Variation of WC grain shape with carbon content in the WC–Co alloys during liquid-phase sintering

WC–Co hard metals have faceted WC grains dispersed in a ductile cobalt-rich matrix. The effect of carbon (C) content on the shape of WC grain in the WC–Co metals during liquid-phase sintering is investigated in this work. The WC grain shape varies with the C content and, more importantly, the shape change occurs reversibly with the C content.     

A new approach to fabrication of gradient WC–Co hardmetals

WC–Co hardmetals with gradient structure comprising neither η-phase nor grain growth inhibitors were produced for the first time by regulating the WC re-crystallisation and carbon content in their near-surface layer and core. Hardmetals with low Co contents in the surface region were obtained by selective carburisation of the near-surface zone of green articles with the original low carbon content and their consequent liquid-phase sintering. The surface region of such gradient hardmetals has a hardness of up 150 Vickers units higher and fracture toughness significantly superior than those of the core. Gradient hardmetals with high Co contents in the surface region were obtained by selective decarburisation of the near-surface zone of green articles with the original high carbon content and their consequent liquid-phase sintering. The new approach for fabrication of gradient WC–Co materials appears to be a unique tool for increasing both the hardmetal hardness and fracture toughness.     

Properties of WC–8Co hardmetals with plate-like WC grains prepared by plasma-assisted milling

WC-8C0 hardmetals with different proportions of prismatic WC grains and plate-like WC grains were directly produced through sintering the W-C-8C0 elemental powder mixture which was fabricated by dielectric barrier discharge plasma(DBDP)-assisted milling.The morphology of prepared WC-8C0 hardmetals,geometry and the preferential orientation of plate-like WC were investigated by X-ray diffraction(XRD) and scanning electron microscopy(SEM) analysis.The results demonstrate that the microstructure and mechanical properties of the sintered hardmetals are related to the morphology of W grain which is dependent on DBDP-milling time.The DBDP for 1 h(DBDP-1 h)-milled W-C-Co powder contains granular W particles that tend to form prismatic WC grains,while the DBDP for 3 h(DBDP-3 h)-milled powder contains lamellar W particles that generate plate-like WC grains.By adjusting the weight ratio of DBDP-1 h powder and DBDP-3 h powder in W-C-8C0 mixture,the proportion of plate-like WC in the hardmetals can be controlled,and relatively high transverse rupture strength(TRS) is obtained as the proportion of plate-like WC grain in the hardmetals is about 35%in present experimental condition.     

Influence of Ca ions and temperature on the corrosion behavior of WC–Co hardmetals in alkaline solutions

The effect of temperature and Ca ions on the corrosion behavior of hardmetals was investigated in 0.1 M NaOH and 0.05 M Ca(OH)₂ alkaline electrolytes using impedance spectroscopy, potentiodynamic polarization and surface analytical techniques. It was found that calcium containing alkaline solutions efficiently decrease the anodic currents up to 5 times by forming a calcium containing deposit on the top of the WC–Co hardmetal surface, which remains stable even at higher temperatures (40 °C and 60 °C). This positive influence of Ca ions is predominant under polarization in the range from 0 to +0.85 V (Ag/AgCl) but is not apparent under OCP conditions. In NaOH, however, the corrosion resistance strongly decreases at higher temperatures as compared with the room temperature. At the slightly elevated temperature in 0.1 M NaOH the Co binder phase loses its passivity and is almost completely washed out of the compound material. A WC skeleton remains on the surface and hence the ductility in the hardmetal is lost. In the end, the material could completely fail under such operating conditions. Also the Ni alloyed binder loses its strong passivation ability at the elevated temperatures.     

A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid.This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.     

Near-nano WC–Co hardmetals: Will they substitute conventional coarse-grained mining grades?

Development of nanostructured hardmetals is a task of great importance. Nevertheless, in spite of some “euphoria” with respect to nanograined hardmetals, their potential application ranges are yet not clear. In some works, near-nano and nano hardmetals are believed can potentially substitute conventional medium- and coarse-grained WC–Co grades. In the present work near-nano hardmetals with WC mean grain size of nearly 200 nm and Co contents of 10–33 wt.% were produced and examined with respect to their hardness, fracture toughness, transverse rupture strength and wear-resistance. The near-nano hardmetal with 10% Co having a hardness of 20 GPa and fracture toughness of 9.5 MPa m^1/2 is characterised by exceptionally high wear-resistance obtained by use of the ASTM B611 test in comparison with an ultra-fine grade with 10%. The wear-resistance of the near-nano hardmetals in the ASTM B611 test significantly decreases with increasing the Co content and the wear rates of the difference between the wear rates of the grades with 10% and 33% Co is equal to nearly 44 times. The near-nano hardmetals with 25%, 28% and 33% Co having a moderate hardness and high fracture toughness corresponding to conventional coarse and ultra-coarse-grained mining grades have a very low wear-resistance in laboratory tests on concrete-cutting, granite-cutting and percussion drilling of quartzite. A number of grades with the very similar hardness of 13 ± 0.2 GPa, WC mean grain sizes varying from 0.2 to 4.8 μm and Co contents varying from 3% to 25% were produced and examined by use of the ASTM B611 test. The wear-resistance of the near-nano grade with 25% Co is found to be lower by more than three times compared to the coarsest grade with 3% Co at almost the same hardness. In this case, in spite of the very similar hardness of all the samples, the proportion of the soft binder phase on the surface subjected to abrasive particles when performing the test is significantly higher for the near-nano grade compared to the coarse- and ultra-coarse grained hardmetals. Thus, near-nano and presumably nano hardmetals are expected to never substitute conventional medium- and coarse-grained mining grades. The only application range, where near-nano and nano hardmetals can potentially substitute conventional grades, is an application range of hardnesses of above 18 GPa.     

Effect of Cooling Rate on the Formation and Morphology of (W,V)C_x in VC-doped WC–Co Cemented Carbide

The grain growth retardation mechanism and the effect of cooling rate on VC-doped WC–Co cemented carbides were investigated in this work.WC–30Co and WC–30Co–VC were prepared by powder metallurgy,liquid-phase sintering at 1400 °C and followed by water quenching([150 °C/s) or furnace cooling(*0.083 °C/s).Based on the results of electron probe microanalysis(EPMA),we found that WC concentration in the Co binder was independent of VC doping during liquid-phase sintering,hence barely contributing to the retardation of WC grain growth.In contrast,the(W,V)C_x phase formed at the WC/Co interfaces played a major role in retarding WC grain growth during liquid-phase sintering.The effect of cooling rate on the morphology of(W,V)C_xwas revealed by high-resolution transmission electron microscopy(HRTEM) and energy-dispersive spectroscopy(EDS).In the water-quenched WC–30Co–VC,(W,V)C_xprecipitates were found as thin layers at the WC/Co interfaces.In contrast,both thin layers of similar thickness and nanoparticles of(W,V)C_x were observed in the furnace-cooled counterpart.These observations listed above suggested that thin(W,V)C_xlayers were stable structures effectively suppressing the growth of WC grains and their thickness remained independent of the cooling rate.The(W,V)C_xnanoparticles,however,may be inhibited through rapid cooling,ensuring the VC-doped WC–Co cemented carbides desired toughness.     

Abnormal grain growth in Al–3.5Cu

Significant abnormal grain growth has been observed in an Al–3.5 wt.% Cu alloy at temperatures where the volume fraction of small CuAl₂ particles was less than about 0.01. The initial fine-grained material had a weak crystallographic texture and there was no indication that any special boundaries were involved in the abnormal growth. Island grains isolated within the abnormal grains also showed no indication of special orientation relationships with their surrounding grains. Measurements indicated that the island grains initially had a size advantage over other matrix grains. The fraction of pinning phase was much lower at abnormal grain boundaries than at boundaries in the fine-grained matrix into which they were growing. A variety of simulations were made, including attempts to model that difference in pinning phase distribution, but none of these were successful in predicting abnormal grain growth.     

Electrochemical corrosion behaviors of straight WC–Co alloys: Exclusive variation in grain sizes and aggressive media

The electrochemical corrosion behaviors of straight WC–10Co cemented carbides with grain sizes of 1.2, 2.6, 6.1 and 8.2 μm, were comparatively investigated in the solutions of NaOH (pH = 13), Na₂SO₄ (pH = 7) and H₂SO₄ (pH = 1) respectively. To insure a sole variable of WC grain sizes, specific magnetic saturation values of the alloys are adjusted to be identical. The results show a good linear dependence for R_ct (charge transfer resistance) and I_corr (corrosion current density) against the grain sizes. A high sensitivity of the grain sizes to both R_ct and I_corr are identified in NaOH and H₂SO₄. In the solutions of NaOH and Na₂SO₄, the alloys with smaller WC grain sizes exhibit better corrosion resistances, while the alloys with larger WC grain sizes exhibit better corrosion resistances in H₂SO₄. Additionally, in terms of the corrosiveness, NaOH is the weakest and H₂SO₄ is the most aggressive for all the alloys. The corrosion mechanisms were discussed in light of the SEM surface observation, X-ray photoelectron spectroscope analysis and the electrical equivalent circuits for electrochemical impedance spectroscopy.     

Viscosity of WC–Co compacts during sintering

This paper describes experiments performed on WC–Co compacts in order to measure the viscosities of a Newtonian constitutive law commonly used to simulate sintering. An intermittent loading method is used during two series of experiments. The first series are dedicated to determining the axial viscosity and takes place in a loading-dilatometer. The second one takes advantage of a video-extensometry device and provides results about the viscous Poisson's ratio. The axial viscosity is obtained as a function of relative density and temperature. Viscosity shows strong exponential increase with increasing density during isothermal conditions but decreases from 10 to 1 GPa·s between 1100°C and 1325°C during a conventional sintering cycle. Viscous Poisson's ratio shows low values at low densities and increases to 0.5 at full density.     

Effect of stress ratio on fatigue lifetime and crack growth behavior of WC–Co cemented carbide

Two types of fatigue tests, a rotating bending fatigue test and a three- or four-point bending fatigue test, were carried out on a fine grained WC–Co cemented carbide to evaluate its fatigue crack growth behavior and fatigue lifetime. From successive observations of the specimen surface during the fatigue process, it was revealed that most of the fatigue lifetime of the tested WC–Co cemented carbide was occupied with crack growth cycles. Using the basic equation of fracture mechanics, the relationship between the fatigue crack growth rate (da/dN) and the maximum stress intensity factor (K_max) was derived. From this relation, both the values of the threshold intensity factor (K_th) and the fatigue fracture toughness (K_fc) of the material were determined. The fatigue lifetime of the WC–Co cemented carbide was estimated by analysis based on the modified linear elastic fracture mechanics approach. Good agreement between the estimated and experimental fatigue lifetimes was confirmed.     

The grain size dependence of flow stress in a Cu–26Ni–17Zn alloy

The effect of grain size in the range 15–120 μm on flow stress was studied at room temperature to investigate the Hall–Petch relationship in a Cu–26Ni–17Zn alloy. It was found that the Hall–Petch relation is valid for the alloy. The Hall–Petch constants, σ_oε and k_ε are related to true strain (ε) in such a way that σ_oε is proportional to ε and k_ε to ε^1/2. An equation for flow stress as a function of true strain and grain size has been derived from these results. Dislocation density model for grain size strengthening is found valid for this alloy. Solid solution strengthening in the Cu–26Ni–17Zn alloy is attributed to the interaction of nickel and zinc atoms with screw dislocations and the effective interaction is more due to modulus mismatch than size misfit.     

Effect of Ti content in Ag–Cu–Ti activated filler metal on dissimilar joint formation of sialon and WC–Co alloy by laser brazing

Laser brazing was carried out for dissimilar joining of sialon and a WC–Co alloy. Eutectic type Ag–Cu alloys as filler metals with different Ti content ranging from 0 to 2·8 mass-% were used to investigate the effects of Ti on the interface structure and strength of the joint. The filler metal sheet was sandwiched between a sialon block and a WC–Co alloy plate, and a laser beam was irradiated selectively on the WC–Co alloy plate. The brazed joint was obtained using the filler metal containing >0·3 mass-%Ti. TiN, Ti₅Si₃, and Cu₄Ti layers were formed at the interface of sialon and brazed metal as compound layers. The shear strength of the brazed joint increased with increasing Ti content in the filler metal in the range 0·3–1·7 mass-%, reaching a maximum value of 106 MPa. However, the strength decreased when the Ti content became higher than 1·7 mass-%.     

Co evolutions for WC–Co with different Co contents during pretreatment and HFCVD diamond film growth processes

Systematical researches were accomplished on WC–Co with different Co contents (6%, 10% and 12%, mass fraction). Based on the XPS and EDX, from orthogonal pretreatment experiments, it is indicated that the acid concentration, the time of the acid pretreatment and the original Co content have significant influences on the Co-removal depth (D). Moreover, the specimen temperature, original Co content and Co-removal depth dependences of the Co evolution in nucleation, heating (in pure H₂ atmosphere) and growth experiments were discussed, and mechanisms of Co evolutions were summarized, providing sufficient theoretical bases for the deposition of high-quality diamond films on WC–Co substrates, especially Co-rich WC–Co substrates. It is proven that the Co-rich substrate often presents rapid Co diffusion. The high substrate temperature can promote the Co diffusion in the pretreated substrate, while acts as a Co-etching process for the untreated substrates. It is finally found that the appropriate Co-removal depth for the WC–12%Co substrate is 8–9 μm.     

Creep behaviour of cemented carbides — Influence of binder content,binder composition and WC grain size

The high-temperature creep behaviour of cemented carbides was evaluated for a wide variety of binder contents, binder compositions and WC grain sizes at temperatures ranging from 700 °C to 950 °C. The creep behaviour was characterised using compressive high-temperature experiments. The results show that the above mentioned microstructural parameters as well as the binder composition have a significant influence on the samples' plastic deformation. Based on these findings, a structured creep behaviour control map may be established for future materials development, aiding in the design of new high performing cemented carbides for challenging technical applications.     

Dynamic grain growth and particle coarsening in Al–3.5Cu

Grain growth and particle coarsening in Al–3.5Cu at a temperature of 450 °C has been studied. Plastic deformation of this Zener-pinned system at strain rates of 10^?3 and 10^?4 s^?1 led to an increase in both the grain growth and particle coarsening rates. The results of mechanical tests and metallography, including in situ studies, showed that the material was deforming primarily by intragranular slip. The dynamic grain growth was ascribed to the geometric effect of deformation on the Zener pinning, and the rate sensitivity of the growth to the dynamic particle coarsening. The principal effect of deformation on particle coarsening was concluded to be increased diffusion due to the dislocation content.     

High temperature mechanical properties of WC–Co hard metals

Understanding of the load situation and consequently the lifetime of cutting tools made of WC–Co hard metal requires quantitative data for thermo-mechanical properties. For the elevated temperatures present in application, these data are currently rather rare. The present work does discuss elastic material properties up to 1100 °C and compressive yield strength up to 900 °C, both as a function of Co content. The fracture toughness was determined as a function of the WC grain size and Co content up to 800 °C. Young's modulus and yield strength decrease with increasing temperature. A significant rise in fracture toughness was observed at 800 °C with increasing Co content and decreasing WC grain size. A possible reason for this increase is an increase in the plastic zone size at elevated temperatures.     

Modification of computer simulation of normal grain growth

A set of principles on transition probability was supplied for the physical process of grain growth. In accord with these principles, a modified transition probability considering the influence of temperature was put forward to simulate the normal grain growth relying on temperature and second phase particles. The modified transition probability correctly reflects the dependence of grain growth on the temperature. The effect of different shapes of second phase particles on the grain growth process was taken into account using the modified transition probability. The relationship between the area fraction of second phase particles and the limit of grain size of the matrix was given. The microstructural evolution patterns employed to 2-D were given. The results agree well with the real grain growth process. All these suggest that the modified transition probability is better than the conventional one.     

Simultaneous strengthening and toughening effects in WC–(nanoWC–Co)

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