The properties of cemented carbides depend not only on the grain size of WC but also significantly on the phase composition, microstructure, and their distribution in the alloy. In actual production, due to factors such as raw materials and sintering processes, the alloy typically exhibits a complex microstructure. Therefore, this paper primarily discusses the phase composition and phase transformation process in WC-based carbides from a thermodynamic perspective, based on the W-Co-C phase diagram.

Phase Composition of WC-Co Cemented Carbides

Figure 1 shows the vertical section of the W-Co-C ternary phase diagram along the Co-WC line. Taking a WC-60%Co alloy as an example:

Before liquid phase formation, the solubility of WC in Co increases with temperature.

At the eutectic temperature (~1340°C), a liquid phase with eutectic composition begins to form in the sintered body.

During sintering at 1400°C and subsequent holding, the sintered body consists of a liquid phase and residual WC solid phase.

Upon cooling, WC first precipitates from the liquid phase. Below the eutectic temperature, the WC-based carbides forms a two-phase structure of WC + γ.

Figure 1: Vertical Section of the W-Co-C Ternary Phase Diagram Along the Co-WC Line

In actual production, the composition of sintered bodies often deviates from the vertical section of the Co-WC line. Consequently, the alloy is not simply composed of γ+WC two phases. As shown in Figure 2 , the carbon-rich side of the γ+WC two-phase region borders the γ+WC+C three-phase region and the γ+C two-phase region, while the carbon-deficient side borders the γ+WC+η three-phase region. Only when the carbon content of the sintered body varies strictly within the γ+WC two-phase region can the WC-based carbide avoid the formation of a third phase. Otherwise, it may lead to carbon inclusions or the formation of carbon-deficient η phase.

Since the strength of the alloy is closely related to the structure and composition of the γ phase, while the presence of η phase may degrade toughness, extensive research has been conducted on the γ and η phases, as well as phase transformation processes, in an effort to control the phase composition of WC-Co alloys and improve their overall performance.

γ Phase Composition and Phase Transformation in WC-based carbides

As shown in Figure 2, the composition of the γ phase depends on the carbon content of the alloy, while its tungsten content increases with decreasing carbon content. When the alloy’s carbon content lies at the boundary between the γ+WC two-phase region and the γ+WC+η three-phase region, the γ phase exhibits the highest tungsten concentration. Conversely, when free carbon is present and the carbon content aligns precisely with the Co-WC cross-section (i.e., the theoretical carbon content of 6–12 wt.%), the γ phase contains the lowest tungsten concentration.

The tungsten concentration in the γ phase is also influenced by the cooling rate: slower cooling results in lower tungsten content, while rapid cooling leads to higher tungsten retention. This occurs because faster cooling suppresses the diffusion-driven precipitation of tungsten from the γ phase, locking in a non-equilibrium concentration. Additionally, higher sintering temperatures increase the tungsten solubility in the liquid phase, thereby raising the tungsten content in the γ phase at a given cooling rate. However, under sufficiently slow cooling, thermodynamic equilibrium dictates that the γ phase composition becomes independent of the sintering temperature.

In WC-Co cemented carbides, the γ phase is a cobalt-based solid solution of W and C. It exists either as discrete γ grains separated by grain boundaries or as isolated γ domains unevenly distributed within the matrix. Both γ grains and domains typically exhibit equiaxed or near-equiaxed morphologies. Notably, the volume fraction of γ domains increases with higher cobalt content in the WC-based carbide.

 

Factors Influencing γ Phase Transformation in WC-based carbides

Effect of Internal Stresses

The mismatch in thermal expansion coefficients between WC phase (384×10??/°C) and γ phase (1.25×10??/°C) generates microstructural stresses during cooling (tensile in γ phase, compressive in WC phase).

Increased cooling rate or quenching suppresses W diffusion precipitation in γ phase, elevating W concentration in room-temperature γ phase while reducing hcp γ phase content.

Cryogenic treatment (below Ms point) induces W supersaturation in γ phase, enlarging the free energy difference between fcc and hcp γ phases. Concurrently, enhanced internal stresses promote Ms transformation, markedly increasing hcp γ phase fraction—particularly pronounced in low-Co alloys.

Impact of Cobalt Content

In low-Co alloys (e.g., WC-8Co), thin γ phase layers (<0.3 μm) facilitate W diffusion to WC grains, lowering W concentration in γ phase. This raises the Ms point, favoring hcp γ phase formation during cooling and yielding higher room-temperature hcp γ phase content.

 

η Phase in WC-based carbides

Formation Mechanism and Morphology of η Phase

Due to the narrow carbon content range in the WC-γ two-phase region (Fig. 2), carbon deficiency in raw materials or sintering decarburization often leads to η phase formation (e.g., M?C-type Co?W?C, Co?W?C, and M??C-type Co?W?C). Among these, Co?W?C is most common.

Formation process

Heterogeneous nucleation: γ phase nucleates along WC-γ interfaces using WC grain surfaces as nucleation sites, facilitated by slow W diffusion from WC to γ phase and high W concentration at phase boundaries. γ phase tends to fill surface defects (high-energy sites) of WC grains.

Carbon loss and η phase precipitation

Rapid C diffusion in γ phase causes C depletion when WC dissolves, resulting in W/C ratio imbalance (room temperature [W]/[C]≈284).

During sintering (1350-1500°C), excessive C loss leads to W-rich γ phase, precipitating carbon-deficient η phase (intermediate phases like Co?W and Co?W?C form first, transforming to η phase at high temperatures).

Phase equilibrium and morphology

η phase growth consumes W and C, driving WC dissolution until equilibrium is reached.

η phase morphology is influenced by γ liquid phase flow (e.g., cross-shaped single crystals).

Key point: Carbon imbalance is the primary cause of η phase formation, with γ phase nucleation dependent on WC interfaces and high-temperature C loss driving η phase precipitation.

 

Factors Influencing η Phase Formation

Carbon content is critically important for η phase formation. In the WC+γ+η three-phase region:

Higher carbon content maintains W and C concentrations in γ phase closer to equilibrium, hindering η phase nucleation.

Mild carbon deficiency: η phase growth relies on dissolution of WC microcrystals in γ interlayers, resulting in η phases enveloping undissolved WC grains with regular geometries.

Severe carbon deficiency: Significant deviation from equilibrium W/C ratio in γ phase promotes extensive WC dissolution, leading to dispersed particulate η phase distribution.

 

Cobalt content effects

High-Co alloys contain more γ phase with better fluidity, facilitating W and C diffusion. While η phase nucleation is difficult, growth is easier, forming coarse, clustered grains.

 

WC grain size effects

Coarser WC grains promote η phase nucleation but slow growth, resulting in dispersed particulate phases.

 

Sintering process effects

Faster cooling reduces dwell time at η phase critical temperature, suppressing η phase formation.

Higher sintering temperatures increase γ liquid phase quantity, favoring coarse η phase grains, but excessive temperatures may keep γ liquid away from η phase boundaries, inhibiting η phase growth.

 

Conclusions

A comprehensive understanding of the phase transformation processes during the sintering of WC-based carbides is crucial for optimizing production processes, controlling phase composition and microstructure in the alloys, thereby creating favorable conditions for manufacturing high-performance WC cemented carbides.