3.2.2 The Distribution of W in the γ Phase

In the ideal equilibrium state, the distribution of W in the γ phase should be uniform. However, due to non-equilibrium factors such as the sintering cooling rate, compositional segregation may occur. For instance, near the WC/Co phase interface, the concentration of W might slightly increase. Such microscale compositional inhomogeneities can also influence the initiation and propagation behavior of cracks.

3.2.3 Cobalt Phase Crystal Structure and Its Transformation

Pure cobalt exists as ε-Co with a hexagonal close-packed (hcp) structure at room temperature and normal pressure, but it transforms into α-Co with a face-centered cubic (fcc) structure above approximately 417°C. This transformation is reversible.

In WC-Co cemented carbides, the situation becomes more complex:

Stabilization of the fcc phase: During the cooling process after high-temperature sintering, elements such as W and C—particularly C, which are dissolved in Co—exert a strong stabilizing effect on the fcc-structured α-Co. These elements significantly lower the transformation temperature for the fcc-to-hcp phase transition. As a result, in rapidly cooled cemented carbides, the high-temperature fcc-Co phase can largely remain undercooled and retained even at room temperature. Consequently, the bonding phase in conventional cemented carbides is predominantly composed of the fcc structure.

The relationship between the formation of the hcp phase and carbon content: The fcc-to-hcp phase transformation is a martensitic transformation that is highly sensitive to composition. The lower the carbon content in the alloy, the higher the tungsten (W) content in the bonding phase, and the stronger the tendency for the fcc-to-hcp transformation becomes. This is because a high concentration of W weakens the stabilizing effect of C on the fcc phase. Consequently, in carbon-deficient alloys, in addition to fcc-Co, a significant amount of hcp-Co phase may also appear.

Stacking faults are widespread in the bonding phase, even when the material hasn’t fully transformed into the hcp phase—this can be regarded as an early stage of the fcc-to-hcp transition. Moreover, the density of stacking faults is also correlated with carbon content: the lower the carbon content, the higher the fault density.

The crystal structure of the cobalt phase significantly influences its properties. Compared to hcp-Co, fcc-Co has more slip systems, resulting in better ductility and toughness. On the other hand, hcp-Co and high-density stacking faults tend to impede dislocation movement, enhancing the strength and hardness of the bonding phase—but at the expense of toughness. Consequently, carbon-deficient alloys not only become brittle due to the potential presence of the η phase, but their bonding phase itself also exhibits reduced toughness because of the coexistence of the hcp phase and dense stacking faults. By carefully controlling the alloy to maintain an optimal carbon-rich state, it is possible to ensure that the bonding phase predominantly adopts the highly ductile fcc structure, thereby preserving the overall toughness of the alloy.

3.3 The Influence of WC Carbon Content Changes on Hard Alloy Grain Size

The carbon content of the alloy also significantly influences the growth behavior of WC grains during the sintering process. At its core, this mechanism stems from how carbon content affects the dissolution-precipitation process of WC in liquid cobalt—a key pathway for grain growth during the later stages of sintering (Ostwald ripening).

High carbon content promotes grain growth: When the alloy has a higher carbon level (within the two-phase region), the carbon concentration in the liquid Co phase also increases. This, in turn, enhances the solubility of WC in the liquid Co phase. Higher solubility means that during sintering, the mass transfer process—where small grains dissolve and larger grains grow—proceeds more rapidly and vigorously. Consequently, elevated carbon levels typically accelerate the growth of WC grains, especially in the absence of effective grain-growth inhibitors.

Carbon deficiency inhibits grain growth: Conversely, when the alloy has a low carbon content, the carbon concentration in the liquid-phase Co becomes low, which also reduces the solubility of WC. As a result, the rate of material diffusion slows down, diminishing the driving force for grain growth. Therefore, under carbon-deficient conditions, there is indeed a certain degree of inhibition on grain growth. However, this suppressive effect often comes at the cost of forming the η phase, making it an undesirable trade-off.

This influence is particularly critical when producing ultrafine-grained or nanocrystalline cemented carbides. To achieve fine grain sizes, in addition to using ultrafine WC raw powder and adding grain growth inhibitors, it is also essential to maintain extremely precise control over the alloy's carbon content. Typically, a relatively low carbon level is chosen—just enough to prevent the formation of the η phase—while still ensuring that grain coarsening during sintering is minimized as much as possible.

3.4 The Influence of Carbon Content Variations on the Properties of Cemented Carbides Since carbon content profoundly affects the alloy's phase composition, binder phase structure, and grain size, it naturally follows that it also exerts a comprehensive impact on the alloy's macroscopic properties.

3.4.1 The Influence on Alloy Strength

Strength—especially flexural strength—is most sensitive to structural defects.

Carbon deficiency (η phase): The η phase is the "silent killer" of cemented carbide strength. Its inherently brittle nature, combined with its mismatched deformation behavior relative to the matrix, makes it an ideal initiation site for cracks. Even a trace amount of the η phase can lead to a sharp decline in the alloy's flexural strength.

Carbon-rich (graphite): The graphite phase is soft and porous, and its presence effectively creates microvoids and cracks within the alloy. Consequently, the emergence of graphite also leads to a significant reduction in flexural strength.

The flexural strength of the alloy reaches its peak within the (WC + γ) two-phase region. Deviating from this optimal carbon content—whether by carbon deficiency or carbon excess—results in a deterioration of strength.

3.4.2 The Influence on Alloy Hardness

Carbon-deficient (η phase): The η phase exhibits extremely high hardness, even surpassing that of WC. Consequently, when the η phase is uniformly dispersed throughout the alloy, it significantly boosts the alloy’s macroscopic hardness. However, this remarkable increase in hardness comes at the severe expense of nearly all toughness, rendering the material entirely impractical for engineering applications.

Carbon-rich (graphite): Graphite is one of the softest of all phases, and its presence can reduce the hardness of the alloy.

Within the two-phase region: Inside the (WC + γ) two-phase zone, even slight variations in carbon content have a relatively minor impact on hardness, primarily influencing the alloy through changes in the degree of solid-solution strengthening of the binder phase and the size of the WC grains. Generally, alloys with slightly higher carbon content (closer to the graphite boundary) tend to have a softer binder phase and slightly coarser grains, resulting in a marginally lower hardness.

3.4.3 The Influence on Alloy Density

The density of an alloy is the weighted average of the densities of its constituent phases.

The density of WC is approximately 15.7 g/cm³.

The density of Co is approximately 8.9 g/cm³.

The density of the η phase (such as Co₃W₃C) typically ranges around 11–13 g/cm³, which is lower than that of WC.

Graphite has a density of approximately 2.2 g/cm³, which is significantly lower than that of the other phases. Therefore:

Carbon deficiency causes WC and Co to transform into the lower-density η phase, slightly reducing the overall density of the alloy.

Carbon enrichment leads to the precipitation of extremely low-density graphite, which significantly reduces the overall density of the alloy.

In the two-phase region, the alloy exhibits the highest density. Therefore, density measurement is also an effective method for detecting whether sintered alloys undergo phase transformations or contain defects such as porosity.

3.4.4 The Influence on Alloy Corrosion Resistance

Cemented carbide corrosion is typically electrochemical, occurring primarily on the cobalt (Co) binder phase.

The ideal microstructure: The two-phase structure of (WC + γ) exhibits the best corrosion resistance. The bonding phase uniformly coats the chemically inert, high-WC grains, forming an effective protective layer.

Carbon deficiency (η phase): The electrode potential of the η phase differs from both WC and the γ phase. Its presence creates multiple microscopic corrosion galvanic couples within the alloy, accelerating the corrosion of the bonding phase (which is preferentially corroded).

Carbon-rich (graphite): Graphite also exhibits an electrode potential distinct from other phases, which can lead to the formation of corrosion galvanic couples, accelerating the corrosion process.

The binder phase composition: Carbon-deficient alloys have a high W content in their binder phase, which typically enhances their corrosion resistance. However, this beneficial effect is often overshadowed by the negative impact introduced by the η phase.

In summary, to achieve optimal corrosion resistance, it is equally important to strictly control the carbon content within the two-phase region, ensuring a uniform microstructure free of any tertiary phases.

3.4.5 The Influence on the Lattice Constant of the γ Phase

The lattice constant of the γ phase (fcc-Co solid solution) is highly sensitive to the solute atoms dissolved within it. Since the radius of W atoms is larger than that of Co atoms, when W dissolves into Co, it causes the Co lattice to expand, thereby increasing the lattice constant.

As previously mentioned, the carbon content of the alloy determines the solubility of W in Co:

Low carbon → High W solubility → Increased lattice constant of the γ phase

High carbon → Low W solubility → γ-phase lattice constant decreases

Research shows that the lattice constant of the γ phase exhibits a nearly linear relationship with the total carbon content of the alloy. This relationship holds significant practical value. By precisely measuring the lattice constant of the γ phase using X-ray diffraction (XRD), it is possible to deduce the carbon equilibrium state of the alloy. This approach provides a powerful analytical tool for non-destructive, accurate monitoring and evaluation of the carbon content in cemented carbides.

3.5WC Carbon Content vs. Relative Saturation Magnetization of Cemented Carbides

Saturation magnetization is the maximum magnetization a material can achieve under an external magnetic field strong enough. In cemented carbides, magnetism primarily originates from the Co binder phase. Pure Co exhibits exceptionally high saturation magnetization. However, when non-magnetic W atoms dissolve into Co to form a solid solution, they dilute Co's magnetic moments, leading to a reduction in the saturation magnetization of the binder phase. The more W that dissolves, the more pronounced the decline in saturation magnetization becomes.

We define the ratio of the saturation magnetization of cemented carbide to that of pure Co with the same content as the relative saturation magnetization, typically expressed as a percentage (%σs). This value directly reflects the degree of W dissolution in the binder phase.

Combining the previous discussion on how carbon content affects the solubility of W, we can establish the following chain of relationships:

Increased total carbon content in the alloy → increased liquid-phase carbon potential → decreased solubility of W in Co → reduced [W] in the bonding phase → increased σs of the bonding phase → increased alloy %σs

Alloy's total carbon content ↓ → Liquid-phase carbon potential ↓ → Solubility of W in Co ↑ → Carbon concentration [W] in the bonding phase ↑ → Yield strength σs of the bonding phase ↓ → Alloy's percentage yield strength ↓

Therefore, the relative saturation magnetization is positively correlated with the total carbon content of the alloy.

A high σs value (close to 100%) indicates that very little W is dissolved in the bonding phase, suggesting the alloy is carbon-rich and may already be approaching or even exhibiting graphite formation.

A low σs value indicates that a large amount of W is dissolved in the bonding phase, suggesting the alloy is carbon-deficient and may be approaching or already exhibiting the η phase.

In the (WC+γ) two-phase region, %σs will have a moderate, ideal range.

This clear correlation makes the measurement of relative saturation magnetization the most commonly used, fastest, and most effective method for carbon balance control in industrial production. By measuring %σs, it becomes possible to sensitively determine whether a product is "under-carbon," "over-carbon," or "just right," thereby guiding process adjustments and enabling precise classification of product quality.

3.6 Carbon Content's Influence on Alloy Bending and Deformation

Here, the bending and deformation primarily refer to the macroscopic shape changes that occur in cemented carbide products (especially those with elongated or thin-sheet-like forms) after sintering. Ideally, the sintering process should result in uniform shrinkage while preserving the original shape of the part.

Changes in carbon content, particularly uneven carbon distribution, are one of the key factors leading to sintering deformation.

Phase Transformation Volume Effect: The formation of the η phase and the precipitation of graphite both involve volume changes. If, in different regions of a component, uneven carbon content—caused, for instance, by variations in pressing density or non-uniform sintering atmospheres leading to localized decarburization or carburization—results in differing degrees of phase transformation, this will lead to inconsistent shrinkage across the part. Consequently, the entire component may bend, warp, or twist. For example, if decarburization occurs on one surface of the part, forming the η phase, the shrinkage behavior of that area will differ from the normal microstructure within the material, ultimately causing deformation.

Sintering shrinkage behavior: Carbon content influences the temperature at which the liquid phase appears (the eutectic temperature) and the amount of liquid phase formed, thereby affecting the sintering shrinkage rate and the final density. Uneven carbon distribution can lead to inconsistent shrinkage processes across different regions, potentially causing macroscopic deformation as well.

Therefore, to produce cemented carbide parts with high dimensional accuracy and uniform shapes, it is essential not only to ensure that the overall carbon content remains within the correct range but also to guarantee a uniform distribution of carbon throughout the entire volume of the part. This places extremely stringent demands on the uniform mixing of raw materials, the consistent density of the green compact, and the stability of the atmosphere inside the sintering furnace.

Via: "My Cemented Carbide Journey" – Xiao Zhou