa.

The strengthening effect of μ phase can be explained by the Orowan bypass mechanism, which was calculated using Equations (17) and (18) [14]:

$${∆\sigma }_{P_i}={\displaystyle \frac{0.4MGb}{\pi \sqrt{1-\vartheta }}}·{\displaystyle \frac{\ln (2\overline{r_i}/b)}{L_{Pi}}}$$

(17)

$${∆\sigma }_P=\sqrt{{{∆\sigma }_{P_1}}^2+{∆{\sigma }_{P_2}}^2}$$

(18)

where $\vartheta$ is Poisson’s ratio (0.24) [14]; $\overline{r_i}=\sqrt{2/3}{r}_i$, $r_i$ is the mean particle radius of μ phases (1.270 μm for coarse μ phase and 0.072 μm for fine μ phase); The mean interparticle distances ($L_P$) for coarse and fine μ phase were measured to be 3.412 μm and 0.401 μm, respectively. As a result, the contribution to yield strength from μ phase was ~138.4 MPa.

From the analysis mentioned above, the calculated sum of the yield strength was approximately 627.7 MPa, which was close to the experimental value with a negligible discrepancy.

4.3. Deformation Mechanisms

To elucidate the deformation mechanism of cold-rolled CoFeNiW_0.3 annealed at 800 °C, the microstructures of this MEA before and after deformation are depicted in Figure 6. From Figure 6a, CoFeNiW_0.3 annealed at 800 °C exhibited a distinct partially recrystallized microstructure. Many dislocations were distributed in non-recrystallized regions, while recrystallized regions were relatively clear without obvious dislocations. After deformation, dislocation motions were effectively hindered by μ phase, which significantly piled up around μ phase particles (as given in Figure 6b). No obvious stacking faults or deformation twins were found in deformed CoFeNiW_0.3. Therefore, it can be concluded that the deformation mechanism of cold-rolled CoFeNiW_0.3 annealed at 800 °C was mainly attributed to dislocation motions.

5. Conclusions

In this work, the effect of W contents and annealing temperatures was studied. After thermal-mechanical processing, CoFeNi and CoFeNiW_0.1 still maintained a single FCC structure. However, coarse μ particles with an average diameter of ~2 μm appeared in CoFeNiW_0.3 MEA after homogenizing. After annealing at 800 °C and 900 °C, fine particles emerged in CoFeNiW_0.3, with the average diameters of ~144 nm and ~225 nm, respectively. W additions can significantly hinder the recrystallization process of CoFeNiW_x MEAs. When CoFeNiW_0.3 MEA was subjected to 800 °C, the alloy showed a partially recrystallized microstructure with the coexistence of non-recrystallized and recrystallized regions. This alloy can exhibit better comprehensive mechanical properties. During the deformation process, dislocation motions were the controlled deformation mechanism. Our work can provide a better understanding of the microstructure evolution and mechanical properties of W-added MEAs, and give a reference for designing both MEAs and HEAs.