{"id":23475,"date":"2025-04-17T14:16:00","date_gmt":"2025-04-17T06:16:00","guid":{"rendered":"https:\/\/www.meetyoucarbide.com\/?p=23475"},"modified":"2025-04-17T14:25:09","modified_gmt":"2025-04-17T06:25:09","slug":"phase-of-wc-based-carbide","status":"publish","type":"post","link":"https:\/\/www.meetyoucarbide.com\/vi\/phase-of-wc-based-carbide\/","title":{"rendered":"How Phase Transformations Shape the Properties of WC-based Carbides"},"content":{"rendered":"
The properties of cemented carbides depend not only on the grain size of Nh\u00e0 v\u1ec7 sinh<\/a> 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.<\/p>\n

\"\"<\/p>\n

Phase Composition of WC-Co Cemented Carbides<\/h1>\n

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:<\/p>\n

Before liquid phase formation, the solubility of WC in Co increases with temperature.<\/p>\n

At the eutectic temperature (~1340\u00b0C), a liquid phase with eutectic composition begins to form in the sintered body.<\/p>\n

During sintering at 1400\u00b0C and subsequent holding, the sintered body consists of a liquid phase and residual WC solid phase.<\/p>\n

Upon cooling, WC first precipitates from the liquid phase. Below the eutectic temperature, the WC-based carbides forms a two-phase structure of WC + \u03b3.<\/p>\n

\"\"<\/p>\n

Figure 1: Vertical Section of the W-Co-C Ternary Phase Diagram Along the Co-WC Line<\/p>\n

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 \u03b3+WC two phases. As shown in Figure 2 , the carbon-rich side of the \u03b3+WC two-phase region borders the \u03b3+WC+C three-phase region and the \u03b3+C two-phase region, while the carbon-deficient side borders the \u03b3+WC+\u03b7 three-phase region. Only when the carbon content of the sintered body varies strictly within the \u03b3+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 \u03b7 phase.<\/p>\n

Since the strength of the alloy is closely related to the structure and composition of the \u03b3 phase, while the presence of \u03b7 phase may degrade toughness, extensive research has been conducted on the \u03b3 and \u03b7 phases, as well as phase transformation processes, in an effort to control the phase composition of WC-Co alloys and improve their overall performance.<\/p>\n

\"WC-based<\/p>\n

\u03b3 Phase Composition and Phase Transformation in WC-based carbides<\/a><\/h1>\n

As shown in Figure 2, the composition of the \u03b3 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 \u03b3+WC two-phase region and the \u03b3+WC+\u03b7 three-phase region, the \u03b3 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\u201312 wt.%), the \u03b3 phase contains the lowest tungsten concentration.<\/p>\n

The tungsten concentration in the \u03b3 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 \u03b3 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 \u03b3 phase at a given cooling rate. However, under sufficiently slow cooling, thermodynamic equilibrium dictates that the \u03b3 phase composition becomes independent of the sintering temperature.<\/p>\n

In WC-Co cemented carbides, the \u03b3 phase is a cobalt-based solid solution of W and C. It exists either as discrete \u03b3 grains separated by grain boundaries or as isolated \u03b3 domains unevenly distributed within the matrix. Both \u03b3 grains and domains typically exhibit equiaxed or near-equiaxed morphologies. Notably, the volume fraction of \u03b3 domains increases with higher cobalt content in the WC-based carbide.<\/p>\n

 <\/p>\n

Factors Influencing \u03b3 Phase Transformation in WC-based carbides<\/h2>\n

Effect of Internal Stresses<\/h3>\n

The mismatch in thermal expansion coefficients between WC phase (384\u00d710\u207b\u2076\/\u00b0C) and \u03b3 phase (1.25\u00d710\u207b\u2075\/\u00b0C) generates microstructural stresses during cooling (tensile in \u03b3 phase, compressive in WC phase).<\/p>\n

Increased cooling rate or quenching suppresses W diffusion precipitation in \u03b3 phase, elevating W concentration in room-temperature \u03b3 phase while reducing hcp \u03b3 phase content.<\/p>\n

Cryogenic treatment (below Ms point) induces W supersaturation in \u03b3 phase, enlarging the free energy difference between fcc and hcp \u03b3 phases. Concurrently, enhanced internal stresses promote Ms transformation, markedly increasing hcp \u03b3 phase fraction\u2014particularly pronounced in low-Co alloys.<\/p>\n

Impact of Cobalt Content<\/h3>\n

In low-Co alloys (e.g., WC-8Co), thin \u03b3 phase layers (<0.3 \u03bcm) facilitate W diffusion to WC grains, lowering W concentration in \u03b3 phase. This raises the Ms point, favoring hcp \u03b3 phase formation during cooling and yielding higher room-temperature hcp \u03b3 phase content.<\/p>\n

 <\/p>\n

\u03b7 Phase in WC-based carbides<\/h1>\n

Formation Mechanism and Morphology of \u03b7 Phase<\/h2>\n

Due to the narrow carbon content range in the WC-\u03b3 two-phase region (Fig. 2), carbon deficiency in raw materials or sintering decarburization often leads to \u03b7 phase formation (e.g., M\u2086C-type Co\u2083W\u2083C, Co\u2082W\u2084C, and M\u2081\u2082C-type Co\u2086W\u2086C). Among these, Co\u2083W\u2083C is most common.<\/p>\n

Formation process<\/h3>\n

Heterogeneous nucleation: \u03b3 phase nucleates along WC-\u03b3 interfaces using WC grain surfaces as nucleation sites, facilitated by slow W diffusion from WC to \u03b3 phase and high W concentration at phase boundaries. \u03b3 phase tends to fill surface defects (high-energy sites) of WC grains.<\/p>\n

Carbon loss and \u03b7 phase precipitation<\/h3>\n

Rapid C diffusion in \u03b3 phase causes C depletion when WC dissolves, resulting in W\/C ratio imbalance (room temperature [W]\/[C]\u2248284).<\/p>\n

During sintering (1350-1500\u00b0C), excessive C loss leads to W-rich \u03b3 phase, precipitating carbon-deficient \u03b7 phase (intermediate phases like Co\u2083W and Co\u2086W\u2086C form first, transforming to \u03b7 phase at high temperatures).<\/p>\n

Phase equilibrium and morphology<\/h3>\n

\u03b7 phase growth consumes W and C, driving WC dissolution until equilibrium is reached.<\/p>\n

\u03b7 phase morphology is influenced by \u03b3 liquid phase flow (e.g., cross-shaped single crystals).<\/p>\n

Key point: Carbon imbalance is the primary cause of \u03b7 phase formation, with \u03b3 phase nucleation dependent on WC interfaces and high-temperature C loss driving \u03b7 phase precipitation.<\/p>\n

 <\/p>\n

Factors Influencing \u03b7 Phase Formation<\/h2>\n

Carbon content is critically important for \u03b7 phase formation. In the WC+\u03b3+\u03b7 three-phase region:<\/p>\n

Higher carbon content maintains W and C concentrations in \u03b3 phase closer to equilibrium, hindering \u03b7 phase nucleation.<\/p>\n

Mild carbon deficiency: \u03b7 phase growth relies on dissolution of WC microcrystals in \u03b3 interlayers, resulting in \u03b7 phases enveloping undissolved WC grains with regular geometries.<\/p>\n

Severe carbon deficiency: Significant deviation from equilibrium W\/C ratio in \u03b3 phase promotes extensive WC dissolution, leading to dispersed particulate \u03b7 phase distribution.<\/p>\n

 <\/p>\n

Cobalt content effects<\/h3>\n

High-Co alloys contain more \u03b3 phase with better fluidity, facilitating W and C diffusion. While \u03b7 phase nucleation is difficult, growth is easier, forming coarse, clustered grains.<\/p>\n

 <\/p>\n

WC grain size effects<\/h3>\n

Coarser WC grains promote \u03b7 phase nucleation but slow growth, resulting in dispersed particulate phases.<\/p>\n

 <\/p>\n

Sintering process effects<\/h3>\n

Faster cooling reduces dwell time at \u03b7 phase critical temperature, suppressing \u03b7 phase formation.<\/p>\n

Higher sintering temperatures increase \u03b3 liquid phase quantity, favoring coarse \u03b7 phase grains, but excessive temperatures may keep \u03b3 liquid away from \u03b7 phase boundaries, inhibiting \u03b7 phase growth.<\/p>\n

\"\"<\/p>\n

 <\/p>\n

Conclusions<\/h1>\n

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.<\/p><\/div>\n

<\/p>","protected":false},"excerpt":{"rendered":"

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…<\/p>","protected":false},"author":2,"featured_media":23476,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[79],"tags":[],"class_list":["post-23475","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-materials-weekly"],"jetpack_featured_media_url":"https:\/\/www.meetyoucarbide.com\/wp-content\/uploads\/2025\/04\/d009b3de9c82d158c1c0e3ef5d5236d5bd3e4297.webp","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/posts\/23475","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/comments?post=23475"}],"version-history":[{"count":6,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/posts\/23475\/revisions"}],"predecessor-version":[{"id":23485,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/posts\/23475\/revisions\/23485"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/media\/23476"}],"wp:attachment":[{"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/media?parent=23475"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/categories?post=23475"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.meetyoucarbide.com\/vi\/wp-json\/wp\/v2\/tags?post=23475"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}