Cemented carbides rely heavily on the cobalt (Co) phase’s morphology and hardness for wear resistance — a low-hardness, poor-wear-resistance binder phase severely shortens their service life. For instance, WC-Co cemented carbides in mining and construction tools experience rapid Co phase wear during soft rock excavation, exposing WC grains over time. These exposed grains become prone to damage and detachment under operational stress.
Given this, researching Co phase strengthening and hardening mechanisms is critical to boosting the durability and reliability of WC-Co cemented carbides. This article summarizes the latest progress in Co phase-strengthened WC cemented carbides, establishes connections between post-treatment, composition, microstructure, and mechanical properties, and reveals the core principles behind Co phase strengthening.
Secondary Phase Strengthening
In WC-Co cemented carbides, the Co binder phase exists in two allotropic forms: face-centered cubic (fcc) α-Co and hexagonal close-packed (hcp) ε-Co. α-Co is a high-temperature phase, while ε-Co is a low-temperature phase, with a solid-state transformation temperature of approximately 420°C between the two. ε-Co is thermodynamically more stable at room temperature, but during the cooling process after alloy sintering, fine WC grains and solute atoms exert a pinning effect on the Co phase grain boundaries. This inhibits the crystal transformation from α-Co to ε-Co, resulting in a large amount of retained α-Co structure in the cemented carbide at room temperature.
From a performance perspective, ε-Co has fewer slip systems, a lower friction coefficient, and excellent self-lubricating properties. It can significantly enhance the overall wear resistance of cemented carbides, serving as both an important microstructural form for Co binder phase strengthening and a key pathway for the Co phase’s own secondary phase strengthening.
Inducing ε-Co Transformation via Deep Cryogenic Treatment
Deep cryogenic treatment can further promote the martensitic transformation from α-Co to ε-Co: the low-temperature environment provides sufficient driving force for the phase change. After deep cryogenic treatment, the martensitic transformation of the Co phase induced by low temperature, combined with the increase in residual compressive stress, can effectively improve the hardness, flexural strength, fatigue life, and wear resistance of cemented carbides.
(Residual compressive stress refers to the internal stress of the alloy after sintering, deep cryogenic treatment, or cutting deformation. It is a stress that “squeezes” the internal lattice and tightly binds the grains together.)
Effect of Alloying Elements on Stacking Fault Energy
Alloying elements exert differential effects on the stacking fault energy (SFE) of the Co phase: elements such as Cr, Re, Ru, and Rh reduce the SFE, facilitating the transformation from α-Co to ε-Co; elements like Fe, Ni, and Pd increase the SFE, stabilizing the α-Co structure; Mo has a relatively weak impact on the SFE. Doping with elements such as Ta, Re, La, Si, and Cr can all reduce the SFE, promoting the α-Co→ε-Co phase transformation. The role of these elements is summarized in Table 1.
(Stacking fault energy is the energy required for atomic stacking faults to occur in metal crystals. A lower SFE means atoms can more easily undergo the martensitic transformation from α-Co to ε-Co.)
θ Phase Strengthening
In WC-Co cemented carbides, the η phase is a typical carbon-deficient phase, and its formation is closely related to the alloy’s carbon content: when carbon is sufficient, W atoms dissolved in the liquid phase combine with C to form a stable WC phase at the WC grain boundaries; when the carbon content is below the two-phase region and carbon supply is insufficient, some W atoms that cannot form WC react with Co atoms to generate tungsten-cobalt-carbon ternary compounds, which are conventional η phases.
Although conventional η phases can slightly increase the hardness of the Co phase, they cause a sharp decline in the alloy’s fracture toughness, creating a contradiction of “increased hardness but collapsed toughness” — an inherent flaw of traditional carbon-deficient phases. In contrast, the θ phase (chemical formula Co₂W₄C) is essentially a special type of η phase. It can precipitate uniformly in the Co binder phase as nanoscale particles, enhancing the alloy’s performance through a “dispersion strengthening” mechanism without causing embrittlement of the Co phase.
What is Nanodispersion Strengthening?
In the cemented carbide matrix, countless ultra-fine nanoscale hard particles precipitate spontaneously, dispersing uniformly within the Co phase and along grain boundaries — similar to sprinkling a layer of ultra-fine “strengthening hard spots” into the Co phase.
These small particles do not agglomerate or clump but distribute evenly, serving three key functions:
- Blocking dislocation slip to prevent lattice deformation;
- Pinning grain boundaries to inhibit grain growth;
- Improving hardness, strength, and wear resistance without embrittling the material.
Microstructural Characteristics of θ Phase-Strengthened Cemented Carbides
Researchers have developed a new type of θ phase dispersion-strengthened Co-based cemented carbide. The microstructure of this cemented carbide consists of extra-coarse WC particles and a Co phase reinforced by nanophases. As clearly observed in dark-field TEM images, corresponding CSAED patterns, and HRTEM characterizations (Figure 1 (a)): the lattice of the nano-precipitated θ phase particles perfectly matches that of the Co matrix, with no obvious interface between them. This coherent interface ensures strong bonding between the θ phase and Co matrix, avoiding the formation of microcracks and achieving a key breakthrough of “strengthening without embrittlement.”
Fig.1 Nanostructure（a）and mechanical properties（b）of nano-precipitation-strengthened WC-Co
Performance of θ Phase Strengthening
Compared with traditional extra-coarse-grained cemented carbides with the same Co content and similar WC particle size, θ phase-strengthened cemented carbides exhibit significantly higher flexural strength and hardness while maintaining equivalent fracture toughness, with greatly improved wear resistance. As intuitively shown in the comparison curve of wear resistance and fracture toughness (Figure 1 (b)): conventional WC-Co cemented carbides (black curve) exhibit an inverse hyperbolic relationship where “wear resistance and fracture toughness are mutually exclusive,” while nanoscale θ phase-strengthened WC-Co cemented carbides (red curve) have successfully broken this inherent limitation.
Grain Boundary Strengthening
The microstructure of WC-Co cemented carbides consists of a hard phase (WC) and a binder phase (Co), with grain boundaries acting as the “interface” between these two phases. The core principle of grain boundary strengthening lies in its blocking effect on “dislocation slip.” Impurity atoms tend to accumulate at grain boundaries, making it difficult for dislocations (defects in atomic arrangement) to cross the boundaries; instead, dislocations pile up at the boundaries, thereby making the cemented carbide more resistant to deformation and harder.
Principles and Influencing Factors of Grain Boundary Formation
From the perspective of structure-performance relationships, the finer the WC grains, the more grain boundaries form between WC and Co. The increased number of grain boundaries provides more obstacles to dislocation movement, enhancing the strengthening effect.
The natural WC/Co interface has poor coherency, with a coherency degree of only 20% — “coherency” can be understood as the “matching degree” of atomic arrangement at the interface between two phases. A lower matching degree results in weaker interface bonding and limited strengthening effects.
Regulation of Grain Boundaries via Alloying
To optimize grain boundary strengthening, regulation can be achieved by adding other metal carbides (such as VC, Cr₃C₂, TiC, TaC, NbC, etc.): at high sintering temperatures, these added carbides melt and mix with the liquid Co phase. During the cooling process, the Co phase “dissipates” the solute atoms dissolved in the liquid phase, allowing the precipitated carbides to form a thin (W,M)Co nanolayer at the WC/Co interface. The role of this nanolayer is to inhibit excessive WC grain growth, thereby increasing the density of grain boundaries.
Relevant studies have statistically analyzed the coherency of WC/Co interfaces after doping with MC (M=V, Cr, Ti, Ta, Nb) (Figures 2 (b), (c) and Figure 3(a)). It was found that the more significantly WC grain growth is inhibited, the lower the interface coherency rate; however, as the grain boundary density increases, the hardness of the cemented carbide increases significantly. Among these, cemented carbides doped with Cr₃C₂ achieve a balance between high hardness and high transverse fracture strength due to their minimal impact on WC/Co interface coherency.
Fig.2 Bonder phase and WC interface microstructure and interface atomic fracture work（a）HRTEM of WC1- x thin layer at WC-8Co phase boundary；（b）V-doped WC-10Co interface microstructure；（c）atomic interface fracture work
Fig.3 WC-12Co cemented carbides with various inhibitors（a）interface conformity rate of WC/Co；（b）average grain size；（c）Vickers hardness；（d）fracture toughness；（e）TRS
Solid Solution Strengthening
Solid solution strengthening is the most commonly used method for strengthening the Co binder phase in WC-Co cemented carbides. It enhances the performance of the Co phase through atomic-scale structural changes by dissolving other elemental atoms into the Co phase. When foreign atoms integrate into the Co lattice, they act like “foreign objects of varying sizes” inserted into the neatly arranged Co atomic queue, causing slight “distortion” of the original regular lattice. This lattice distortion makes the Co phase resistant to slip deformation, directly improving the strength and hardness of the solid solution binder phase.
Key Solute Elements and Their Strengthening Effects
Different solute elements have distinct strengthening effects and applicable scenarios for the Co phase:
Ruthenium (Ru)
Not only promotes the martensitic transformation of Co from fcc to hcp (synergizing with secondary phase strengthening) but also improves the fluidity of the Co phase during liquid-phase sintering. It directly hardens the binder phase through solid solution-induced lattice distortion. This strengthening effect simultaneously enhances the hardness and fracture strength of cemented carbides, making the Co phase more wear-resistant and less prone to wear. However, as a precious metal, Ru’s high cost limits its large-scale application.
Rhenium (Re)
Exhibits similar performance to Ru. When dissolved in the Co phase, it forms a Co-Re (hcp) solid solution, while increasing the number of high-angle grain boundaries and CSL grain boundaries (Σ=2 and Σ=13a) in the alloy. This significantly improves the hardness and Young’s modulus of cemented carbides, with a wear rate only one-fifth that of traditional WC-Co cemented carbides. A more prominent advantage is Re’s ability to improve the high-temperature mechanical properties of the alloy: the hardness reduction of traditional WC-Co cemented carbides at 300°C and 500°C is almost twice that of WC-Co-Re cemented carbides. Therefore, it is particularly suitable for cutting tools used in processing heat-generating materials such as nickel-based superalloys, with tool life extended by 150%.
Copper (Cu)
Forms a solid solution with Co, inhibiting the dissolution of the WC hard phase and the solid-phase diffusion of W and C elements in the Co phase. Selecting an appropriate Cu doping ratio can improve the alloy’s hardness and wear resistance while ensuring toughness, with relatively low cost.
Non-metallic-related elements
Non-metallic elements or their compounds can also achieve solid solution strengthening. For example, after doping WC-6Co with CrSi₂, Cr and Si atoms do not dissolve in WC grains but form a Co(W,C,Cr,Si) solid solution. This not only reduces the SFE of the Co phase and induces martensitic transformation but also enhances the material’s hardness and compressive strength through the synergistic effect of solid solution strengthening and secondary phase strengthening. When Al is dissolved in the Co phase, the hardness, densification degree, and flexural strength of WC-Co cemented carbides are also significantly improved.
Wniosek
This article compares the advantages, disadvantages, and strength-toughness matching characteristics of different Co phase strengthening pathways, including grain boundary strengthening, solid solution strengthening, ε-Co secondary phase strengthening, θ phase dispersion strengthening, and doping modification. Essentially, all strengthening methods modify the structure and performance of the binder phase through four key dimensions: Co phase lattice distortion, martensitic transformation, nanoscale secondary phase precipitation, and
WC/Co interface coherency regulation. The difference lies in the varying degrees of impact each method has on hardness and fracture toughness.
By sorting out the pros and cons of various Co phase strengthening methods, the following conclusions are clear:
- Solid solution strengthening results in minimal loss of toughness;
- Grain boundary strengthening achieves significant hardness improvement but weakens toughness;
- ε-Co martensitic transformation enables simultaneous enhancement of strength and toughness;
- Nanoscale θ phase dispersion strengthening hardens the Co phase without sacrificing toughness;
- Oxide doping offers the optimal balance of strength and toughness.
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