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简体中文 High-entropy carbide is a new material that solves big problems for traditional cemented carbides. These old materials struggle with costly, hard-to-get cobalt (Co) and can’t easily balance hardness and toughness. This new carbide, made of five metals and carbon, is hard and stable. A key study shows we can control its performance by changing carbon levels or using different metal binders like iron (Fe) or nickel (Ni).
The Formula Transformation of Cemented Carbides
The “Key Points” of High-entropy karbit
Traditional cemented carbides use WC (tungsten carbide) as the hard phase and Co as the binder phase, similar to “mixing stones with cement” — WC particles provide hardness, while Co metal fills the gaps and imparts toughness. However, Co is a scarce and toxic resource, making the search for alternative materials an urgent industry need.
The emergence of high-entropy carbide couldn’t be more timely. “High-entropy” refers to the mixing of multiple elements in equimolar ratios to form a stable single solid solution phase, much like multiple metal elements “huddling together for warmth.” This structure not only retains the high hardness of ceramics but also enhances stability through synergistic effects between elements. Among these, (Ti,Nb,Ta,Mo,W)C is a research hotspot. It contains five metal elements — titanium, niobium, tantalum, molybdenum, and tungsten — and even molybdenum and tungsten, which tend to form special carbides, can stably exist in a face-centered cubic (FCC) structure, making it an ideal new hard phase.
Carbon Content and Binder Phase: Two Key Control Knobs
The performance of cemented carbides relies entirely on the tacit cooperation between the “hard phase” and the “binder phase,” with carbon content being the core knob to adjust this cooperation. Excessive carbon leads to the precipitation of free graphite, making the material brittle; insufficient carbon, on the other hand, forms brittle intermetallic compounds (such as η phase), which also impairs performance.
The other key knob is the binder phase. Researchers have chosen Fe and Ni to replace Co, which are not only more cost-effective and environmentally friendly but also have distinct “personalities”: Fe has a low liquidus formation temperature (approximately 1095℃), leading to easy carbon loss during sintering; Ni has a higher liquidus formation temperature (approximately 1330℃), ensuring more stable carbon content and forming a stable two-phase region in a narrow temperature range. These differences directly result in significant variations in the final material performance.
How to Prepare High-Entropy Carbides in Experiments?
The experimental process of this study perfectly demonstrates the core steps of preparing high-end cemented carbides via powder metallurgy, with precise control embedded in each step:
Preparing High-Entropy Carbide Powders with Three Carbon Contents
The starting point of the experiment is synthesizing (Ti,Nb,Ta,Mo,W)C powders (abbreviated as HEC-L, HEC-M, HEC-H) with three carbon contents: low (47.5 mol%), medium (52.5 mol%), and high (57.5 mol%).
Raw material proportioning: Ti, Nb, Ta, Mo, and W metal powders are weighed in equimolar ratios, and graphite powder is added according to the target carbon content to ensure the precise ratio of carbon to metal elements.
Mechanical alloying: The mixed powders are placed in a ball mill with cemented carbide balls and high-speed ball-milled for 40 hours under argon protection. During ball milling, metal particles are crushed by impact and fully mixed with carbon powder to form ultra-fine composite powders while avoiding oxidation.
Vacuum sintering synthesis: The ball-milled powders are placed in a graphite mold and sintered at 1800℃ in a vacuum environment for 2 hours. At high temperatures, carbon and metals undergo carbothermal reduction reactions to form high-entropy carbides. At this stage, differences between different carbon contents become apparent: medium-carbon HEC-M forms a pure single-phase FCC structure, low-carbon HEC-L precipitates body-centered cubic (BCC) metal phases and M₂C phases, while high-carbon HEC-H exhibits hexagonal MC carbides and free carbon. This result is intuitively verified by scanning electron microscopy (SEM) images (Figure 3):
the three images show the micro-morphologies of HEC-L, HEC-M, and HEC-H powders, clearly displaying differences in particle shapes among powders with different carbon contents. The EDS element distribution maps below further confirm that, except for special phase regions, the five metal elements are relatively uniformly distributed in the powders.
Figür 3 Scanning electron microscopy (SEM) micrographs of as-synthesized HEC powders:
a) HEC-L powder; b) HEC-M powder; c) HEC-H powder.
Combining with Fe/Ni Binder Phases and Secondary Sintering Forming
Next, the three HEC powders are respectively mixed with 20wt% Fe powder or Ni powder to prepare the final cermets, a process similar to “kneading dough to make steamed buns”:
Mixing and forming: HEC powders are mixed with Fe/Ni powders in a ball mill, with a small amount of dispersant (such as alcohol) added, and low-speed ball-milled for 8 hours to ensure uniform mixing. Subsequently, the mixture is compacted under 200MPa pressure to form disc-shaped green compacts, much like pressing flour into steamed bun embryos.
Degreasing and pre-sintering: The green compacts are placed in an inert atmosphere furnace, first held at 500℃ for 2 hours to remove residual dispersants, then heated to 900℃ for pre-sintering to initially harden the compacts and prevent deformation during subsequent sintering.
Final sintering: Vacuum liquid-phase sintering is adopted. The HEC-Fe system is heated to 1150℃ (slightly higher than Fe’s liquidus temperature), and the HEC-Ni system to 1380℃ (slightly higher than Ni’s liquidus temperature), with a 1-hour holding period. At this stage, the metal binder phase melts, fills the gaps between HEC particles, and promotes uniform grain growth through dissolution-precipitation mechanisms, ultimately forming near-fully dense cermets.
Multi-Dimensional Characterization to Unveil the Material’s “True Face”
After sintering, the materials undergo a series of “high-tech tests” to analyze their performance:
XRD phase analysis: X-ray diffractometer irradiates the samples, and phase composition is determined by the position of diffraction peaks. For example, single-phase HEC shows a single FCC characteristic peak, while additional diffraction peaks appear if secondary phases are present. The XRD patterns in Figure 2 clearly demonstrate this difference: HEC powders with low, medium, and high carbon contents, as well as their corresponding Fe/Ni-based cemented carbides, exhibit varying numbers and positions of diffraction peaks, intuitively confirming the influence of different carbon contents and binders on phase composition — for instance, medium-carbon HEC-M powder shows a single diffraction peak, while low-carbon HEC-L powder displays additional secondary phase diffraction peaks.
şekil 2 X-ray diffraction (XRD) patterns of HEC-Fe-based (blue), HEC-Ni-based (green) cemented carbides, and corresponding HEC powders (black):
a) Low-carbon HEC-L powder and its derived cemented carbides HLF, HLN;
b) Medium-carbon HEC-M powder and its derived cemented carbides HMF, HMN;
c) High-carbon HEC-H powder and its derived cemented carbides HHF, HHN.
Mechanical property testing: Vickers hardness tester measures hardness (unit: HV10), and fracture toughness (unit: MPa·m¹/²) is calculated via indentation method. Hardness reflects the material’s wear resistance, while toughness indicates its impact resistance — the balance between the two is the core criterion for evaluating cemented carbides.
How Carbon Content and Binder Phase Affect Performance?
After a series of experiments, researchers have drawn several key conclusions that directly guide the future design of cemented carbides:
Medium Carbon Content is the “Golden Ratio”
In materials science, carbon content is a key indicator of material composition, usually expressed as mass percentage or atomic percentage. “Medium carbon content” refers to a specific range relative to low and high carbon contents, and in the research system of high-entropy carbides (HEC), it is the intermediate value among the three set carbon contents. This content range is crucial because, among the three carbon contents, medium-carbon HEC-M is the only formulation that can form a single-phase high-entropy carbide.
When combined with either Fe or Ni, medium-carbon samples have a more uniform microstructure without redundant brittle phases or free carbon. In particular, the medium-carbon HEC-Ni (HMN) achieves a perfect balance of hardness and toughness, with a hardness of 1200 HV10 and a fracture toughness of approximately 10.0 MPa·m¹/², making it the optimal solution in terms of comprehensive performance. This conclusion is highly consistent with the thermodynamic phase diagram predictions in Figure 1: Figures 1a, 1b, and 1c show the phase diagrams of the Ti-Nb-Ta-Mo-W-C, Ti-Nb-Ta-Mo-W-C-Fe, and Ti-Nb-Ta-Mo-W-C-Ni systems, respectively. Green, blue, and red marks correspond to the positions of low, medium, and high carbon content samples. The diagrams clearly show that medium-carbon HEC-M is located in the most stable single-phase region, while low and high carbon content samples are in regions prone to precipitating secondary phases or free carbon, forming a perfect correspondence between thermodynamic calculations and experimental results.
Şekil 1 Phase diagrams of the Ti-Nb-Ta-Mo-W-C system (a), Ti-Nb-Ta-Mo-W-C-Fe system (b), and Ti-Nb-Ta-Mo-W-C-Ni system (c).
The mechanical property comparison chart in Figure 8 further confirms this trend: as carbon content increases, the hardness of both systems generally decreases while toughness improves. Ni-based samples have higher overall toughness than Fe-based ones, and the medium-carbon Ni-based sample (HMN) shows a balanced bar chart, intuitively reflecting its optimal comprehensive performance.
Şekil 8 Mechanical properties of HEC-Fe-based and HEC-Ni-based cemented carbides.
Differences Between Fe and Ni Binder Phases Lead to Performance Differentiation
HEC-Fe system: A large number of M₆C-type η phases precipitate in low and medium carbon contents. These brittle phases act like “impurities,” disrupting the uniform distribution of HEC grains in the binder phase; in high carbon content, η phase formation is inhibited, but the proportion of binder phase increases significantly, leading to decreased hardness and improved toughness. Overall, Fe-based samples have higher hardness but lower toughness.
HEC-Ni system: No η phase forms in any carbon content, and the secondary phase is only (W,Mo)ₓCᵧ, with grain size comparable to HEC, resulting in a more uniform microstructure. Ni-based samples have slightly lower hardness than Fe-based ones but superior toughness, especially the medium-carbon sample, which becomes the preferred choice for balancing performance and stability.
Figure 4 Optical microscopy (OM) micrographs of the prepared HEC-Fe-based (left) and HEC-Ni-based (right) cemented carbides.
These microstructural differences are vividly displayed in the characterization images: Optical microscopy images in Figure 4 macroscopically show the contrast between Fe-based and Ni-based cemented carbides, making the influence of binders on structure more intuitive. SEM images in Figure 5 further zoom in on the details: in left Fe-based samples, low and medium carbon contents show scattered η phase particles, while high-carbon HEC-Fe exhibits rose-like aggregated carbon; right Ni-based samples have uniform structures overall, with high-carbon HEC-Ni showing flaky discrete carbon — these morphological differences directly explain the performance gap between the two systems.
Figure 5 SEM micrographs of the microstructures of the prepared cemented carbides:
a), b) HLF and HLN based on HEC-L, respectively;
c), d) HMF and HMN based on HEC-M, respectively;
e), f) HHF and HHN based on HEC-H, respectively.
Energy-dispersive spectroscopy (EDS) provides deeper insights: Figure 6’s analysis results accurately show the element distribution in HMF and HMN cemented carbides, verifying that HEC grains form a “core-shell structure” — cores rich in heavy elements like molybdenum and uniform element distribution in shells. Figure 7 captures a unique mesoscopic two-phase region in HEC-Ni samples, where large-sized HEC grains and sparse Ni binder phases distribute uniformly, a feature not found in Fe-based samples, which is attributed to Ni’s higher liquidus temperature and less sufficient dissolution-precipitation during sintering.
Figure 6 Energy-dispersive spectroscopy (EDS) analysis results of the microstructures of cemented carbides prepared with HEC-M as the starting powder:
a) HMF cemented carbide; b) HMN cemented carbide.
Figure 7 SEM micrograph and EDS elemental mapping of the mesoscopic biphasic region in the low-carbon HEC-Ni-based cemented carbide (HLN).
Çözüm
The value of this research on high-entropy carbide-based cemented carbides lies not only in revealing the influence laws of carbon content and binder phase but also in laying the foundation for their industrial application. In the future, optimizing compositions (such as reducing molybdenum and tungsten contents) and improving sintering processes (such as adopting spark plasma sintering, SPS) are expected to produce more stable single-phase high-entropy carbides.
For industry, this means that cemented carbides can be “customized” according to needs: medium-carbon HEC-Fe is suitable for cutting tools requiring high hardness, while medium-carbon HEC-Ni is a better choice for mining drill bits needing high toughness. The replacement of Co with Fe and Ni not only reduces costs but also solves resource and environmental problems, driving the cemented carbide industry towards a green and efficient direction.
