Effect of Cr₃C₂ Addition on Microstructure and Mechanical Properties of WC-CoNiFe Cemented Carbides

Abstract

In traditional cemented carbides, Co is mainly used as a binder. Recently, replacing Co with medium- to high-entropy alloys has shown significant improvements in hardness, fracture toughness, high-temperature oxidation resistance, and corrosion resistance, making it a research focus globally. Both the typical grain refiner Cr₃C₂ and medium- to high-entropy alloy binders affect the WC grain size in cemented carbides. This study investigates the synergistic grain refinement mechanism of Cr₃C₂ and medium- to high-entropy alloy binders on WC grains and their impact on the microstructure and mechanical properties of cemented carbides. The results show that increasing the Cr₃C₂ addition refined WC grains in the WC-CoNiFe alloy, increased coercivity, and enhanced hardness, with transverse rupture strength first increasing and then decreasing. The alloy achieved optimal performance at 0.6 wt.%Cr₃C₂, with a hardness of 91.25 HRA and transverse rupture strength of 3883.2 MPa.

1. Introduction

Traditional cemented carbides, prepared using powder metallurgy, consist of WC as the hard phase and metals like Co and Ni as the binder phase. These alloys exhibit excellent wear resistance, high hardness, high strength, and superior fracture toughness, making them widely used in cutting tools, molds, and mining tools [1,2,3]. The performance requirements of cemented carbides vary by application, with one key method to enhance mechanical properties being the control of WC grain size [4,5]. Studies have shown that refining WC grains to submicron and nanoscale significantly improves the mechanical properties, extending the service life of carbide components and tools [6,7]. However, fine grains have high surface energy, leading to rapid Ostwald ripening during sintering, where fine grains quickly dissolve and reprecipitate on larger grains, causing continuous grain growth (CGG) and coarsening [8]. Additionally, uniformly dispersing fine powder is challenging, which can lead to local carbon segregation and the abnormal grain growth of WC [9]. Therefore, a simple powder metallurgy method is insufficient for producing fine-grained cemented carbides with excellent performance.

Studies have shown that adding grain refiners can slow the CGG rate and suppress abnormal WC grain growth. The mechanisms are mainly categorized into dissolution and segregation theories. The dissolution theory [10] suggests that during sintering, refiners dissolve into the binder phase, altering W and C content, and reducing WC dissolution rates. The segregation theory [11,12] posits that refiners segregate to WC grain boundaries, forming a thin film of complex compounds that limits W and C solubility, thus reducing dissolution and precipitation rates, increasing WC grain boundaries, and refining grains. However, the specific grain refinement mechanism often requires a combined discussion of the selection of grain inhibitors and the composition of the binder phase.

Medium- to high-entropy alloy binder cemented carbides are highly valued for their exceptional mechanical properties. Composed of three or more elements in nearly equal proportions, these alloys exhibit significant lattice distortion and solid solution strengthening, leading to unique microstructures and superior high-temperature mechanical properties [5,13,14]. Research indicates that these alloys offer enhanced hardness [15,16], fracture toughness [17,18], corrosion resistance, and oxidation resistance [19,20,21] compared to traditional cobalt binder cemented carbides, making them promising for engineering applications.

Studies have shown that replacing Co with CoNiFe mid-entropy alloys as binders can improve the mechanical properties of cemented carbides. Chang et al. [22] found that nano WC−Co−Ni−Fe exhibits superior fracture toughness and comparable hardness to nano WC−Co. Gao et al. [23] reported that the hardness of WC−Co−Ni−Fe increases with higher Fe/Ni ratios compared to WC−Co. Qian et al. The authors of [5,17] observed that WC−CoNiFe outperforms WC−Co in flexural strength, fracture toughness, and fatigue strength across various WC grain sizes. Additionally, Co, Ni, and Fe, all in the fourth period and group VIII, have similar atomic radii and properties, and readily form stable solid solutions with other metals. Therefore, CoNiFe is commonly used as a principal element in high-entropy alloys. In conclusion, using CoNiFe mid-entropy alloys as binders is a representative approach for studying the effects of grain refiners on medium-/high-entropy alloy binder cemented carbides.

Cr₃C₂ is a typical grain refiner used in the production of cemented carbides. Numerous studies have shown [24,25,26] that the appropriate addition of Cr₃C₂ significantly inhibits the continuous and abnormal grain growth of WC, thereby enhancing the mechanical properties of the cemented carbide within a certain range. For instance, Balbino et al. [27] reported that adding 2 wt.% Cr₃C₂ to WC-8Ni prevented abnormal grain growth of WC in the densified alloy, resulting in an improvement of the alloy’s fracture toughness by approximately 2.0 MPa. m¹/² compared to traditional WC−10Ni. Additionally, Cr₃C₂ exhibits excellent overall mechanical performance as a grain refiner. Yin et al. [12] found that the hardness, flexural strength, and fracture toughness of the cemented carbides doped with Cr₃C₂ were superior to those of the carbides doped with VC and TaC. However, these studies primarily focus on traditional cemented carbides with Co, Ni, and other metals as binders, with the role and mechanism of grain refiners in medium-/high-entropy binder cemented carbides being less explored in current research.

This study prepared WC−CoNiFe by replacing Co with CoNiFe mid-entropy alloy, using YG10 as a reference, and applying typical powder metallurgy parameters. Various amounts of Cr₃C₂ were added to investigate its effects on the grain size and microstructure of WC−CoNiFe during liquid-phase sintering. Hardness, flexural strength, and magnetic properties were tested to explore the impact of Cr₃C₂ on grain refinement and its mechanisms, providing insights into the effects of grain refiners on the microstructure and properties of medium- and high-entropy binder cemented carbides.

2. Materials and Methods

2.1. Powder Preparation and Debinding Sintering

The WC powder used in the experiment was provided by Xiamen Golden Egret Special Alloy Co., Ltd. (Xiamen, China), with a particle size of 0.8 μm. Co powder (particle size 1.5 μm), Ni powder (particle size 2.7 μm), and Fe powder (particle size 3.78 μm) were provided by GEM Co., Ltd. (Shenzhen, China) The Cr₃C₂ powder, with a particle size of 0.97 μm, was provided by Zhuzhou Haokun Hard Materials Co., Ltd. (Zhuzhou, China) The Co powder, Ni powder, and Fe powder were mixed with WC powder in a binder ratio of Co: Ni: Fe = 5:4:1 to form the WC−5Co4Ni1Fe (wt.%) (WC−CoNiFe) powder. Based on this mixture, different amounts of Cr₃C₂ (as shown in

Table 1. Preparation scheme of WC−CoNiFe cemented carbides.

Sample	Composition (wt.%)		Sinter Parameter
	WC−5Co4Ni1Fe	Cr3C2
A	100	0	1430 °C, 1 h
B	99.8	0.2
C	99.4	0.6
D	99.1	0.9
E	98	2.0

) were added. The prepared powders were milled in a planetary ball mill for 24 h using alcohol as the milling medium under argon protection, with a ball/powder weight ratio of 4:1 and a milling speed of 280 r/min.

The wet-milled powders were dried at 80 °C for 1 h in an electric heating blast drying oven. After drying, the powders were mixed with wax (2 wt.% of the total powder mass), granulated, sieved, and then pressed into green compacts. The green compacts were sintered at 1430 °C for 1 h to obtain the WC−CoNiFe cemented carbide sintered compacts.

2.2. Material Characterization

The microstructure of the sintered bodies was characterized using a scanning electron microscope (SEM, Tescan Mira 4, Brno, Czech). The metallographic structure of the samples was observed with an optical microscope. The coercivity (${\mathrm{H}}_{\mathrm{c}}$) and magnetic saturation of the samples were measured using a SKY-IV coercivity meter (Xianyou Co., Ltd, Changsha, China) and an ACoMT automatic cobalt magnetic measurement instrument (Xianyou Co., Ltd, Changsha, China). The phase composition of the alloy was analyzed using a D/MAX-2250 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan). The samples were processed into 5 mm × 5 mm × 30 mm rectangular bars, and three-point bending performance tests were conducted using a universal testing machine. The hardness of the samples was tested using a KH3000 Rockwell hardness tester (NYSE: ROK, Milwaukee, WI, USA) with a pressure of 60 kg and a dwell time of 2 s, recording the average value of five tests.

3. Results and Discussions

3.1. Effect of Different Cr₃C₂ Contents on the Phase Composition of the Alloy

Figure 1 shows the XRD patterns of the five groups of the WC−CoNiFe cemented carbide samples.

Table 2. Physical characteristics of the alloy.

Sample	ω(Cr3C2) /%	Porosity	Uncombined Carbon	Mean Size (μm)
A	0	A00 B00	C00/E00	1.28
B	0.2	A00 B00	C00/E00	0.97
C	0.6	A00 B00	C00/E00	0.9
D	0.9	A02 B00	C00/E00	0.72
E	2.0	A04 B00	C00/E00	0.54

provides data on the microstructural characteristics of the alloys. As can be seen from the figure and table, all five groups of the samples are primarily composed of the WC phase and binder phase, with no η phase or graphite phase detected. Typically, in WC−Co cemented carbides, the binder Co usually exhibits an HCP structure. In this study, the binder CoNiFe exhibited a single FCC structure, with the three peaks from left to right corresponding to (111), (200), and (220). Additionally, the diffraction peak intensities and angles of the four Cr₃C₂-doped samples were almost identical to those of the undoped sample, indicating that to a certain extent, the addition of Cr₃C₂ has no significant impact on the phase structure characteristics of WC−CoNiFe.

3.2. Effect of Cr₃C₂ Content on the Microstructure of the Alloy

Figure 2 shows the backscattered scanning electron microscope images of WC−CoNiFe. As seen from the images, the undoped WC grains exhibit a rectangular plate-like or multi-angular prismatic shape. With the addition of Cr₃C₂, the proportion of equilateral triangular prismatic WC grains gradually increases. This is because after adding Cr₃C₂, Cr precipitate on the two preferential precipitation planes {$10\overline{1}0$} and (0001) at the WC/binder interface, forming a carbide thin film that inhibits the growth of these two planes. Consequently, the {$01\overline{1}0$} plane grows preferentially, causing the WC grains to transform into equilateral triangular prisms [12,28].

Additionally, as shown in Figure 2a, the undoped sample A has coarse grains with an average grain size of 1.28 μm. Since the grain growth was not restricted by refiners, the Ostwald ripening rate was relatively fast, resulting in fewer fine grains smaller than 0.5 μm compared to the other four Cr₃C₂−doped WC−CoNiFe alloys. The samples B and C, with 0.2 wt.% and 0.6 wt.% Cr₃C₂, respectively, showed relatively refined WC grains with a mixed distribution of coarse and fine grains. The samples D and E, with 0.9 wt.% and 2.0 wt.% Cr₃C₂, respectively, exhibited significant grain refinement, with the sample E having an average grain size of 0.54 μm.

The grain size variation can also be reflected in the changes in coercivity. Typically, the grain size in WC−Co cemented carbides is negatively correlated with the coercivity ${\mathrm{H}}_{\mathrm{c}}$, as described by the following Equation (1) [5,29]:

$${\mathrm{H}}_{\mathrm{c}}=1.44+0.04{\mathrm{m}}_{\mathrm{Co}}+{\displaystyle \frac{12.47-0.37{\mathrm{m}}_{\mathrm{Co}}}{{\mathrm{d}}_{\mathrm{WC}}}}$$

(1)

where ${\mathrm{H}}_{\mathrm{c}}$ is the coercivity, ${\mathrm{d}}_{\mathrm{WC}}$ is the WC grain size, and ${\mathrm{m}}_{\mathrm{Co}}$ is the mass fraction of Co. In this study, as shown in Figure 3, the coercivity of the undoped sample A and the sample E with 2.0 wt.% Cr₃C₂ are 12.04 H_c/(ka∙m⁻¹) and 22.06 H_c/(ka∙m⁻¹), respectively. The addition of Cr₃C₂ significantly increased the coercivity of the alloy, with the increase in coercivity being more pronounced as the Cr₃C₂ content increased.

In summary, with the increase in the Cr₃C₂ addition, the degree of WC grain refinement gradually increases. When the Cr₃C₂ content is equal to or greater than 0.9 wt.%, the grain inhibition effect of Cr₃C₂ on WC-CoNiFe is significant.

Figure 3 shows the curve of WC−CoNiFe cobalt magnetism and coercive magnetism with different content of Cr₃C₂. As seen from the figure, with the increase in the Cr₃C₂ content, the coercive force of the cemented carbide continuously increases, which matches the degree of grain refinement. Correspondingly, the cobalt magnetism of the alloy decreases gradually with the increasing Cr₃C₂ content. Previous studies [30,31] have shown that the addition of Cr₃C₂ often leads to an increase in the W content and a significant decrease in the C content in the cemented carbide binder, resulting in a decrease in the cobalt magnetism. The significant reduction in carbon content in the binder restricts the dissolution and diffusion of WC during sintering, slowing down the rate of WC grain dissolution and reprecipitation growth, ultimately causing grain refinement, which corresponds to the grain refinement trends described above.

In this study, the change in the cobalt magnetism intensity was relatively slow, with the maximum difference being only 0.53%. This is primarily related to the method of the Cr₃C₂ addition. Since Cr₃C₂ was added to the WC−CoNiFe composite powder without balancing the carbon content, its addition increased the total carbon content in the alloy. As cobalt magnetism in cemented carbides is inversely correlated with the W content in the binder phase to some extent [32], an increase in C content in the binder phase leads to a decrease in W content. Therefore, the overall increase in the carbon content in this study resulted in a decrease in the W content in the binder phase and a relative increase in the cobalt magnetism, mitigating the impact of the Cr₃C₂ addition on the decrease in cobalt magnetic strength. Additionally, the cobalt magnetism still showed a slow decline with the increasing Cr₃C₂ content, further indicating that the mechanism of Cr₃C₂ in inhibiting the grain growth of WC−CoNiFe is closely related to the changes in the W and C content in the alloy binder.

3.3. Effect of Cr₃C₂ Addition on the Mechanical Properties of the Alloy

Figure 4 shows the distribution curves of transverse rupture strength and hardness for the five sample groups. As seen from the figure, with the increase in the Cr₃C₂ content, the hardness of the alloy continuously increases. According to

Table 3. Summary of the microstructure and properties of the WC−CoNiFe cemented carbides with different Cr₃C₂ contents.

Sample	ω(Cr3C2)/%	Magnetic Saturation/%	Coercivity, Hc /(ka∙m−1)	Hardness (HRA)	TRS/MPa
A	0	6.81	12.04	90.1 ± 0.11	3533.86
B	0.2	6.73	15.83	90.5 ± 0.16	3632
C	0.6	6.7	16.25	91.25 ± 0.15	3883.2
D	0.9	6.57	17.33	91.4 ± 0.2	3769
E	2.0	6.28	22.06	92.3 ± 0.12	1613

, the hardness of the alloy without Cr₃C₂ is 90.2 HRA, which increases to 92.5 HRA after the addition of 2 wt.% Cr₃C₂. Simultaneously, the transverse rupture strength of the alloy gradually increases with the increasing Cr₃C₂ content, reaching a maximum value of 3883.2 MPa at a Cr₃C₂ content of 0.6 wt.%. However, as the Cr₃C₂ content continues to increase, the transverse rupture strength decreases, and the rate of decline increases with further addition of Cr₃C₂. When the Cr₃C₂ content reaches 2 wt.%, the transverse rupture strength drops to 1613.4 MPa.

According to the Hall/Petch equation, the hardness of cemented carbides is negatively correlated with the WC grain size. As the WC grains are refined, the hardness of the cemented carbides increases [33,34]. In this study, as the Cr₃C₂ content increases, the degree of WC grain refinement also increases, leading to an increase in hardness. Simultaneously, due to the grain refinement, the number of grain boundaries per unit volume in the alloy increases, and the crack propagation of WC and Co is restricted. Therefore, with the addition of Cr₃C₂, the grain refinement leads to an increase in the transverse rupture strength of the alloy, which gradually reaches a peak. However, as the amount of Cr₃C₂ increases, the flexural strength of the alloy significantly decreases. This phenomenon will be analyzed in detail in conjunction with Figure 5.

Figure 5 shows the bending fracture morphologies of the WC−CoNiFe cemented carbides with different Cr₃C₂ contents. From Figure 5, it can be observed that the fracture mode of the alloy is mainly intergranular fracture, with some transcrystalline fracture. Figure 5a shows that the alloy without Cr₃C₂ predominantly fractures along the WC/binder grain boundaries, with some coarse grains fracturing transgranularly. When the Cr₃C₂ content is 0.2 wt.% to 0.9 wt.%, there is no significant difference in the fracture modes of the alloys. However, when the Cr₃C₂ content is 2 wt.%, the proportion of the transcrystalline fracture decreases significantly. This is because the binder inhibits the abnormal growth of the WC grains, significantly reducing the average grain size. The finer grains are less likely to cause stress concentration and thus reduce the likelihood of transcrystalline fracture.

Additionally, as seen in Figure 5, when the Cr₃C₂ content is 2.0 wt.%, there are noticeable pores at the WC grain boundaries. This is because the Cr₃C₂ refiner dissolves in the CoNiFe binder, affecting the fluidity of the liquid phase during sintering. The higher the refiner content, the poorer the binder’s fluidity, resulting in the incomplete filling of the pores during liquid phase sintering, ultimately leaving some porosity in the alloy microstructure [35,36]. Notably, when the Cr₃C₂ content reaches 0.9 wt.%, small pores appear in some WC grains, as indicated by the yellow arrows in Figure 5d. In alloys with 2.0 wt.% Cr₃C₂, these pores are larger and more numerous. This indicates that Cr₃C₂ decreases liquid phase flowability [37], preventing the timely expulsion of bubbles during liquid phase sintering. As the grains grow, these bubbles remain trapped within the WC grains, forming spherical pores. The number of pores in the alloy microstructure increases proportionally with the Cr₃C₂ content. Consequently, when the Cr₃C₂ content exceeds 0.9 wt.%, and especially at 2 wt.%, numerous pores form in the alloy. These pores lead to stress concentration under load, promoting crack initiation and propagation, which significantly reduces the flexural strength.

The microstructure and properties of traditional WC−Co and WC−MEPA cemented carbides are shown in Table 4. It is noteworthy that the choice of raw materials, forming techniques, sintering temperature, and atmosphere can all affect the performance of the samples. However, as shown in Table 4, the cemented carbides prepared by adding Cr₃C₂ to WC−CoNiFe exhibit excellent comprehensive mechanical properties. When the grain size and binder content are similar, their mechanical properties are comparable to the optimal mechanical properties of the traditional WC−Co cemented carbides.

In summary, incorporating Cr₃C₂ into WC−CoNiFe cemented carbide significantly changes the alloy’s microstructure and properties. With the increase in the Cr₃C₂ content, the WC−CoNiFe grains gradually refine, leading to a continuous increase in the alloy’s hardness. The transverse rupture strength first increases but then gradually decreases due to the influence of pores. When the transverse rupture strength reaches its peak, the alloy exhibits excellent comprehensive mechanical properties. Additionally, the WC−CoNiFe cemented carbides obtained in this study exhibit fundamental mechanical properties comparable to those of the traditional WC−Co cemented carbide cutting tools [38,39]. Furthermore, they offer lower raw material costs compared to the WC−Co cemented carbides.

Table 4. Properties with Co and medium-/high-entropy binder cemented carbides (Partial).

Materials Composition (wt.%)	Mean Grain Size of WC (μm)	Hardness	TRS (MPa)
WC−10Co [40]	0.8	90.8 HRA	3700
WC−10Co−0.6Cr3C2 [40]	<0.8	91.8 HRA	3975
WC−10Co [35]	0.46	91.8 HRA	2818
WC−12Co−0.2Cr3C2 [35]	0.39	92.9 HRA	3483
WC−10Co [5]	1.38	1605 HV1	1677
WC−5Co4Ni1Fe [5]	1.32	1506 HV1	1772
WC−10Co7Ni2Fe1Cr−4.5C [41]	1~2	1080 HV30	1800
WC−10Co7Ni2Fe1Cr−4.9C [41]	1~2	950 HV30	3450
WC−5Co4Ni1Fe [This work]	1.28	90.1 HRA	3533.86
(WC−5Co4Ni1Fe)−0.6Cr3C2 [This work]	0.9	91.25 HRA	3883.2

Table 4. Properties with Co and medium-/high-entropy binder cemented carbides (Partial).

Materials Composition (wt.%)	Mean Grain Size of WC (μm)	Hardness	TRS (MPa)
WC−10Co [40]	0.8	90.8 HRA	3700
WC−10Co−0.6Cr3C2 [40]	<0.8	91.8 HRA	3975
WC−10Co [35]	0.46	91.8 HRA	2818
WC−12Co−0.2Cr3C2 [35]	0.39	92.9 HRA	3483
WC−10Co [5]	1.38	1605 HV1	1677
WC−5Co4Ni1Fe [5]	1.32	1506 HV1	1772
WC−10Co7Ni2Fe1Cr−4.5C [41]	1~2	1080 HV30	1800
WC−10Co7Ni2Fe1Cr−4.9C [41]	1~2	950 HV30	3450
WC−5Co4Ni1Fe [This work]	1.28	90.1 HRA	3533.86
(WC−5Co4Ni1Fe)−0.6Cr3C2 [This work]	0.9	91.25 HRA	3883.2

4. Conclusions

In this study, different amounts of Cr₃C₂ refiner were added to WC−CoNiFe cemented carbide to investigate the effects of Cr₃C₂ on the phase composition and microstructural evolution of WC−CoNiFe cemented carbides during sintering. The corresponding relationships between the microstructure, hardness, transverse rupture strength, and related magnetic properties were analyzed, leading to the following conclusions:

(1)

Cr₃C₂ effectively inhibits the grain growth of WC−CoNiFe. The undoped sample has coarse and uneven grains with a tendency for abnormal growth; with the increase in the amount of refiner, the degree of WC grain refinement in the alloy increases.

(2)

As the Cr₃C₂ content increases, the coercivity of the alloy increases, and the cobalt magnetism decreases, further indicating that Cr₃C₂ has a significant refinery effect on WC grain growth. The inhibition mechanism is closely related to the W and C content in the binder.

(3)

With the increase in Cr₃C₂ content and the refinement of grains, the hardness of the alloy continuously increases, while the transverse rupture strength first increases and then decreases. The decrease in transverse rupture strength is due to Cr₃C₂ reducing the fluidity of the binder, preventing pores from being fully filled, which subsequently leads to a decrease in the alloy’s crack propagation resistance. Additionally, the WC−CoNiFe presented in this study exhibits mechanical properties comparable to the traditional WC−Co cemented carbide cutting tools and may offer lower raw material costs and better engineering application potential.