Sintering and mechanical properties of cemented carbides based on tungsten carbide and multicomponent metallic alloys.

Cemented carbides are composite materials used in a wide variety of applications requiring the right combination of mechanical strength and wear resistance under harsh environments (i.e. metal cutting and shaping, civil engineering, mining, valves for the chemical industry, etc). The most common compositions comprise tungsten carbide grains bonded with a cobalt based metallic matrix. The reason is twofold. On the one hand, WC-Co materials are relatively easy to sinter to full density state with the adequate processing methodology and, on the other, a wide range of useful properties can be obtained by changing the WC grain size and the WC/Co ratio. Nevertheless, the use and availability of cobalt are presently jeopardized by both its new classification as toxic substance (REACH regulations) and the growing demand of this metal for making Li-ion batteries for electrical vehicles. The present thesis is focused on studying the sintering behavior and mechanical properties of WC-metal systems in which pure cobalt is replaced by different combinations of metals. Two promising candidates have been found: WC-NiCoCrTiAl cemented carbides These materials were designed starting from WC-NiCoCr compositions with a Ni/Co ratio equal to one. The main challenge was to increase the hardness of these compositions since it is too low compared with that of WC-Co grades. This was achieved firstly by alloying the binder phase with aluminum and, afterwards, inducing gamma prime precipitation by aging treatments. Two different aluminum containing compounds were investigated in order to avoid catastrophic oxidation of aluminum during PM processing: AlN and TiAl3. The latter produced the best results concerning sinterability and precipitation hardening effects. WC-Ni-Co-Cr-Ti-Al materials were obtained in fully dense form by using HIP after sintering technique, a process compatible with industrial processing technologies like Sinter HIP. Aging experiments show that hardness peaks occur at lower temperatures as the Al content of the binder phase increases. Apart from hardness, transverse rupture strength (TRS) was also measured in selected WC-NiCoCrTiAl compositions in both as-HIPed and solution-aged conditions. Results are only 15% lower than those reported for WC-Co materials with similar WC grain sizes and WC/metal ratios. These results also suggest that, like in as-cast Ni superalloys, the properties of the binder phase would be retained at temperatures below those used in aging treatments. WC-FeNiCoCr cemented carbides WC-Fe-Ni-Co-Cr compositions were designed following an alternative approach. In this case, the aim was to obtain a metallic binder with no precipitation of free carbon or any secondary carbide (including those of chromium). This was achieved by starting from WC-Fe-Ni-Co-Cr3C2 powder mixtures with a constant proportion between Fe, Ni and Co equal to 40/40/20. Chromium and carbon contents have been modified in order to find the upper and lower bounds defining the “so-called” carbon windows. In addition, shrinkage kinetics have been thoroughly studied in order to define a robust sintering process for both coarse and submicron WC powders. Results of calorimetric experiments have been used to improve the description of the W-C-Fe-Ni-Co quinary system for 40Fe-40Ni-20Co composition by means of ThermoCalc® software. In this case, mechanical tests confirmed that the values of hardness and transverse rupture strength are within tolerances of those reported for WC-Co grades with similar binder contents and WC grain sizes, provided that precipitation of undesired phases is avoided.Los carburos cementados son materiales compuestos utilizados en una amplia variedad de aplicaciones que requieren la combinación correcta de resistencia mecánica y resistencia al desgaste en entornos hostiles (es decir, corte y conformado de metales, ingeniería civil, minería, válvulas para la industria química, etc). Las composiciones más comunes comprenden granos de carburo de tungsteno unidos con una matriz metálica de cobalto. La razón es doble. Por un lado, los materiales WC-Co son relativamente fáciles de sinterizar a un estado de densidad completa con la metodología de procesamiento adecuada y, por otro lado, una amplia variedad de propiedades útiles se pueden obtener variando el tamaño de grano de WC y la relación WC/Co. Sin embargo, el uso y la disponibilidad de cobalto está actualmente en peligro tanto por su nueva clasificación como sustancia tóxica (normativa REACH) como por la creciente demanda de este metal para la fabricación de baterías de iones de litio para vehículos eléctricos. La presente tesis se centra en el estudio de la sinterización y las propiedades mecánicas de sistemas WC-metal en los que el cobalto puro es reemplazado por diferentes combinaciones de metales. Se han encontrado dos candidatos prometedores: Carburos cementados WC-NiCoCrTiAl Estos materiales fueron diseñados a partir de composiciones WC-NiCoCr con una relación Ni/Co igual a uno. El principal reto fue aumentar la dureza de estas composiciones ya que es muy baja en comparación con los grados WC-Co. Esto se logró en primer lugar mediante la aleación de la fase ligante con aluminio y, posteriormente, induciendo la precipitación gamma prima mediante tratamientos de envejecimiento. Se investigaron dos fuentes de aluminio distintas para evitar una oxidación catastrófica del aluminio durante el procesamiento PM: AlN y TiAl3. Este último produjo los mejores resultados en cuanto a la sinterabilidad y los efectos de endurecimiento por precipitación. Los materiales WC-Ni-Co-Cr-Ti-Al fueron obtenidos en forma totalmente densa mediante el uso de HIP después de la sinterización, un proceso compatible con tecnologías de procesamiento industrial como Sinter HIP. Los experimentos de envejecimiento muestran que la dureza alcanza su punto máximo a temperaturas más bajas a medida que aumenta el contenido de Al de la fase ligante.Además de la dureza, la resistencia a la ruptura transversal (TRS) también se midió en composiciones seleccionadas de WC-NiCoCrTiAl tanto en condiciones de HIPed como de envejecimiento en solución. Los resultados son solo el 15% más bajos que los reportados para materiales WC-Co con tamaños de grano de WC y proporciones WC/metal similares. Estos resultados sugieren también que, al igual que en las superaleaciones base Ni, las propiedades de la fase ligante se mantendrían a temperaturas inferiores a las utilizadas en los tratamientos de envejecimiento. Carburos cementados WC-FeNiCoCr Las composiciones WC-Fe-Ni-Co-Cr fueron diseñadas siguiendo un enfoque alternativo. En este caso, el objetivo era obtener un ligante metálico sin precipitación de carbono libre ni ningún carburo secundario (incluidos los de cromo). Esto se logró partiendo de mezclas en polvo WC- Fe-Ni-Co-Cr3C2 con una proporción constante entre Fe, Ni y Co igual a 40/40/20. Los contenidos en cromo y carbono han sido modificados para encontrar los límites superior e inferior que definen "las llamadas" ventanas de carbono. Además, la cinética de contracción se ha estudiado a fondo para definir un proceso robusto de sinterización para polvos de WC gruesos y submicra. Los resultados de los experimentos calorimétricos se han utilizado para mejorar la descripción del sistema quinario W-C-Fe-Ni-Co para la composición 40Fe-40Ni-20Co mediante el software ThermoCalc®. En este caso, las pruebas mecánicas confirmaron que los valores de dureza y de resistencia a la ruptura transversal están dentro de las tolerancias reportadas para los grados WC-Co con contenidos de ligante y tamaños de grano de WC similares, siempre que se evite la precipitación de fases no deseadas

Densification of WC-Fe-Ni-Co-Cr cemented carbides processed by HIP after sintering: effect of WC powder particle size

Shrinkage, liquid formation and mass losses of WC-19 vol% FeNiCoCr alloys during sintering have been inves- tigated in compositions either with coarse or submicron WC powders. Mass losses detected by thermogravimetry are compatible with carbothermal reduction of the different oxides present in the powder mixtures. Hardness and fracture toughness of materials based on submicron WC powders are within tolerances of those reported for WC- Co materials with similar microstructures. However, fracture strength is approx. 25% lower

Fabrication and arc erosion behavior of Ag-SnO2-ZnO electrical contact materials

This study investigated the synthesis of Ag-SnO2-ZnO by powder metallurgy methods and their subsequent electrical contact behavior. The pieces of Lambda g-SnO2-ZnO were prepared by ball milling and hot pressing. The arc erosion behavior of the material was evaluated using homemade equipment. The microstructure and phase evolution of the materials were investigated through X-ray diffraction, energy-dispersive spectroscopy and scanning electron microscopy. The results showed that, although the mass loss of the Ag-SnO2-ZnO composite (9.08 mg) during the electrical contact test was higher than that of the commercial Ag-CdO (1.42 mg), its electrical conductivity remained constant (26.9 +/- 1.5% IACS). This fact would be related to the reaction of Zn2SnO4's formation on the material's surface via electric arc. This reaction would play an important role in controlling the surface segregation and subsequent loss of electrical conductivity of this type of composite, thus enabling the development of a new electrical contact material to replace the non-environmentally friendly Ag-CdO composite

Sintering and mechanical properties of cemented carbides based on tungsten carbide and multicomponent metallic alloys.

Cemented carbides are composite materials used in a wide variety of applications requiring the right combination of mechanical strength and wear resistance under harsh environments (i.e. metal cutting and shaping, civil engineering, mining, valves for the chemical industry, etc). The most common compositions comprise tungsten carbide grains bonded with a cobalt based metallic matrix. The reason is twofold. On the one hand, WC-Co materials are relatively easy to sinter to full density state with the adequate processing methodology and, on the other, a wide range of useful properties can be obtained by changing the WC grain size and the WC/Co ratio. Nevertheless, the use and availability of cobalt are presently jeopardized by both its new classification as toxic substance (REACH regulations) and the growing demand of this metal for making Li-ion batteries for electrical vehicles. The present thesis is focused on studying the sintering behavior and mechanical properties of WC-metal systems in which pure cobalt is replaced by different combinations of metals. Two promising candidates have been found: WC-NiCoCrTiAl cemented carbides These materials were designed starting from WC-NiCoCr compositions with a Ni/Co ratio equal to one. The main challenge was to increase the hardness of these compositions since it is too low compared with that of WC-Co grades. This was achieved firstly by alloying the binder phase with aluminum and, afterwards, inducing gamma prime precipitation by aging treatments. Two different aluminum containing compounds were investigated in order to avoid catastrophic oxidation of aluminum during PM processing: AlN and TiAl3. The latter produced the best results concerning sinterability and precipitation hardening effects. WC-Ni-Co-Cr-Ti-Al materials were obtained in fully dense form by using HIP after sintering technique, a process compatible with industrial processing technologies like Sinter HIP. Aging experiments show that hardness peaks occur at lower temperatures as the Al content of the binder phase increases. Apart from hardness, transverse rupture strength (TRS) was also measured in selected WC-NiCoCrTiAl compositions in both as-HIPed and solution-aged conditions. Results are only 15% lower than those reported for WC-Co materials with similar WC grain sizes and WC/metal ratios. These results also suggest that, like in as-cast Ni superalloys, the properties of the binder phase would be retained at temperatures below those used in aging treatments. WC-FeNiCoCr cemented carbides WC-Fe-Ni-Co-Cr compositions were designed following an alternative approach. In this case, the aim was to obtain a metallic binder with no precipitation of free carbon or any secondary carbide (including those of chromium). This was achieved by starting from WC-Fe-Ni-Co-Cr3C2 powder mixtures with a constant proportion between Fe, Ni and Co equal to 40/40/20. Chromium and carbon contents have been modified in order to find the upper and lower bounds defining the “so-called” carbon windows. In addition, shrinkage kinetics have been thoroughly studied in order to define a robust sintering process for both coarse and submicron WC powders. Results of calorimetric experiments have been used to improve the description of the W-C-Fe-Ni-Co quinary system for 40Fe-40Ni-20Co composition by means of ThermoCalc® software. In this case, mechanical tests confirmed that the values of hardness and transverse rupture strength are within tolerances of those reported for WC-Co grades with similar binder contents and WC grain sizes, provided that precipitation of undesired phases is avoided.Los carburos cementados son materiales compuestos utilizados en una amplia variedad de aplicaciones que requieren la combinación correcta de resistencia mecánica y resistencia al desgaste en entornos hostiles (es decir, corte y conformado de metales, ingeniería civil, minería, válvulas para la industria química, etc). Las composiciones más comunes comprenden granos de carburo de tungsteno unidos con una matriz metálica de cobalto. La razón es doble. Por un lado, los materiales WC-Co son relativamente fáciles de sinterizar a un estado de densidad completa con la metodología de procesamiento adecuada y, por otro lado, una amplia variedad de propiedades útiles se pueden obtener variando el tamaño de grano de WC y la relación WC/Co. Sin embargo, el uso y la disponibilidad de cobalto está actualmente en peligro tanto por su nueva clasificación como sustancia tóxica (normativa REACH) como por la creciente demanda de este metal para la fabricación de baterías de iones de litio para vehículos eléctricos. La presente tesis se centra en el estudio de la sinterización y las propiedades mecánicas de sistemas WC-metal en los que el cobalto puro es reemplazado por diferentes combinaciones de metales. Se han encontrado dos candidatos prometedores: Carburos cementados WC-NiCoCrTiAl Estos materiales fueron diseñados a partir de composiciones WC-NiCoCr con una relación Ni/Co igual a uno. El principal reto fue aumentar la dureza de estas composiciones ya que es muy baja en comparación con los grados WC-Co. Esto se logró en primer lugar mediante la aleación de la fase ligante con aluminio y, posteriormente, induciendo la precipitación gamma prima mediante tratamientos de envejecimiento. Se investigaron dos fuentes de aluminio distintas para evitar una oxidación catastrófica del aluminio durante el procesamiento PM: AlN y TiAl3. Este último produjo los mejores resultados en cuanto a la sinterabilidad y los efectos de endurecimiento por precipitación. Los materiales WC-Ni-Co-Cr-Ti-Al fueron obtenidos en forma totalmente densa mediante el uso de HIP después de la sinterización, un proceso compatible con tecnologías de procesamiento industrial como Sinter HIP. Los experimentos de envejecimiento muestran que la dureza alcanza su punto máximo a temperaturas más bajas a medida que aumenta el contenido de Al de la fase ligante.Además de la dureza, la resistencia a la ruptura transversal (TRS) también se midió en composiciones seleccionadas de WC-NiCoCrTiAl tanto en condiciones de HIPed como de envejecimiento en solución. Los resultados son solo el 15% más bajos que los reportados para materiales WC-Co con tamaños de grano de WC y proporciones WC/metal similares. Estos resultados sugieren también que, al igual que en las superaleaciones base Ni, las propiedades de la fase ligante se mantendrían a temperaturas inferiores a las utilizadas en los tratamientos de envejecimiento. Carburos cementados WC-FeNiCoCr Las composiciones WC-Fe-Ni-Co-Cr fueron diseñadas siguiendo un enfoque alternativo. En este caso, el objetivo era obtener un ligante metálico sin precipitación de carbono libre ni ningún carburo secundario (incluidos los de cromo). Esto se logró partiendo de mezclas en polvo WC- Fe-Ni-Co-Cr3C2 con una proporción constante entre Fe, Ni y Co igual a 40/40/20. Los contenidos en cromo y carbono han sido modificados para encontrar los límites superior e inferior que definen "las llamadas" ventanas de carbono. Además, la cinética de contracción se ha estudiado a fondo para definir un proceso robusto de sinterización para polvos de WC gruesos y submicra. Los resultados de los experimentos calorimétricos se han utilizado para mejorar la descripción del sistema quinario W-C-Fe-Ni-Co para la composición 40Fe-40Ni-20Co mediante el software ThermoCalc®. En este caso, las pruebas mecánicas confirmaron que los valores de dureza y de resistencia a la ruptura transversal están dentro de las tolerancias reportadas para los grados WC-Co con contenidos de ligante y tamaños de grano de WC similares, siempre que se evite la precipitación de fases no deseadas

Hard materials free of WC and Co for tribological applications: WB4 ultrahard ceramics and TiC cermets.

This thesis addresses the problem of finding wear-resistant materials that could substitute WC-Co alloys in specific tribological applications. The main challenge lies in the versatility of these alloys to cover a broad range of physicochemical properties by merely varying the WC/Co ratio and the grain size of the ceramic phase. This is not possible when using other combinations of ceramic and metal since, in general, the interfacial strength is lower than that observed between tungsten carbide and cobalt. The need to find alternatives to the WC-Co system arises from the strategic value of these raw materials, currently controlled by the People's Republic of China. Furthermore, batteries developed for electric vehicle motors contain a significant amount of cobalt, exacerbating the supply risk of this metal in the European Union. This thesis proposes the development of two very different materials that could replace WC-Co alloys in two sectors of interest: materials used in shot blasting or water jet cutting and guiding systems in hot rolling equipment. For the former, tungsten tetraboride, a new metallic boride with hardness exceeding 40 GPa, has been considered, and for the latter, the development of cermets based on titanium carbide and iron is suggested. The first set of results in this thesis focuses on the development of ultrahard materials based on tungsten tetraboride that could substitute WC binderless carbides in applications requiring high erosive wear resistance (i.e., nozzles for water cutting or shot peening). This objective has been addressed applying hot isostatic pressing (HIP) technique to both as-received WB4-B and WB4-B-Ta powder mixtures. Overstoichiometric B/W ratios and Ta additions were selected for their effect on stabilizing the ultrahard WB4 phase, as described in the literature. This is a difficult task, since the stability of WB4 requires a boron activity much higher than that corresponding to a B/W ratio of 4. Porosity removal was more efficient in the alloy containing metallic tantalum, achieving near full density at temperatures 300 ºC lower than those reported so far for these materials. The WB4 phase is better stabilized by HIPing at 1350 ºC than at 1100 ºC. This is due to the formation of TaB2, which at 1100 ºC, likely occurs by direct reaction between metallic Ta and the surrounding WB4 particles. At 1350 ºC, diffusion is enhanced and the reaction between free B and Ta particles becomes more probable. The nanohardness of WB4 HIPed specimens reaches 43 GPa, that is, 43 % higher than the highest reported for binderless WC. Indentation toughness is similar to that reported for WC-1 wt. % Mo2C (5.6 and 6.6 MPa·m1/2 respectively). Contrary to that reported by other authors, it has been confirmed that metallic Ta additions enhance the decomposition of tungsten tetraboride into tungsten diboride and Ta-rich borides. According to XRD data and SEM analyses, this phenomenon is more pronounced when HIPing is made at lower temperatures, which is against thermodynamic calculations. This is probably related to kinetic effects, since direct reaction between free B regions and metallic Ta powders requires higher mobility than that observed after HIPing at 1100 ºC. The latest results were obtained by investigating the possibility of producing cermets based on WB4-B-TaB2 powders with Ni additions. It was confirmed that densification of WB4-B-TaB2 is notably activated by Ni additions, reducing by 250 ºC the temperature needed for porosity removal. This is likely due to the formation of a liquid phase above 1007 ºC, compatible with the formation of Ni4B3 and NiB borides on heating. These borides are mainly formed by direct reaction between free B and Ni powders, although some boron could also be available from the partial decomposition of WB4 into W2B5. As no metallic nickel remains after HIPing, the toughness of these composites is very low. On the other hand, strength and toughness of WB4-B-TaB2-Ni alloys are notably improved by TiAl3 and Zr additions. Although Ni containing borides are still present in these alloys, there are also Ni-Al rich phases free of boron which provide a significant toughening effect, as confirmed by indentation cracking. These B-free regions remain unbroken joining the crack lips, although it progresses forward through boride grains. Highest fracture strength values correspond to the combined addition of TiAl3 and Zr powders to WB4-B-TaB2-Ni mixtures (≈ 1 GPa). However, in these materials WB4 grains are fully decomposed into a combination of mixed borides. The challenge associated with manufacturing components from TiC-Fe alloys for steel wire guiding in hot rolling equipment lies in their difficulty to be sintered. In this thesis, fully dense TiC-Fe-Cr-Mo based cermets have been sintered from two different alloys: TiC-Fe-Cr3C2-Mo and TiC-Fe-Cr3C2-Mo2C. It has been investigated the effect of metallic molybdenum or molybdenum carbide additions as wetting activators during liquid phase sintering of TiC-Fe powder mixtures. The addition of molybdenum in these systems is carried out to increase the wetting properties by the formation of the so-called core-rim structures, typical of TiC based cermets. These structures are formed by Ti-rich cores (α’ phase) surrounded by shells (α’’ phase) comprised of complex cubic carbides. Chromium was added into the powder mixtures as carbide, since its oxidation resistance is higher than that of metallic Cr and it is brittle enough to distributed homogeneously with the other constituents of the alloy during the mixing and milling process. In addition, the effect of pressure in the furnace chamber on porosity removal and on the formation of surface compositional gradients has also been explored. Results show that densification of TiC-Fe-Cr-Mo cermets is strongly affected by the selection of starting powders and the vacuum condition used during the sintering cycle. Liquid phase sintering is enhanced by using a lower pressure during the heating ramp of the sintering cycle (i.e., 10-5 mbar vs. 10-2 mbar), confirming that carbothermal reduction of the most stable oxides present in these materials requires very low oxygen activity. The massive evaporation of the binder phase observed at 10-5 mbar is avoided by injecting Ar in the sintering chamber above 1300 ºC. However, cermets sintered in this condition present some residual porosity due to Ar entrapment. Cermets with Mo2C additions lead to higher densities than those based on metallic Mo. This is likely due to the finer particle size of the former which likely accelerate diffusion kinetics associated to the formation of “core-rim” structures. Carbon losses after sintering are also higher in compositions based on Mo2C additions, suggesting that these powders enhance carbothermal reduction of oxides present in the surface of starting powders. Significant migration of the binder phase towards the cermet surface is observed in TiC-Fe-Cr-Mo cermets when sintering is carried out at low vacuum levels (i. e., 10-2 mbar). Compositional gradients produced by this migration are likely related to oxidation occurring once the closed porosity state is reached. Best results are obtained with 10-5 mbar of pressure and injection of 1.2 bar of Ar at 1300 ºC (named HV-1 cycle) and with 10-2 mbar of pressure without Ar injection (named LV-2 cycle). These two cycles lead to materials with low porosity levels, low mass losses and homogeneous microstructures. These materials achieve significant hardening through air-quenching from 950 ºC, resulting in properties suitable for hot wear applications. Another key aspect of TiC-Fe-Cr-Mo based cermets is the carbon partitioning between the ceramic and the metallic phase during sintering, since the latter is susceptible to hardening by austenitizing and quenching. It has been found that relatively low differences in the carbon content of the FeCrMo binder phase induces significant microstructural changes both after sintering and after subsequent thermal treatment. Thus, the alloy with higher carbon content (that with metallic Mo) presents higher precipitation of Cr-rich M7C3 carbides at the (Ti1-x,Mox)yCz - metal interface and certain amount of retained austenite. In the alloy with lower carbon content (that with Mo2C), there is no retained austenite, and the precipitation of Cr-rich carbides is less abundant. By means of EDS-TEM analyses of the metallic matrix of TiC-Fe-Cr-Mo cermets after sintering, it was found to be alloyed with Cr and Mo. The contents of these elements are similar to those reported for hot work steels (i.e., AISI H11 or H13). However, these metallic matrices are very different as they do not contain Si, Mn or V. With the aim of studying the hardenability of these materials, thermal treatments have been carried out after sintering using a quenching dilatometer. Volumetric changes associated to phase transformations have been measured by austenitizing at 950 ºC for 20 min and subsequently cooling at different rates (from 100 ºC/s to 0.1 ºC/s). Carbon differences are observed to shift bainitic transformation, with its onset found at approximately 1 ºC/s. As expected, this transformation is shifted towards higher temperatures as the total carbon content of the alloy decreases. CCT diagrams built on the basis of dilatometric results confirm that bainitic transformation occurs at higher cooling rates than in standard hot work steels. Anyhow, the metallic matrices present in these TiC-Fe-Cr-Mo cermets are air-quenchable at approximately 1 ºC/s, inducing a hardness increase of 30 % with respect to that of as-sintered materials. Precipitation of M7C3 and M23C6 carbides is observed by means of SEM/TEM analyses and XRD measurements. These findings agree with thermodynamic calculations made by Thermocalc® software. Although it still unclear, preliminary results suggest that the precipitation of M23C6 carbides is related to the progression of the bainitic transformation

Effect of milling conditions and binder phase content on liquid phase sintering of heat treatable WCNi-Co-Cr-Al-Ti cemented carbides

The binder phase of WC based cemented carbides has been alloyed by adding two different aluminium compounds, AlN and TiAl3, to mixtures comprised of WC, Ni, Co and Cr3C2 powders. A more efficient alloying effect is obtained by TiAl3 additions likely due to its higher dissolution rate during liquid phase sintering. Shrinkage and melting phenomena are strongly affected by the energy of the milling process and the amount of metallic additions. The use of higher milling rotation speed induces higher oxidation of the powder mixtures and the subsequent formation of a higher volume fraction of alumina particles after sintering. Densification and WC grain growth are hindered by increasing the Al addition. Thus, full densification of alloys with higher Al additions require the use of HIP after standard vacuum sintering cycles. As-HIPed WC-Ni-Co-Cr-Al-Ti samples present a binder phase with precipitation of gamma prime similar to that found in as-cast Ni superalloys. The size, volume fraction and morphology of these precipitates has been modified by applying a standard solution treatment (1150 °C-2 h) followed by fast air cooling and subsequent aging at 600 °C and different dwelling times. Age hardening effects have been confirmed in the composition consisting of WC-12 wt% Co-12 wt% Ni-1.7 wt% Cr3C2-5 wt% TiAl3 after 100 h at this temperature

Effect of chromium and carbon contents on the sintering of WC-Fe-Ni-Co-Cr multicomponent alloys

WC-Fe-Ni-Co-Cr cemented carbides have been obtained by liquid phase sintering from WC-Fe-Ni-Co-Cr3C2 powder mixtures. Taking the 40wt%Fe-40wt%Ni-20wt.%Co alloy as a reference, new binder phases has been prepared by introducing controlled amounts of Cr and C, via Cr3C2 and C black powders respectively. As described for WC-Co-Cr materials, Cr additions are observed to reduce the eutectic temperatures of the WC-Fe-Ni-Co system. First liquids detected on heating exhibit wide temperature melting ranges, which become narrower and are displaced to higher temperatures on repeated heating and cooling cycles. Apart from the decarburization associated to the carbothermal reduction of powder oxides, this phenomenon could be also associated to the homogeneization of the chemical composition of these multicomponent binder phases, which is faster as C content decreases. Correlation between experimental melting and solidification temperature ranges and those predicted by Thermocalc (R) is better as Cr content increases. Experimental C windows, defined in this work by the absence of free C or. phases, are located at C contents higher than those estimated by Thermocalc (R). Although the 40wt.%Fe-40wt.%Ni-20wt.%Co alloy is austenitc, BCC phases are partially stabilized at low C and high Cr contents. Although these compositions are free from. phases or free C, a precipitation of Cr-rich carbides is found at the WC-metal interface. These precipitates are not observed in the alloy with 0.75 wt% Cr (i.e. 5 wt% of the nominal metal content) and 5.39 wt%C. This C content is 0.17 wt% higher than that predicted for precipitation of M7C3

Densification of WC-Fe-Ni-Co-Cr cemented carbides processed by HIP after sintering: effect of WC powder particle size

Shrinkage, liquid formation and mass losses of WC-19 vol% FeNiCoCr alloys during sintering have been inves- tigated in compositions either with coarse or submicron WC powders. Mass losses detected by thermogravimetry are compatible with carbothermal reduction of the different oxides present in the powder mixtures. Hardness and fracture toughness of materials based on submicron WC powders are within tolerances of those reported for WC- Co materials with similar microstructures. However, fracture strength is approx. 25% lower

Microstructure, mechanical properties and fracture behavior of NiCoCrTiAl and FeNiCoCr new alternative binders for WC based hardmetals

Hardness and fracture strength of WC-Ni-Co-Cr-Ti-Al cemented carbides have been measured at room temperature in as-HIPed and solution-aged conditions. These treatments are applied to modify the size of gamma prime precipitates, which are intrinsically formed within the metallic binder of these ceramic-metal composites during the sintering process. Compositions containing approx. 28-29 vol% metal content exhibit hardness values in the range of those reported for similar grades of WC-Co hardmetals. Optimized aluminum additions lead to materials with fracture strength values only 15% lower than those reported for the same WC-Co commercial references. These results suggest gamma prime precipitation hardening as a potential strategy for improving the performance of WC-Co materials at high temperatures. Regarding Fe-Ni-Co-Cr alloys are potential candidates for partial substitution of Co content in WC-based hardmetals. It has been investigated WC coarse grade with 15 wt%(FeNiCoCr). The Cr content has been adjusted in order to avoid the precipitation of M7C3 carbides. Within the corresponding carbon windows, fracture strength values range from 2.8 GPa to 3.0 GPa. These values are within the range of standard WC-Co grades with similar binder contents and WC grain sizes

Reactive sintering of WC-Ni-Co-Cr-Ti-Al cemented carbides and precipitation of gamma prime in their metallic binder phases

Cemented carbides processed from WC-Ni-Co-Cr3C2-TiAl3 powder mixtures by HIP after sintering present a homogeneous precipitation of gamma prime in their metallic binder phases. Other reaction products include alumina precipitates and (Ti-x,W1-x)C mixed carbides. The formation of these phases is consistent with the highly exothermic reactions detected by DSC during the heating ramp of the sintering cycle. The amount of alumina and gamma prime phases present in the sintered materials depends on the oxygen content of the powder mixtures. Refinement of gamma prime precipitates depends critically on the cooling rate after HIPing. Nanoindentation experiments have confirmed the presence of age hardening effects of samples which were previously solution treated. As expected, gamma prime precipitation is induced at lower temperatures in compositions with higher Al contents in the binder phase