Effects of Zr Alloying on Microstructure Evolution and Mechanical Properties of CoCrNi Medium Entropy Alloy

Abstract

Alloying provides an effective approach to designing metallic materials with unique microstructures and enhanced performance. In this work, we developed a series of (CoCrNi)_100−xZr_x (where x = 1, 2, 3, 4, and 5) medium entropy alloys (MEAs) by vacuum arc-melting method. The effects of Zr addition on the microstructures and mechanical properties of (CoCrNi)_100−xZr_x MEAs were systematically investigated. Due to the negative mixing enthalpy of Zr with Co, Cr, and Ni, lamellar C15 Laves-phase precipitates formed within the ductile FCC matrix. As the Zr content increases, the alloys exhibit higher strength but become more brittle at room temperature. Among the (CoCrNi)_100−xZr_x MEAs series, the CoCrNiZr₃ MEA shows an excellent balance between strength and ductility, achieving a compressive yield strength of 610 MPa and a hardness of 249 HV, respectively, while maintaining a good ductility beyond 45%. Microstructural analysis using scanning electron microscope and transmission electron microscope suggests that this outstanding strength-ductility balance of CoCrNiZr₃ MEA arises from the synergistic effect of precipitation strengthening and solid solution strengthening. These findings not only provide deeper insight into the interaction between different strengthening mechanisms but also offer valuable guidance for designing high-performance multi-component alloys through strategic alloying.

1. Introduction

High-performance metallic materials have long been a cornerstone of technological advancements. However, conventional alloys, typically composed of one or two base elements, increasingly fail to meet the growing demand for versatile, high-performance materials in specialized applications. This challenge highlights the urgent need for an innovative approach to alloy design. Over one decade, high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), developed with entropy as a key designing principle, have attracted significant interest and discussion within the materials science community [1,2,3]. Unlike conventional alloys, HEAs and MEAs usually consist of three to five elements in nearly equal atomic ratios. Due to their unique features, including high entropy, lattice distortion, sluggish diffusion, and the cocktail effect [4], HEAs/MEAs tend to solidify into a single-phase solid solution containing multiple components. To date, these alloys have exhibited an exceptional combination of properties, including excellent low-temperature ductility [5,6,7], superior high-temperature mechanical performance [8,9], outstanding corrosion resistance [10,11,12], and remarkable electrocatalytic performance [13,14]. For example, the face-centered cubic (FCC) CrCoNi MEA demonstrates a high strength up to 1.3 GPa, a failure strain of 90%, and a fracture toughness (K_JIc) value of 275 MPa·m^1/2 at cryogenic temperatures [7]. Despite these advantages, FCC HEAs/MEAs generally suffer from relatively low strength, particularly in terms of yield strength at room temperature, which remains a critical limitation for their engineering application as structural materials.

Alloying has long been employed to tailor the microstructures and properties of metallic materials. Recent studies have proposed an alloying strategy that introduces elements capable of reacting with the matrix elements of FCC-type HEAs/MEAs to form hard second phases [15,16,17,18,19,20,21,22]. This approach effectively activates strengthening mechanisms such as solid solution hardening, precipitation strengthening, and fine-grained strengthening, thereby achieving a superior strength-ductility balance [19,20]. For example, the addition of Al to CoCrNi MEAs promotes the formation of Al-Ni-rich B2 precipitates, which induces several strengthening mechanisms including substitutional solid solution (SSH), grain refinement, and B2-phase precipitation strengthening [20]. Similarly, Zhao et al. proposed an Al/Ti co-alloying strategy for CoCrNi MEA, which successfully introduces nano-scale L1₂-(Ni, Co, Cr)₃(Ti, Al)-type particles in the ductile matrix [21]. In contrast to the CoCrNi MEA, the CoCrNiAl₃Ti₃ MEA shows a substantial precipitation-hardening effect, induced by the nano-scale reinforcement that integrates both heterogeneous and homogeneous mechanisms. Consequently, the precipitation-strengthened CoCrNi MEA achieves a 70% increase in yield strength and a 44% increase in tensile strength while maintaining exceptional ductility exceeding 40%. Additionally, the addition of elements such as Nb [23], Mo [24], and Ta [25] to CoCrNi MEA can lead to the formation of hard Laves or BCC second phases, which has been shown to effectively improve the balance between strength and ductility. Hence, the careful selection of alloying elements is crucial for optimizing the strength-ductility balance of CoCrNi MEA.

Previous studies have suggested that Zirconium (Zr) is frequently employed as an alloying element to facilitate the formation of hard secondary phases [26,27,28,29,30]. For example, Ren et al. [28] successfully developed a Zr-alloyed AlCoCrFeNi HEA with a dual-phase structure consisting of BCC and C15 Laves phases. The synergistic effect of multiple strengthening mechanisms, including solid solution hardening, precipitation hardening, and grain refinement strengthening, induced by the formation of the Laves phase, results in a remarkable enhancement in wear resistance. Similarly, Huo et al. [27] revealed that the addition of an appropriate amount of Zr element to CoCrFeNi HEA leads to the formation of a eutectic structure comprising FCC and Laves phases, thereby significantly enhancing its mechanical properties at both room and elevated temperatures. Moreover, Qi et al. [30] reported that proper Zr alloying not only significantly enhances the compressive strength but also improves the corrosion resistance of the CoCrFeNi matrix. The studies selected Zr as an alloying element primarily based on the following two considerations. First, Zr plays a crucial role in forming hard Laves phases, such as (Co, Cr, Ni)₂Zr with a C15 Laves-type structure [27,30,31], primarily due to its low negative mixing enthalpies with the base elements in the CoCrNi MEA, as shown in Table 1. Second, the significant atomic size differences between Zr and these elements, also summarized in Table 1, contribute to solid solution strengthening and promote the formation of precipitate phases.

Hence, alloying CoCrNi MEA with Zr may represent a feasible strategy for enhancing its strength. Recently, Bhukya et al.’s study [29] has demonstrated that in a Zr-doping CoCrNi alloy after high-pressure torsion followed by heat treatment, the Ni-Zr-rich phases are uniformly dispersed within the FCC matrix, leading to grain refinement and dislocation pinning, which significantly enhances the tensile strength of CoCrNi MEA. Herein, to investigate the effect of further Zr addition on the microstructure and compressive strength of CoCrNi MEA, this work prepared a series of (CoCrNi)_100−xZr_x (x = 1, 2, 3, 4, and 5 at. %) alloys. Systematic analyses were performed to examine their phase composition and microstructure, including the size, morphology, and chemical composition of the precipitated phases, elucidating the evolutionary trends as a function of Zr content. Additionally, the mechanical properties and the underlying strengthening mechanisms were thoroughly investigated.

2. Materials and Methods

Alloy ingots with a nominal composition of (CoCrNi)_100−xZr_x (x = 0, 1, 2, 3, 4, and 5 at. %, denoted as Zr₀, Zr₁, Zr₂, Zr₃, Zr₄, and Zr₅, respectively) were synthesized via arc melting under an Ar atmosphere using Co, Cr, Ni, and Zr, each with a purity of at least 99.99%. To ensure homogeneous mixing of the elements, each arc-melted alloy ingot was flipped and remelted at least five times. Specimens with various sizes were then cut from the alloy ingots using electric discharge machining for subsequent tests. Cylindrical specimens with a diameter of 4 mm and a height of 6 mm were cut and used for compression tests, while plate specimens with dimensions of 5 × 5 × 1 mm³ were prepared for phase composition and microstructural observation.

The phase compositions of the obtained alloys were identified by a Japanese Rigaku ULTIMA IV X-ray diffractometer (XRD) with Cu K_α radiation at a scanning rate of 5°/min, scanning from 30 to 90°. Microstructural characterization was conducted using a Hitachi SU8010 scanning electron microscope (SEM) and a American FEI Tecnai G2 F30 transmission electron microscope (TEM) equipped with an Oxford energy dispersive spectrometer (EDS). For enhanced SEM characterization, the samples were sequentially ground, electrolyte-polished, and etched using a perchloric acid-alcohol solution in a volume ratio of 1:9. Thin foils for TEM observation were first mechanically ground to a thickness of 60 μm and then ion-beam thinned inside a cold station at 100 K.

Room-temperature static compression tests were conducted using an MTS SilentFlo 515 test rig at a strain rate of 0.5 mm/min, with three specimens tested per composition. Hardness measurements were conducted using a American HYHVS-1000T Vickers hardness tester with a 10 N load applied for 15 s. For each sample, at least nine indentations were measured to obtain an average value. The volume fractions of the Laves phase in the alloys were quantified from micrographs using the commercial software ImageJ 1.0.

3. Results and Discussion

3.1. Phase Analysis

Figure 1a presents the XRD patterns of (CoCrNi)_100−xZr_x MEAs. As indicated by the hollow diamonds, only characteristic peaks of the FCC structure are observed in the Zr-free CoCrNi MEA [20]. When 1 at. % Zr is added into CoCrNi MEA, the XRD pattern remains similar to that of the Zr-free MEA. According to Bhukya’s study, the absence of diffraction peaks corresponding to the second phase is due to its low content, making it undetectable by XRD [29]. However, when the Zr content exceeds 2 at. %, additional diffraction peaks appear at approximately 36° and 44°, as marked by the hollow circles. These peaks are tentatively identified as C15 Laves phase according to the PDF card of the (ZrX₂, X = Ni or Co) compound [30], which needs to be further confirmed by TEM observation. The diffraction peak position of (111)_FCC for each composition is extracted from Figure 1a and plotted as a function of Zr content in Figure 1b. For Zr-free CoCrNi, the peak is located at 43.98°. Upon the addition of 1 at% Zr, the peak shifts to a lower 2θ of 43.81°. With 2 at% Zr, it moves to a higher 2θ of 53.83°, then to 43.95° for Zr₃. As the Zr content increases further, the peak continues to shift toward lower 2θ values. The corresponding lattice constants, derived from the diffraction angles, are also provided in Figure 1a. Since the atomic radius of Zr is larger than those of Cr, Co, and Ni (Table 1), its dissolution into the CoCrNi FCC solid solution induces significant lattice distortion, leading to an increased lattice constant and, consequently, a smaller diffraction angle. Therefore, the variation in lattice constant, as shown in Figure 1b, can be attributed to the phase evolution with increasing Zr content.

Although available phase diagram information for HEAs is limited, various theories and models have been proposed to accurately describe their phase evolution [32,33,34]. Based on the parameters δ and ${\Delta H}_{mix}$, Guo et al. [33] suggested that the phase diagram of HEAs can be divided into three regions: a solid solution region, a solid solution + intermetallic compound region, and an amorphous alloy region. Here, δ represents the atomic size mismatch among the constituent elements [35], while ${\Delta \mathit{H}}_{\mathit{m}\mathit{i}\mathit{x}}^{\mathit{A}\mathit{B}}$ denotes the weight-averaged mixing enthalpies of different alloying element pairs, calculated using the Miedema Model (Table 1) [36]. The values of δ and ${\Delta \mathit{H}}_{\mathit{m}\mathit{i}\mathit{x}}^{\mathit{A}\mathit{B}}$ are adapted from the reference [28,33], and data from this work are also included in Figure 1c. It can be clearly observed that, aside from the Zr-free alloy, the data point corresponding to the Zr₁ alloy also falls within the solid solution region (blue-shaded area), indicating that most Zr atoms in the Zr₁ alloy exist in the form of solute atoms. This explains why its lattice constant increases sharply from 3.5626 to 3.5760 nm, as shown in Figure 1b, as the dissolution of Zr atoms into an FCC solid solution matrix induces severe lattice distortion. It should be emphasized that not all Zr atoms entered the lattice interstices; a small fraction precipitated to form a minor second phase, as later evidenced by SEM observations. When the Zr content reaches 2 at.%, the Zr₂ alloy falls within the intermetallic compound region, as indicated by the green dashed ellipse region in Figure 1c, suggesting that most Zr atoms begin to precipitate as the Laves phase. This precipitation accounts for the observed decrease in lattice constant. As the Zr content further increases to 4 at% and 5 at%, the number of Zr atoms occupying interstitial sites increases again, leading to a subsequent expansion of the lattice constant.

Table 1. The mixing enthalpy ${\Delta H}_{mix}^{AB}$ (kJ·mol⁻¹) of atom pairs [36] and atomic sizes (Å) of the elements.

Elements	$\mathbf{Mixing}\mathbf{Enthalpy},{\Delta \mathit{H}}_{\mathit{m}\mathit{i}\mathit{x}}^{\mathit{A}\mathit{B}}$				Atomic Radius
	Co	Cr	Ni	Zr
Co	/	−4	0	−41	1.28
Cr		/	−7	−12	1.25
Ni			/	−49	1.24
Zr				/	1.6

Table 1. The mixing enthalpy ${\Delta H}_{mix}^{AB}$ (kJ·mol⁻¹) of atom pairs [36] and atomic sizes (Å) of the elements.

Elements	$\mathbf{Mixing}\mathbf{Enthalpy},{\Delta \mathit{H}}_{\mathit{m}\mathit{i}\mathit{x}}^{\mathit{A}\mathit{B}}$				Atomic Radius
	Co	Cr	Ni	Zr
Co	/	−4	0	−41	1.28
Cr		/	−7	−12	1.25
Ni			/	−49	1.24
Zr				/	1.6

Figure 1. (a) XRD patterns of (CoCrNi)_100−xZr_x MEAs, (b) the diffraction peak position of (111)_FCC and corresponding lattice constant as functions of Zr content, and (c) the $\delta -{\Delta H}_{mix}$ plot adopted from the reference [28,33], incorporating data from this work.

Figure 1. (a) XRD patterns of (CoCrNi)_100−xZr_x MEAs, (b) the diffraction peak position of (111)_FCC and corresponding lattice constant as functions of Zr content, and (c) the $\delta -{\Delta H}_{mix}$ plot adopted from the reference [28,33], incorporating data from this work.

3.2. Microstructural Evolution

Figure 2 presents the optical micrographs, SEM images, and corresponding elemental mappings of as-cast (CoCrNi)_100−xZr_x MEAs. The microstructure of the base CoCrNi MEA has been extensively reported in previous studies [23] and is therefore not discussed here. Upon Zr alloying, a typical hypoeutectic structure, consisting of primary FCC dendrites and interdendritic regions, is observed in all Zr₁–Zr₅ MEAs, as shown in the optical micrographs in Figure 2(a1–e1). SEM observations reveal bright intermetallic phases in the interdendritic regions, which are identified as Laves phase based on the XRD analysis. Notably, although the Laves phase is present in the Zr₁ alloy, its corresponding XRD diffraction peak is absent, likely due to the limited volume fraction of the phase. At higher magnification, as shown in Figure 2(a3–e3), the interdendritic regions show a eutectic-like structure composed of intermetallic Laves-phase lamellae and closed-cell FCC phases. The Laves-phase lamellae show a coarse and semi-connected shape. The elemental mapping obtained via EDS, as shown in Figure 2(a4–e4), indicates that the dendritic regions are enriched in Co, Cr, and Ni while being nearly Zr-free. In contrast, the Laves phase is enriched in Ni, Co, and Zr but depleted in Fe and Cr.

Table 2. Chemical compositions (at. %) of the as-cast (CoCrNi)_100−xZr_x MEAs analyzed by EDS, along with the volume fractions of Laves phase (f_Laves, %) measured using ImageJ software.

Alloys	Region	Co	Cr	Ni	Zr	fLaves
Zr1	Nominal	33	33	33	1	11.8
	DR	33.73	34.48	31.79	0
	IR	19.13	6.66	48.3	25.91
Zr2	Nominal	32.66	32.67	32.67	2	16.1
	DR	35.46	33.99	30.43	0.12
	IR	21.36	6.09	46.88	25.67
Zr3	Nominal	32.33	32.33	32.34	3	22.7
	DR	34.85	39.54	25.5	0.11
	IR	20.48	5.51	48.7	25.31
Zr4	Nominal	32	32	32	4	36.1
	DR	36.18	35.75	27.97	0.1
	IR	18.99	3.66	48.27	29.08
Zr5	Nominal	31.66	31.67	31.67	5	43.2
	DR	36.24	37.53	26.2	0.03
	IR	17.05	6.92	50.6	25.43

summarizes the nominal composition and EDS results of the dendritic and interdendritic regions of (CoCrNi)_100−xZr_x MEAs. The EDS results of the Laves phase basically match the stoichiometry type of Zr(Co, Ni)₂. Additionally, the volume fractions of the Laves phase ($f_{Laves}$) in each composition were measured from micrographs and summarized in Table 2. As suggested by the values of $f_{Laves}$, the volume fraction of the Laves phase increases progressively with increasing Zr content.

To gain further nano-scale structural details, TEM characterization was performed on the Zr₃ MEAs. Figure 3 presents the bright-field (BF) TEM images and the corresponding selected-area electron diffraction (SAED) patterns of the Zr₃ MEAs. As shown in Figure 3a, a dual-phase structure comprising an FCC matrix and Laves phase precipitates is clearly observed. To examine the coherence between the two phases, high-resolution TEM (HRTEM) imaging was conducted at their interface (indicated by the red square in Figure 3a), as shown in Figure 3b. Figure 3c,d display the SAED patterns obtained from regions A and B in Figure 3b, respectively. The diffraction spots in Figure 3c,d were identified as corresponding to the FCC and Laves phases, respectively, which is consistent with the phase identification results obtained from the earlier XRD analysis.

3.3. Mechanical Properties

In this study, hardness and compressive mechanical properties were measured to evaluate the effects of Zr addition on the mechanical behavior of as-cast (CoCrNi)_100−xZr_x MEAs. Figure 4a represents the compressive stress-strain curves of (CoCrNi)_100−xZr_x MEAs at room temperature. The compressive yield strength extracted from stress-strain curves and the hardness are plotted as functions of Zr content in Figure 4b. For the Zr-free CoCrNi MEA, this alloy exhibits excellent compressive ductility but relatively low compressive yield strength and hardness, as extensively reported in previous studies [6,7]. With the addition of Zr (0 to 3 at%), the compressive yield strength increases from 194 MPa to 610 MPa, and the hardness increases from 175.14 HV to 249.87 HV, while the fracture strain decreases to approximately 47%. As a hard but brittle intermetallic compound, the introduction of the Laves phase undoubtedly strengthens the alloy but significantly deteriorates its ductility. Hence, the observed variations in the mechanical properties of (CoCrNi)_100−xZr_x MEAs can be attributed to the formation of Zr (Co, Ni)₂ Laves phase. Notably, as the Zr content continues to increase, the compressive yield strength exhibits a slight increase, while the fracture strain decreases significantly, and the hardness rises sharply. This behavior may be attributed to solid solution strengthening caused by the re-dissolution of a substantial amount of Zr atoms, as indicated by Figure 1b.

To quantify the effect of the Laves phase on mechanical properties, the compressive yield strength and hardness are plotted as functions of the Laves phase volume fraction in Figure 5a and Figure 5b, respectively. For (CoCrNi)_100−xZr_x MEAs with a composite-like structure, where the hard Laves phase is uniformly dispersed within the ductile FCC matrix, the overall strength can be predicted using the rule-of-mixtures approach [23]. Based on the isostrain assumption, the overall strength is determined by the individual strength and volume fraction of the matrix and intermetallic phase, which can be expressed by the following equation:

$${\sigma }_{Overall}={\sigma }_{Intermetallic}·V_{Intermetallic}+{\sigma }_{Matrix}·V_{Matrix}$$

(1)

where ${\sigma }_{Intermetallic}$ and ${\sigma }_{Matrix}$ represent the compressive yield strength of the intermetallic phase and the matrix, while $V_{Intermetallic}$ and $V_{Matrix}$ denote their respective volume fractions. Given that $V_{Intermetallic}+V_{Matrix}=1$, Equation (1) can be rewritten as:

$${\sigma }_{Overall}={\sigma }_{Matrix}+V_{Intermetallic}·({\sigma }_{Intermetallic}-{\sigma }_{Matrix})$$

(2)

As shown in Figure 5a,b, a clear linear relationship is observed, suggesting that the composite model effectively explains strength enhancement. This demonstrates the feasibility of designing high-performance alloys for engineering applications by integrating a soft FCC matrix with hard intermetallic compounds.

To explore the effect of Zr addition on material failure, the fracture surfaces of Zr₃ and Zr₅ MEA after compressive testing were examined using SEM, and the results are shown in Figure 6. The fracture surface of Zr₃ MEA exhibits a small amount of microvoids and torn edges, as indicated by the yellow arrows in Figure 6a, which are characteristic features of ductile fracture. At the same time, visible cracks and cleavage steps can also be observed, which are commonly associated with brittle fractures. These features suggest that the fracture morphology of the Zr₃ MEA represents a transition from ductile to brittle fracture. When the Zr content increased to 5 at. %, microvoids and torn edges are no longer apparent, and the fracture surface appears much smoother, indicating there is no obvious plastic deformation. Meanwhile, cleavage steps dominate the entire fracture morphology, as shown in Figure 6b, further suggesting the poor ductility of Zr₅ MEA. These observations align well with the compressive test results presented in Figure 4a.

4. Conclusions

In this study, the evolution of microstructure and mechanical properties in as-cast (CoCrNi)_100−xZr_x MEAs was characterized and analyzed. The influence of Zr addition on phase formation, microstructure, and mechanical properties was investigated. Based on the results, the following conclusions can be drawn:

(1)

The addition of the Zr element into the CoCrNi MEAs modifies the initial phase constitution, resulting in the formation of an ordered Zr-rich Laves phase dispersed within an FCC solid solution matrix.

(2)

With increasing Zr content, the (CoCrNi)_100−xZr_x MEAs exhibit progressively higher strength but significantly reduced ductility. Among this series, the Zr₃ MEA achieves a relatively balanced combination of strength and ductility.

(3)

The gradual increase in Zr content leads to a progressively higher volume fraction of the Zr-rich Laves phase, which primarily accounts for the enhancement of hardness and compressive yield strength, as well as the reduction in compression fracture strain.