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Sökning: L773:0263 4368 OR L773:2213 3917

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1.
  • Angseryd, Jenny, 1979, et al. (författare)
  • Nanostructure of a cubic BN cutting tool material
  • 2015
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 49:1, s. 283-287
  • Tidskriftsartikel (refereegranskat)abstract
    • Advanced microscopy techniques, laser assisted atom probe tomography and electron energy loss spectroscopy in transmission electron microscopy are used to investigate in detail the microstructure of a polycrystalline cubic boron nitride tool material. During sintering at high pressure superhard cubic boron nitride combines with hard ceramic Ti(C,N) to form a dense material. Diffusion in Ti(C,N) and reactions between Ti(C,N), cBN and additives result in the formation of several nitride, oxide and boride phases in a complex mixed microcrystalline and nano- crystalline microstructure.
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2.
  • Gren, Martin, 1989, et al. (författare)
  • A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides
  • 2020
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 87
  • Tidskriftsartikel (refereegranskat)abstract
    • 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.
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3.
  • Jakob, Severin, 1991, et al. (författare)
  • Effect of boron doping on grain boundary cohesion in technically pure molybdenum investigated via meso-scale three-point-bending tests
  • 2023
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 113
  • Tidskriftsartikel (refereegranskat)abstract
    • Molybdenum has numerous advantageous functional and high-temperature properties. However, plastic deformation as well as structural applications are limited due to a propensity for brittle, intercrystalline failure, especially at low temperatures. It is well known that oxygen segregations have a detrimental effect, whereas it is assessed that carbon and/or boron have a beneficial effect on grain boundary cohesion. An advanced approach for the improvement of these interfaces is segregation engineering, e.g. the addition of cohesion enhancing elements segregating to the grain boundaries. To investigate early stages of crack formation, three-point bending tests on recrystallized commercially pure and boron micro-doped molybdenum were conducted between −28 °C and room temperature. The tensile-loaded top surface of the specimens was examined post-mortem close to the final fracture area via scanning electron microscopy. The occurring separations of grains are investigated for distinct features and the chemical composition of the interface is complementary measured by atom probe tomography.
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4.
  • Jakob, Severin, 1991, et al. (författare)
  • Evolution of nano-pores during annealing of technically pure molybdenum sheet produced from different sintered formats
  • 2023
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 110
  • Tidskriftsartikel (refereegranskat)abstract
    • Molybdenum is a refractory metal with no phase transformation in the solid state and a high melting point. It is therefore an excellent structural material for various high temperature applications. Especially in this field of operation, significant creep resistance is essential. To achieve this, a microstructure with grains in the range of millimeters is desired. However, as demonstrated in the present study, the onset temperature for secondary recrystallization, which would lead to a beneficial grain size, is among other things dependent on the initial dimensions of the sintered part. One possible reason for the different microstructural evolutions is the influence of residual pores in sub-micrometer size. Sheets were thus fabricated via three different production routes employing the same initial Mo powder to exclude chemical variation as an influencing factor. The samples were investigated by in-situ small-angle X-ray scattering at a synchrotron radiation source with two different heating rates. Additionally, selected annealed samples were studied ex-situ with high energy X-rays. The apparent volume fraction of pores is compared to a volatilization model for the vaporization of typical accompanying elements and the induced thermal expansion.
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5.
  • Johansson, Sven A.E., et al. (författare)
  • CCBuilder: a software that produces synthetic microstructures of WC-Co cemented carbides
  • 2019
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 78, s. 210-218
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, the software CCBuilder and its methods to generate synthetic three-dimensional (3D) microstructures of WC-Co cemented carbides are presented. Generating synthetic microstructures is critical, as it enables accurate virtual testing where influences from microstructural parameters can be investigated in a systematic and controlled way. The model of the microstructure is based on a voxel description of the material volume and the WC grains are represented as truncated triangular prisms. The grains are inserted into the simulation domain and to obtain a realistic microstructure a combination of packing and a Monte Carlo Potts simulation is used. The orientations of all individual WC grains are known and it is shown that the microstructure is built up of two interpenetrating networks, the WC grains and the Co phase. Numerical studies are presented to show how input parameters to CCBuilder, such as grain size, grain shape, packing strategy, voxel density, system size, and Monte-Carlo steps affect the resulting simulated microstructure. The obtained microstructures are geometrically realistic and the contiguity of the carbide phase shows very good agreement with published experimental data.
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6.
  • Long, Jianzhan, 1979, et al. (författare)
  • Influences of Cr contents on oxidation behavior of WC-Co-Ni-Cr cemented carbides at 900 °C
  • 2024
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - 0263-4368 .- 2213-3917. ; 118
  • Tidskriftsartikel (refereegranskat)abstract
    • Modification of binder composition is an effective way to improve the oxidation resistance of WC-Co-Ni-Cr cemented carbide. Since the adjustment of Ni and Cr contents in the Co-Ni-Cr binder alters the oxidation behavior and oxidation products, WC-Co-Ni-Cr cemented carbide is expected to achieve excellent oxidation resistance in severe service conditions. In this paper, the influences of Cr contents in Co-Ni-Cr binder phase with a fixed total amount of Co and Ni on the oxidation behavior of WC-Co-Ni-Cr cemented carbides at 900 °C was investigated. The oxide morphology and phase distribution were focused. The oxidation mechanism of WC-Co-Ni-Cr cemented carbide was discussed. It has been shown that Co-rich regions are preferentially oxidized in the initial stage of oxidation. As oxidation proceeds (60 min), the dominant oxidation product on the alloy surface is NiO. The oxidation resistance of the WC-Co-Ni-Cr cemented carbide increases with increasing Cr content in the binder. Compared to the series of WC-2Co-11Ni-xCr, WC-6.5Co-6.5Ni-xCr presents a better overall oxidation resistance due to the formation of Cr2O5 in the middle of the oxide layer.
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7.
  • Meingast, Arno, et al. (författare)
  • High resolution STEM investigation of interface layers in cemented carbides
  • 2018
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 72, s. 135-140
  • Tidskriftsartikel (refereegranskat)abstract
    • Cemented carbides with sub-micron grain size have increased the need to restrict grain growth during sintering. Commonly used inhibitors like V, Ti, and Cr have been observed to form interface layers in the interfaces between WC grains and the Co binder. Atomistic modeling has predicted the composition and thickness of the interface layers. Earlier, the interface layers have been characterized qualitatively using high resolution transmission electron microscopy (TEM). To get more information about the structure and composition of the interface layers in a Ti containing cemented carbide in this work, Z contrast imaging and spectroscopy using scanning transmission electron microscopy (STEM) have been combined. Elemental maps revealing the structure of the interface layers will be presented.
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8.
  • Mühlbauer, Georg, et al. (författare)
  • Transition of W2C to WC during carburization of tungsten metal powder
  • 2018
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 72, s. 141-148
  • Tidskriftsartikel (refereegranskat)abstract
    • The properties of cemented carbides are heavily influenced by the tungsten carbide (WC) powders used in its production. Overall particle size of the WC is most influential, however, in the medium- and coarse-grained tungsten carbide particle range the internal crystallographic structure of the polycrystalline WC is important as well. To investigate the crystal formation of WC and it's resulting structure, selected crystal growth experiments were performed. The transition of ditungsten carbide (W 2 C) into tungsten carbide (WC) during carburization of tungsten powder was studied. The experimental strategy was to obtain W 2 C along with WC within the growing carbide particles. Electron microscopy methods were utilized to study the growth mechanism during conversion of W 2 C into WC, and to detect the existence of preferred crystallographic orientations during the transition process.
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9.
  • Oro, R., et al. (författare)
  • Optimizing the synthesis of ultrafine tungsten carbide powders by effective combinations of carbon sources and atmospheres
  • 2017
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 63, s. 9-16
  • Tidskriftsartikel (refereegranskat)abstract
    • Nanostructured WC powders can provide technologically attractive properties due to the fine microstructures obtained after sintering. Either W or WO3 powders are used for the industrial production of WC. In both cases, the contact area between carbon and tungsten precursors has a critical influence on the reaction temperatures, which in turn affects grain growth and agglomeration of particles. Different methods have been studied to increase the reaction rates by enhancing the contact between reactants: carbon coating of tungsten powder, solid-gas reactions of tungsten powders with atmospheres containing CH4, or mechanical activation followed by thermal activation of tungsten and carbon precursors. In this work WC-powders were obtained by mechanical activation of tungsten and carbon precursors followed by thermal activation of these mixes at temperatures up to 1100 °C. A systematic study has been carried out combining two dissimilar carbon sources (graphite and carbon black), with different atmosphere compositions (Ar, Ar-50H2, Ar-10CO) and studying the evolution of phases at different stages of the synthesis. The results show how the efficiency of the interaction between carbon sources and atmospheres affects the completion of the synthesis. The synthesis of WC from WO3 in H2 containing atmospheres is enhanced when using carbon black sources, however in CO containing atmospheres the most effective interaction is with graphite.
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10.
  • Petisme, Martin, 1981, et al. (författare)
  • Molecular dynamics simulation of WC/WC grain boundary sliding resistance in WC–Co cemented carbides at high temperature
  • 2015
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 49:1, s. 75-80
  • Tidskriftsartikel (refereegranskat)abstract
    • Grain boundary sliding is thought to be an important deformation mechanism in WC-Co based cemented carbides at high temperature. It has been assumed that when the WC skeleton breaks up and grain boundaries are infiltrated by Co, sliding is facilitated. In this work, molecular dynamics simulations with classical interatomic potentials were used to perform simulations of grain boundary sliding at two model WC/WC grain boundaries. Shear stresses were calculated for different numbers of Co layers in the grain boundary at the constant sliding velocity 10 m/s for T = 500 K, 1000 K, 1500 K, and 2000 K. It was found that in all considered cases, about 6 layers of Co in the grain boundary were sufficient to significantly facilitate sliding. The shear stresses that were obtained are an order of magnitude lower with a Co film (>= 6 layers) compared to the most stable configurations containing half a monolayer of Co for T
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11.
  • Prieto, E., et al. (författare)
  • Novel high entropy alloys as binder in cermets: From design to sintering
  • 2021
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 99
  • Tidskriftsartikel (refereegranskat)abstract
    • In recent years a new group of alloys has emerged breaking with the classical alloying concepts of physical metallurgy, high entropy alloys (HEA). Their main characteristic is that these alloys present 4 or 5 main elements increasing the entropy of the system and favouring the formation of a single phase. The disordered solid solution leads to develop an alloy with improved properties, in particular high thermal stability, hardness and strength. These properties make this group of alloys attractive as potential candidates for alternative binders in hard materials. In this work, two new compositions have been designed with the aim of obtaining a single BCC phase, reducing the cost and minimizing the presence of critical elements using elements that can present good potential properties for a cermet and with low toxicity and price such as Al, Cr, Mo, Ni, Fe and Ti. The design has been made based on the composition calculation applying the HEA phase formation empirical rules from literature in combination with thermodynamic simulations by Calphad method. The viability of the compositions has been studied through the processing of the compositions by casting and the study of wettability and solubility at high temperature on the hard phase of TiCN. Once the chosen compositions have been validated as competitive binders, cermets have been consolidated by spark plasma sintering (SPS) and the influence of the compositions on the mechanical properties of the compound materials has been studied.
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12.
  • Qiu, Ren, 1993, et al. (författare)
  • Schmid factor analysis for chip flow induced plastic deformation of textured cubic carbonitride coatings
  • 2022
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 108
  • Tidskriftsartikel (refereegranskat)abstract
    • In high-speed metal machining, cutting tools in the form of cemented carbide inserts coated with thin wear-resistant coatings are commonly used. These coatings are often made of metal carbonitrides with cubic rock salt crystal structure and different growth textures. However, the influence of the crystallographic texture of the coatings on their wear by plastic deformation due to the chip flow during machining needs to be revealed further. In this work, in order to analyse the ability of polycrystalline fibre-textured coatings with a rock salt structure to undergo plastic deformation, a method was developed for calculating Schmid factors of such textured coatings as a function of the loading angle of an external force. The Schmid factors were calculated for coatings with 100 and 211 growth textures, and {100} <110>, {110}<110> and {111}<110> as possible slip systems. For the {111}<110> slip systems, the Schmid factors are not much influenced by the force angle and coating texture, which is contrary to the {100}<110> and {110}<110> slip systems. The simulations were compared to wear on the rake face of two textured Ti(C,N) coatings after short longitudinal turning tests. The variation of the degree of plastic deformation of Ti(C,N) coatings with growth texture and external force angle indicates that the dominant activated slip systems are {110}<110> using the machining conditions applied in this work.
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13.
  • Salmasi, Armin, et al. (författare)
  • Liquid phase migration in cemented carbides – Experiments and modelling
  • 2023
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 112
  • Tidskriftsartikel (refereegranskat)abstract
    • Liquid phase migration (LPM) and its potential impact on the phenomenon known as cobalt capping in cemented carbides have been investigated through experiments on fully densified, pre-sintered samples and simulations. A model for LPM was developed based on interface energies, grain size, and contiguity, and was implemented and integrated with Thermo-Calc software to make predictions about the final microstructure. The model's predictions, when applied to cases with gradients in grain size and volume fraction of the binder phase, were found to somewhat match experimental observations. Additionally, it was obsereved that LPM can cause a phenomenon resembling cobalt capping due to gradients in carbon activity.
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14.
  • Sheikh, Saad, 1987-, et al. (författare)
  • Fracture toughness of cemented carbides : Testing method andmicrostructural effects
  • 2015
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier. - 0263-4368 .- 2213-3917. ; 49, s. 153-160
  • Tidskriftsartikel (refereegranskat)abstract
    • Fracture toughness is one the most important parameters for design applications and performance assessment of WC–Co cemented carbides (hardmetals). Different from hardness, fracture toughness is commonly a property more difficult to evaluate, particularly in brittle materials. A large number of different testing methods have been introduced to evaluate toughness of hardmetals, but in general all of them have either theoretically debatable issues or important experimental difficulties. In this study, three different fracture toughness testing methodologies are investigated: three-point bending on Chevron notched specimen (“reference” baseline), Palmqvist indentation test, and Hertzian indentation method. The work is conducted in several cemented carbide grades with different microstructures, in terms of both WC grain size and Co binder content. It is found that Chevronnotched three-point bending test yields suitable fracture toughness values for a wide range of cemented carbide grades with varying hardness. Concerning indentation methods, the Hertzian one may be particularly recommended, as compared to Palmqvist method, as far as hardness (HV30) drops below 1300. On the other hand, if HV30 is higher than 1300 Palmqvist indentation procedure yields reliable fracture toughness measurements. Experimental findings are finally analyzed and discussed on the basis of two theoretical models proposed in the literature.
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15.
  • Shoja, Siamak, 1980, et al. (författare)
  • Microstructure investigation of textured CVD alumina coatings
  • 2020
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 87
  • Tidskriftsartikel (refereegranskat)abstract
    • This study concerns the interfacial microstructure and texture development in three textured CVD alpha-Al2O3 coatings using X-ray diffraction, transmission Kikuchi diffraction, scanning transmission electron microscopy and energy dispersive X-ray analysis. It is well known that the performance of these types of coatings relies on the degree and type of texture. The aim of this work is to study the microstructure of three different CVD alpha-Al2O3 layers when deposited onto a Ti(C,N,O) bonding layer. The coatings were deposited onto cemented carbide/cobalt substrates (WC/Co). It was observed that grain boundary diffusion of W and Co occurred through the TiN and Ti(C,N) layers to the bonding layer/alpha-Al2O3 interface. This may disturb the alumina layer nucleation and early growth. Interfacial porosity was observed at the bonding layer/alpha-Al2O3 interface. The number of voids that were detected in the (0001) and (01 (1) over bar0)-textured coatings was similar when H2S was not used during the initial deposition step. When H2S was present during the nucleation step deposition of the alpha-Al2O3 more voids were introduced at the interface for the (0001)-textured samples. The alumina grain morphology developed from small (similar to 100 nm) equiaxed grains at the start of the alumina coating to larger (several microns) columnar grains at the top of the coating. The inner part of the alumina coatings had a more random orientation. The texture changed either: (i) gradually over several grains, or (ii) more abruptly from one grain to another; from more random orientations to the desired texture. The desired texture develops earlier when using H2S at the start for the (0001)-textured coatings, while the (01 (1) over bar0)-texture development benefits from the absence of H2S. Thus, in this study, H2S promotes (0001) texture and interfacial void formation.
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16.
  • Soria-Biurrun, Tomas, et al. (författare)
  • Experimental and theoretical study of WC-40Fe-20Co-40Ni
  • 2022
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 102
  • Tidskriftsartikel (refereegranskat)abstract
    • The liquid phase formation temperatures of the quinary system W-C-Co-Fe-Ni with a ratio of Fe:Co:Ni = 40:20:40 were determined by means of DSC analysis. Besides, the experimental C-window of this system with a binder content of 14.3 +/- 2 wt% is accurately defined. Based on the experimental results, a thermodynamic modelling is carried out using the CALPHAD approach. Temperature-composition sections of the W-C-Co-Fe-Ni system with different binder contents are calculated to verify the rationality of the present modelling. There is a good correlation between the experimental and calculated results showing that the experimental data can be well reproduced by the present modelling.
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17.
  • Weidow, Jonathan, 1980, et al. (författare)
  • Analysis of WC with increased Ta doping
  • 2015
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 51, s. 56-60
  • Tidskriftsartikel (refereegranskat)abstract
    • WC powder with increased Ta doping has been produced. Tungsten and tantalum metal powders were cocarburized to yield a mixture of cubic and hexagonal carbide. The carburization was made through a two-step carburization process with (W,Ta)(2)C powder as an intermediate product. X-ray diffraction analysis showed that the lattice parameters of the hexagonal phase in the fully carburized powder were larger than those of pure WC indicating the formation of a mixed crystal carbide, (W,Ta)C. The powder with the largest lattice parameters was investigated in detail. A method to produce atom probe tomography specimens of this powder was developed. The largest Ta solubility, expressed as Ta/(Ta + W), was 0.086 which is more than four times higher than what previously has been observed. In addition, it was found with electron backscatter diffraction that the (W,Ta)C grains had a large fraction of Sigma 2 grain boundaries as well as a small fraction of what was suggested as Sigma 4 grain boundaries. (C) 2015 Elsevier Ltd. All rights reserved.
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18.
  • Weidow, Jonathan, 1980, et al. (författare)
  • Cemented carbides based on WC pre-alloyed with Cr or Ta
  • 2015
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 49, s. 36-41
  • Tidskriftsartikel (refereegranskat)abstract
    • WC powder pre-alloyed with Ta or Cr was used to produce WC-Co based cemented carbides and these were compared with materials produced from pure WC. Atom probe tomography analysis of the pre-alloyed as-sintered materials showed that a high, non-uniform, concentration of Ta or Cr remained in the WC lattice after sintering. The hexagonal (W,Ta)C partially decomposes during sintering leading to the formation of a fcc MC phase. Similarly, the (W,Cr)C partially decomposes, but no Cr-rich precipitates were observed in either of the Cr containing samples. The hardness of the crystallites of the doped materials was significantly lower than the undoped WC, as measured by nano-indentation. The Young's moduli of the doped crystals were significantly lower than the undoped WC in agreement with the lowering of the elastic constants, from ab initio calculations. The microhardness (HV) reflected what would be expected from the average WC grain size giving that the grain boundary surface area is more important for the material hardness than the hardness of the WC crystals themselves in the investigated grain size range.
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19.
  • Weidow, Jonathan, 1980, et al. (författare)
  • Chemistry of binder phase grain boundary in WC-Co based cemented carbide
  • 2013
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 41, s. 366-369
  • Tidskriftsartikel (refereegranskat)abstract
    • A binder phase grain boundary was located in a WC-Co-Cr cemented carbide using electron backscatter diffraction. A transmission electron microscopy specimen was produced including the grain boundary of interest. An energy dispersive X-ray analysis showed segregation of W and Cr to the grain boundary. However, no impurity atoms were found. (C) 2013 Elsevier Ltd. All rights reserved.
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20.
  • Weidow, J., et al. (författare)
  • Impact of crystal defects on the grain growth of cemented carbides
  • 2018
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier Ltd. - 0263-4368 .- 2213-3917. ; 72, s. 199-202
  • Tidskriftsartikel (refereegranskat)abstract
    • Two different WC-Co powder mixtures were produced through a 200 h milling process. One of these mixtures were heat treated with the aim to remove the crystal defects from the WC grains. Image analysis showed that the two powders had the same average WC grain size and grain size distributions. X-ray diffraction showed much more narrow peaks for the material produced from the heat treated powder thus interpreted to have much less crystal defects. The powder mixtures were used to produce cemented carbides. Image analyses showed a clear difference for the WC grain sizes where the material produced from the heat treated powder had a significantly smaller mean WC grain size. It is therefore concluded that the crystal defects are of importance and facilitate WC grain growth during sintering. 
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21.
  • Weidow, Jonathan, 1980, et al. (författare)
  • Optimized production route of WC powder pre-alloyed with Ta for fine grained cemented carbides
  • 2017
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 62:Part B, Special Issue, s. 97-103
  • Tidskriftsartikel (refereegranskat)abstract
    • Powders of hexagonal (W,Ta)C were produced following a two-step carburization process with (W,Ta)(2)C powder as an intermediate product XRD measurements indicate that higher temperatures during the first carburization step increase the fraction of Ta in the (W,Ta)(2)C structure where lower temperatures during the second carburization step seems to increase the fraction of Ta in the (W,Ta)C structure. Lower temperatures during the carburization steps increase the fraction of Sigma 2 WC/WC grain boundaries and cause the formation of what is interpreted as Sigma 4 boundaries. The (W,Ta)C powders can be successfully used to produce fine grained WC-Co based cemented carbides. The pre-alloying with Ta appears to have a softening effect on the material.
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22.
  • Yousfi, Mohamed Amine, 1986, et al. (författare)
  • Creep of un-doped and Cr-doped WC-Co at high temperature and high load
  • 2023
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - 0263-4368 .- 2213-3917. ; 117
  • Tidskriftsartikel (refereegranskat)abstract
    • Un-doped and Cr-doped WC-10 vol% Co cemented carbides with a WC grain size of 1.4 μm have been investigated before and after hot compressive creep tests under an applied load of 900 MPa at 1000 °C and 300 MPa at 1100 °C. The Cr-doped material showed a much higher creep resistance at 1000 °C and a somewhat higher creep resistance at 1100 °C than the un-doped material. Quantitative microscopy showed that WC grain growth occurred in the plane perpendicular to the load axis during creep deformation and that the growth process was slower in the Cr-doped material. In addition, binder phase redistributed and a number of WC grain boundaries were infiltrated with binder phase. This suggests that accommodated WC grain boundary sliding occurred during creep deformation. The formation of intergranular cavities implies that also unaccommodated grain boundary sliding occurred, especially at 1000 °C. It is suggested that WC grain growth perpendicular to the load axis is rate limiting in the creep deformation process, and that Cr segregation to WC/binder phase boundaries hinders grain growth. The weak effect of Cr on creep resistance at 1100 °C at 300 MPa is explained by Cr giving a larger volume fraction of binder phase and therefore a larger number of infiltrated grain boundaries, facilitating grain growth.
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23.
  • Yousfi, Mohamed Amine, 1986, et al. (författare)
  • Deformation mechanisms in a WC–Co based cemented carbide during creep
  • 2015
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 49:1, s. 81-87
  • Tidskriftsartikel (refereegranskat)abstract
    • The microstructure of a WC–Co based cemented carbide has been investigated before and after plastic deformation at high temperatures. The material was fabricated with a Co binder phase content of 16 vol.% and smaller additions of Cr. Hot compressive creep tests were performed under an applied load of 900 MPa at 1000 and 1100 °C. The test bars were deformed to 10 or 20% strain at 1000 °C, and to 7% strain at 1100 °C. Quantitative microscopy using SEM suggests that WC grain growth took place during the creep tests, and that the growth took place preferentially in the plane perpendicular to the load axis. TEM showed that there was an increased dislocation density in the WC grains after creep deformation. Deformation was also associated with a redistribution of the WC grains resulting in the formation of intergranular binder phase lamellae and cavities. EBSD showed that there was an increased spread in the crystallographic orientation of the binder phase after deformation.
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24.
  • Öhman, Mikael, 1983, et al. (författare)
  • Finite element simulations of thermal residual stresses in realistic 3D WC-Co microstructures
  • 2019
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368 .- 2213-3917. ; 85
  • Tidskriftsartikel (refereegranskat)abstract
    • During cooling of WC-Co cemented carbides from sintering temperatures, large internal residual stresses are obtained due to the difference in thermal expansion of WC and Co. The residual stresses depend on the properties of the microstructure, such as volume fractions and morphology, as well as the thermomechanical properties of the constituents. These residual stresses affect the engineering performance and strength of the hardmetal. In this paper, we present finite element simulations of the cooling of realistic synthetic WC-Co microstructures from sintering temperatures to room temperature. The microstructures are generated in voxel grids using a newly developed tool CCBuilder. Finite element analyses are conducted to obtain effective macroscopic thermomechanical properties and thermal residual stresses of WC-Co microstructures in a free sintered specimen. The analyses are performed within the framework of multi-scale computational homogenization using periodic boundary conditions. In the analyses individual anisotropic elastic and thermal properties of WC-grains are accounted for. An excellent agreement between numerically predicted and experimental results for the macroscopic thermomechanical properties for WC-Co is obtained. In addition, the predicted residual stresses in the microstructure after cooling are in good agreement with experimental data. The influence from different constitutive model assumptions (elasticity, elastoplasticity) for the Co-phase on the residual stresses is studied. Finally, the proposed modelling framework is used to study the influence of WC-grain geometry on the residual stress field and our results show a substantial dependency of the residual stresses on the detailed morphology of the microstructure. In conclusion, we demonstrate the power of the presented modelling framework using realistic synthetic 3D microstructures with periodic boundary conditions together with multi-scale computational homogenization. In the future it may be open up strategies for optimal material design of microstructures of WC-Co cemented carbides.
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25.
  • Antonsson, Tomas, et al. (författare)
  • Liquid Ni-Fe penetration and recrystallisation in tungsten
  • 2003
  • Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 21:3-4, s. 159-170
  • Tidskriftsartikel (refereegranskat)abstract
    • Melt penetration in grain boundaries of solid tungsten has been investigated. Solid tungsten rods have been exposed to a nickel-iron melt saturated with tungsten and the penetration depth and the shape of the liquid channels have been examined. The solid tungsten samples have been treated in different ways like cold working, annealing and recrystallisation, before melt exposure. Important parameters for the penetration process are stresses, surface tensions, solution and kinetic effects. A new theoretical model for the penetration mechanism in cold worked samples is proposed. Rapid recovery of the grains in the penetrated areas of the cold worked samples was observed. This is discussed, as well.
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26.
  • Frykholm, R., et al. (författare)
  • Effect of cubic phase composition on gradient zone formation in cemented carbides
  • 2001
  • Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 19:06-apr, s. 527-538
  • Tidskriftsartikel (refereegranskat)abstract
    • Cemented carbide cutting tool inserts with a surface zone depleted of hard cubic carbides and enriched in ductile binder phase have been studied experimentally using scanning electron microscopy (SEM), electron probe micro analysis (EPMA) and transmission electron microscopy (TEM). The results are compared with simulations based on a solution of the multicomponent diffusion equations, coupled with calculations using thermodynamic descriptions of the individual phases. The materials in the study are based on WC-Ti(C,N)-Co, WC-Ti(C,N)-NbC-Co and WC-Ti(C,N)-TaC-Co. The surface zone is formed by creating a gradient in nitrogen activity in the material, leading to an outward diffusion of N. Due to thermodynamical coupling between N and Ti, the outward diffusion of N will lead to an inward diffusion of Ti, and a surface zone depleted in cubic carbides is formed. Additions of elements like Ta or Nb will affect the width of the surface zone. A material with a Ta-containing cubic phase will have a narrower surface zone than a material with a Nb-containing cubic phase. Ta or Nb additions also affect the distribution of the different phases in and adjacent to the surface zone.
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27.
  • Hugosson, Håkan Wilhelm, et al. (författare)
  • The connection between the electronic structure and the properties of binderless tungsten carbides
  • 2003
  • Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 21:02-jan, s. 55-61
  • Tidskriftsartikel (refereegranskat)abstract
    • Using first principles density functional calculations, we here connect the physical properties of binderless tungsten carbides with the theoretical electronic structure as calculated from density functional theory calculations. We show that electronic structure calculations predict and explain various known phenomena in these systems, an example of how such theoretical studies can be a valuable tool in materials science. For example, changes in the energy of formation from 25% substitution with Mo or Ti is used to explain the differences in intermixing between binderless tungsten carbides using TiC or Mo2C as gamma binder phase. Substitutions with Ti atoms are also predicted to stabilize tungsten carbide in the NaCl-structure. A study of the charge density redistribution after substitution is also made.
  •  
28.
  • Hugosson, Håkan Wilhelm, 1972-, et al. (författare)
  • The connection between the electronic structure and the properties of binderless tungsten carbides
  • 2003
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier. - 0263-4368. ; 21:1-2, s. 55-61
  • Tidskriftsartikel (refereegranskat)abstract
    • Using first principles density functional calculations, we here connect the physical properties of binderless tungsten carbides with the theoretical electronic structure as calculated from density functional theory calculations. We show that electronic structure calculations predict and explain various known phenomena in these systems, an example of how such theoretical studies can be a valuable tool in materials science. For example, changes in the energy of formation from 25% substitution with Mo or Ti is used to explain the differences in intermixing between binderless tungsten carbides using TiC or Mo2C as γ binder phase. Substitutions with Ti atoms are also predicted to stabilize tungsten carbide in the NaCl-structure. A study of the charge density redistribution after substitution is also made.
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29.
  • Zhang, Z. Y., et al. (författare)
  • The reduction of cobalt doped ammonium paratungstate to nanostructured W-Co powder
  • 2002
  • Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 20:3, s. 227-233
  • Tidskriftsartikel (refereegranskat)abstract
    • Cobalt doped ammonium paratungstate (Co-doped APT) precursors for the fabrication of cemented carbides were synthesized by co-precipitation, where W and Co are mixed on the atomic scale. The effect of cobalt content on the reduction of W-Co precursor was studied. W-Co precursors with different ratios of tungsten to cobalt were prepared and calcined at 220 degreesC for 5 h in air, followed by a reduction in hydrogen gas at 600 and 650 degreesC for different times. The resulting powders were characterized with respect to phase constituent, specific surface area, particle size, density, and morphology of the reduced powders. It has been found that the addition of cobalt results in a higher reduction rate toward the metallic phases. However, the particle size of the reduced powders increases with increasing cobalt content, while its relative density decreases. The specific surface area of reduced powders at 650 degreesC increases for short reduction times, due to reduction of oxides to metal powder, and decreases for long reduction time due to agglomeration of metal powder. Cobalt plays the role as a catalyst during the reduction processing. On the other hand, it promotes the agglomeration of small particles. However, the completely reduced powders were well nanostructured with an average particle size between 20-50 nm.
  •  
30.
  • Akhtar, Farid (författare)
  • An investigation on the solid state sintering of mechanically alloyed nano-structured 90W--Ni--Fe tungsten heavy alloy
  • 2008
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 26:3, s. 145-151
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, 90W–7Ni–3Fe heavy alloy was investigated for its microstructure development, mechanical properties and fracture behavior after solid state sintering. The nano-sized powders were synthesized by mechanical alloying (MA). The microstructure of solid state sintered heavy alloys consisted of tungsten matrix. The average tungsten grain size in the range of 1.7–3.0 μm was obtained. It was found that the grain size largely affected the mechanical properties. Tensile strength more than 1200 MPa was achieved at a sintering temperature of 1350 °C. Fracture mechanisms based on microscopical observations on the fracture surfaces were studied. Matrix failure, tungsten-intergranular cleavage and tungsten–matrix interfacial separation were found to be the possible failure mechanisms.
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31.
  • Akhtar, Farid, et al. (författare)
  • Effect of WC particle size on the microstructure, mechanical properties and fracture behavior of WC--(W, Ti, Ta) C--6wt% Co cemented carbides
  • 2007
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 25:5-6, s. 405-410
  • Tidskriftsartikel (refereegranskat)abstract
    • This study deals with the microstructure and mechanical properties of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides fabricated by conventional sintering. The conventional WC particles of 4 μm size and ultrafine particles of 0.2 μm were introduced in the system with varying ratio. The ratios of conventional WC particles to ultrafine WC particles were 2:1, 1:1, and 1:2. The microstructures of sintered WC–(W, Ti, Ta) C–9 vol.% Co cemented carbides were sensitively dependent on the ratio of conventional WC particles to ultrafine WC particles. The rim phase increased with the increase in the amount of ultrafine particles. Hardness of WC–(W, Ti, Ta) C–9 vol.% Co cemented carbide increased with increase in the amount of rim phase and decrease in the average grain size of WC particles. The bending strength showed the similar trend of the hardness. The fracture morphologies are reported. The fracture behavior changed from mixed mode to transgranular fracture mode, when the ratio of conventional WC particles to ultrafine WC particles was changed from 2:1 to 1:2.
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32.
  • Andersson, KM, et al. (författare)
  • Oxidation and dissolution of tungsten carbide powder in water
  • 2000
  • Ingår i: International journal of refractory metals & hard materials. - 0263-4368. ; 18, s. 35-43
  • Tidskriftsartikel (refereegranskat)abstract
    • The oxidation and dissolution of tungsten carbide powder dispersed in water was investigated using XPS and leaching studies. We found that the WO3 surface layer on the oxidised tungsten carbide powder dissolves readily at pH>3 with the tungsten concentration increasing linearly with time. Adding cobalt powder to the tungsten carbide suspension resulted in a significant reduction of the dissolution rate at pH<10, when the concentration of dissolved Co is high in the solution phase. Electrokinetic studies indicate that the reduced dissolution rate may be related to the formation of surface complexes; experiments showed that Co species in solution adsorb on the oxidised tungsten carbide powder. The experimental data were discussed in relation to theoretical modelling of the WO3 solution chemistry and the Co2+ adsorption at oxide/water interfaces
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33.
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34.
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35.
  • Beste, Ulrik, et al. (författare)
  • Wear induced material modifications of cemented carbide rock drill buttons
  • 2006
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 24:1-2, s. 168-176
  • Tidskriftsartikel (refereegranskat)abstract
    • The drill crown of a rock drill is made of steel and equipped with WC/Co cemented carbide buttons (or inserts) of different geometries. These rock drill button are exposed to a large number of high load impacts into the rock. The complex and strongly shifting properties of rock minerals lead to a complex mixture of wear mechanisms. These wear mechanisms have recently been mapped by the present authors, and are divided into five classes of deterioration and five classes of material removal mechanisms. In this paper, two important deterioration mechanisms are studied in detail, namely the binder phase degradation and the rock intermixing.Transmission electron microscopy (TEM) has been employed for these high resolution studies. However, rock intermixture and huge internal stresses in the buttons lead to severe difficulties in preparing samples.Therefore, a focused ion beam-instrument (FIB) has been used to cut cross-section samples in the outermost surface on rock drill buttons. These have been investigated in the TEM by EDS, EFTEM, and STEM.Buttons from two rock drills of different history were selected for this investigation. One was used to drill 18 m in a hard rock type (quartzitic granite) and the other to drill 20 m in a much softer rock type (magnetite). Only selected regions of the outermost WC grain layers, which are in a steady state wear mode, were investigated.The crystallographic structure of the Co binder phase was investigated in both buttons, and it was represented mainly by the hcp-Co, but also small extent of fcc-Co. This is suggested to be a result of the mechanical fatigue, following one of two suggested Co-phase transformation series.The rock covers and intermixed zones formed were analysed in detail. The large part of the rock cover was found to be amorphous, containing rock and WC fragments. Adjacent to the WC grains, the rock cover was often found to have a porous structure, where the pores were surrounded by crystalline Co-particles adjacent to a carbon rich area. Apparently, the quartz rock locally melts and sticks very intimately to the WC grains and the porous structure forms during solidification. This feature was further analysed, and it was shown that the amorphous rock is seamlessly connected to WC on the atomic level. It was also stated that the rock cover and intermixed layers are very similar on both buttons, independent of rock type drilled.
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36.
  • Bonvalet, M., et al. (författare)
  • Modelling of prismatic grain growth in cemented carbides
  • 2019
  • Ingår i: International journal of refractory metals & hard materials. - : ELSEVIER SCI LTD. - 0263-4368. ; 78, s. 310-319
  • Tidskriftsartikel (refereegranskat)abstract
    • A mean-field model dealing with prismatic grain growth during liquid phase sintering of cemented carbides with a Co-rich binder is presented. The evolution of the size of an assembly of non-spherical grains is obtained using a Kampmann-Wagner approach and by introducing a constant shape factor between the characteristic lengths of prisms. This factor is a function of interfacial energies of the two kind of facets, basal and prismatic, considered. The growth model is based on three different mechanisms, that can be rate limiting, taking place in series: 2D nucleation of a new atomic layer, mass transfer across the interface and long-range diffusion. The driving force for coarsening is distributed between the different facets. These equations are solved numerically, and the simulation results reveal that the specific abnormal grain growth phenomena experimentally observed in cemented carbides may be reproduced with this new more realistic description of the grain shape contrary to the spherical approach developed in the past. It is also shown that the initial powder size distribution, and more specifically its shape has a strong influence on the distribution of the driving force between the different rate limiting mechanisms and thus on the occurrence of abnormal grain growth. In that case, the self-similarity of the normalized grain size distribution over time is not achieved.
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37.
  • Borgh, Ida, et al. (författare)
  • Effect of carbon activity and powder particle size on WC grain coarsening during sintering of cemented carbides
  • 2014
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 42, s. 30-35
  • Tidskriftsartikel (refereegranskat)abstract
    • Liquid-phase sintering is an important step in the production of cemented carbides. During sintering, the average WC grain size increases, leading to a coarser structure, which affects the performance of the final product. The coarsening occurs by dissolution of small grains and growth of large grains. In the present work, the effect of high carbon activity during sintering on the WC grain coarsening has been evaluated using electron backscattered diffraction (EBSD) and the results have been compared with a previous work where sintering was performed at a lower carbon activity. A more homogeneous grain size distribution was observed in alloys sintered at a high carbon activity. In addition, the effect of the initial powder particle size distribution was investigated. It was found that the coarsening rate of a WC powder with an initial small average grain size is significantly higher as compared to the coarsening rate for a powder with a larger initial average grain size. The results obtained emphasize the importance of considering the complete particle size distribution in order to predict coarsening.
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38.
  • Borgh, Ida, et al. (författare)
  • Influence of nitrogen Gas pressure on the miscibility Gap in the Ti-Zr carbonitride system
  • 2012
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 32, s. 11-15
  • Tidskriftsartikel (refereegranskat)abstract
    • The microstructure of cemented carbides with a gradient structure at the surface consists of WC, cubic carbonitrides and a binder phase. The carbonitrides can, for example, consist of Ti(C,N)-Zr(C,N) where it is reasonable to believe that there is a miscibility gap with Ti-rich and Zr-rich carbonitrides. In the present work, the effect of the N-2-gas pressure on the equilibrium composition of the miscibility gap in the (Ti,Zr)(C,N) system has been investigated. In the study, the carbonitride system is in equilibrium with: WC, liquid binder, graphite and, N-2-gas of different pressures. Both Fe and Co are used as binder phase to study the effect of the binder phase. The results verify that there is a miscibility gap in the carbonitride system and that the region of the miscibility gap will change when N is introduced. There is a critical N-2-gas pressure lower than 0.1 bar and above that pressure the compositions of the carbonitride are rather constant as a result of the formation of a surface rim.
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39.
  • Borgh, Ida, et al. (författare)
  • Microstructure, grain size distribution and grain shape in WC-Co alloys sintered at different carbon activities
  • 2014
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368 .- 0958-0611. ; 43, s. 205-211
  • Tidskriftsartikel (refereegranskat)abstract
    • The properties of cemented carbides strongly depend on the WC grain size and it is thus crucial to control coarsening of WC during processing. The aim of this work was to study the effect of sintering at different carbon activities on the final microstructure, as well as the coarsening behavior of the WC grains, including the size distribution and the shape of WC grains. These aspects were investigated for five WC-Co alloys sintered at 1410 degrees C for 1 h at different carbon activities in the liquid, in the range from the graphite equilibrium (carbon activity of 1) to the eta (M6C) phase equilibrium (carbon activity of 0.33). The grain size distribution was experimentally evaluated for the different alloys using EBSD (electron backscatter diffraction). In addition, the shape of the WC grains was evaluated for the different alloys. It was found that the average WC grain size increased and the grain size distribution became slightly wider with increasing carbon activity. Comparing the two three-phase (WC-Co-eta and WC-Co-graphite) alloys a shape change of the WC grains was observed with larger grains having more planar surfaces and more triangular shape for the WC-Co-graphite alloy. It was indicated that in alloys with a relatively low volume fraction of the binder phase the WC grain shape is significantly affected by impingements. Moreover, after 1 h of sintering the WC grains are at a non-equilibrium state with regards to grain morphology. (C) 2013 Elsevier Ltd. All rights reserved.
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40.
  • Bushlya, V., et al. (författare)
  • Sintering of binderless cubic boron nitride and its modification by β-Si3N4 additive for hard machining applications
  • 2020
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368. ; 86
  • Tidskriftsartikel (refereegranskat)abstract
    • This study present the results of HP-HT sintering, microstructure, properties, and performance of binderless cBN tool material. Within the investigated sintering temperature range of 1900–2600 °C the optimum was found to be 2200–2300 °C. Lower temperature results in incomplete diffusion bonding between cBN grains, while higher temperature results in high degree of recrystallization of initial structure, grain growth, and even formation of hexagonal boron nitride in triple joints. Introduction of stress-inducing β-Si3N4 minor inclusions resulted in high overall mechanical and thermal properties: HK = 41 GPa; KIC = 12.6 MPa·m1/2; λ = 180 W/(m·K). Machining experiments in roughing of hardened tool steels show that binderless cBN material provides high performance in terms of resistance to tool cratering, chipping, and tool fracture.
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41.
  • Canovic, Sead, 1979, et al. (författare)
  • CVD TiC/alumina and TiN/alumina multilayer coatings grown on sapphire single crystals
  • 2010
  • Ingår i: International Journal of Refractory Metals & Hard Materials. - : Elsevier BV. - 0263-4368. ; 28:2, s. 163-173
  • Tidskriftsartikel (refereegranskat)abstract
    • Multilayers of TiC/alpha-Al2O3 and TiN/kappa-Al2O3, consisting of three (1 mm thick) alumina layers separated by thin TiC or TiN (similar to 20 nm thick) layers, have been deposited onto c- and r-surfaces of single crystals of alpha-Al2O3 by chemical vapour deposition (CVD). The aim of this paper is to describe and compare the detailed microstructure of the different multilayer coatings by using transmission electron microscopy (TEM). The general microstructure of the alumina layers is very different when deposited onto different surfaces of alpha-Al2O3 single crystal substrates. On the c-surface the alumina layers grew evenly resulting in growth of single crystal layers of TiC or TiN and alumina throughout the coating. However, when deposited on the r-surface the alumina layers generally grow unevenly. The kappa -> alpha transformation occurs in TiN/kappa-Al2O3 multilayer coatings when deposited on both c- and r-surface substrates. In each layer the transformation starts at the interfacial pores in the upper part and proceeds inwards. The inner layer starts to transform first before the transformation proceeds to the outer layers. The microstructure of the transformed alpha-Al2O3 layers is different compared to as-deposited alpha-Al2O3, e.g. several voids and dislocations are present within the transformed alpha-Al2O3 layers. No pores were observed within the as-deposited alumina layers while a small number of pores was observed at the interfaces below the TiC and TiN layers. However, linkage of pores was observed within the transformed alpha-Al2O3 layers. There are also pores present in the substrate below the innermost TiC and TiN layers. It is believed that they are formed due to chemical etching during the deposition of the TiC and TiN layers. Single crystal TiC/alumina and TiN/alumina layers grow epitaxially on the c-surface substrates with close-packed planes growing on close-packed planes. On the r-surface, epitaxy is present only at some rare locations in alpha(r-sub)-TiC/Al2O3. However, in alpha(r-sub)-TiN/Al2O3 no epitaxy was observed. (C) 2009 Elsevier Ltd. All rights reserved.
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42.
  • Chen, Jinhan, et al. (författare)
  • The effect of hot isostatic pressing on thermal conductivity of additively manufactured pure tungsten
  • 2020
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 87
  • Tidskriftsartikel (refereegranskat)abstract
    • The crack-healing behaviors and microstructure evolution of pure tungsten produced by laser powder bed fusion (LPBF) were studied and compared before and after post hot isostatic pressing (post-HIP) treatment. An average thermal conductivity of 133 W.m(-1).K-1 at room temperature (RT) was obtained after HIP, which was 16% higher than that of as-built sample (115 W.m(-1).K-1). Although the HIP process had little effect on density, it resulted in a large grain size of > 300 mu m accompanied by a decrease in dislocation density and crack healing, which led to a substantial improvement of thermal conductivity of pure tungsten. The positive correlation between relative density and thermal conductivity of as-built tungsten was reported.
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43.
  • Ciurans Oset, Marina, 1993-, et al. (författare)
  • Use of AFM topography images to determine microindentation hardness of cast tungsten carbide powders
  • 2022
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier. - 0263-4368. ; 107
  • Tidskriftsartikel (refereegranskat)abstract
    • Hardness is defined as the resistance of a material to localized plastic deformation. Owing to their non-destructive nature, static indentation hardness tests are widely used in industry. Hardness testing is particularly useful for the mechanical characterization of materials that cannot be tested otherwise, e.g. powdered materials. In this study, challenges related to Vickers microindentation hardness testing of hard brittle cast tungsten carbide (CTC) powders were extensively investigated. Test load was optimized to obtain sufficiently large crack-free indentations allowing for precise measurement of the diagonal lengths. The influence of the operator and imaging technique on the measured hardness value was evaluated. Topography of residual imprints was investigated using atomic force microscopy (AFM) and a systematic and operator bias-free method to locate the indentation vertexes was developed. Results suggested that measurement variability introduced by AFM scanning and post-processing was as low as 3.1% and 1.3% with respect to the mean hardness value, respectively. Since the variability due to the measuring system can be isolated, the homogeneity of powders can be reliably evaluated from the hardness measurements thus obtained.
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44.
  • Edin, Emil, et al. (författare)
  • MD study of C diffusion in WC/W interfaces observed in cemented carbides
  • 2019
  • Ingår i: International journal of refractory metals & hard materials. - : ELSEVIER SCI LTD. - 0263-4368. ; 85
  • Tidskriftsartikel (refereegranskat)abstract
    • WC/Co tool used in the turning of Ti-alloys are subject to rapid crater wear due to chemical processes at the tool chip interface in the form of dissolution/diffusion. It has been observed that a thin layer of bcc W forms on the outermost WC grains in contact with the Ti workpiece meaning that C has diffused away from the WC. The rates involved in this process are of interest for formulating a theory of how wear progresses during turning of Ti-alloys. In this work we investigate the rates involved in this diffusion process by means of classical MD simulations on 6 different WC/W interfaces, 3 with a basal WC surface and 3 with a prismatic WC surface, as a function of C depletion in the outermost WC layer. The results show that all interfaces are stable and that principally no diffusion events of C occur at temperatures below 1373 K for C depletion levels below 30 at.%. At 50 at.% depletion, C diffusion starts occurring regularly and at 70 at.% depletion and above the majority of the C atoms are diffusing except at the lowest temperatures. Additionally, any difference in diffusion rate observed between the basal and prismatic interfaces in their pristine states have vanished at the point of 50 at.% depletion. This all points to a process which is initially slow for each layer of the WC but which increases in speed substantially as C atoms are removed.
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45.
  • Engqvist, H, et al. (författare)
  • A study of grain boundaries in a binderless cemented carbide
  • 1998
  • Ingår i: International journal of refractory metals & hard materials. - : ELSEVIER SCI LTD. - 0263-4368. ; 16:4-6, s. 309-313
  • Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
    • The microstructure of a binderless cemented carbide with <0.5 wt% Co has been analysed using transmission electron microscopy (TEM) in combination with an electron energy loss spectroscopy (EELS) detector and an imaging filter. In particular, microstructu
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46.
  • Eriksson, Mirva, et al. (författare)
  • Spark plasma sintering of wc, cemented carbide and functional graded materials
  • 2013
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 36, s. 31-37
  • Tidskriftsartikel (refereegranskat)abstract
    • Spark plasma sintering (SPS) is an extremely fast solidification technique for compounds that are difficult to sinter within the material group's metals, ceramics, or composites thereof. SPS uses a uniaxial pressure and a very rapid heating cycle to consolidate these materials. The heating is generated by Joule effect when a strong, pulsed electric current passes the conductive graphite die and also through the sample, if conductive. Cemented carbides (hard metals) are mostly used for metal cutting and drilling, wood cutting or rock drilling tools and are consolidated either by pressureless sintering (PLS), hot pressing (HP), or hot isostatic pressing (HIP). With SPS the main benefit is the ability to control the WC grain size due to the short sintering times at high temperature. In addition, unwished reactions between WC and cobalt to form other phases are minimized. By SPS the amount of cobalt can be reduced towards zero in fully dense WC materials. With this technique it is easy to prepare gradient materials where a ductile weldable metal can be joined with the cemented carbide part.
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47.
  • Fazakas, E., et al. (författare)
  • Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys
  • 2014
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 47, s. 131-138
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigated the microstructure and mechanical properties of Ti20Zr20Hf20Nb20X20 (X = V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti20Zr20Hf20Nb20X20 has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti20Zr20Hf20Nb20X20 contains small amount of Cr2Nb and Cr2Hf intermetallic compounds. Ti20Zr20Hf20Nb20X20 alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti20Zr20Hf20Nb20X20 alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10 min) heat treatment at 573, 773, 973 and 1173 K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEM. The theoretically calculated Young's modulus E is in good agreement with the experimental ones.
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48.
  • Feng, Peizhong, et al. (författare)
  • Mechanically activated reactive synthesis of refractory molybdenum and tungsten silicides
  • 2008
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 26:3, s. 173-178
  • Tidskriftsartikel (refereegranskat)abstract
    • In this study, processing of elemental powders mixtures was carried out by mechanical alloying (MA) and heat treatment in vacuum at 700–1000 °C for 1 h. The phase transformation of the powders was investigated by X-ray diffractometer (XRD). The results showed that mechanical alloying promoted the formation of a solid solution of elemental powders. The energy stored in the powders was increased as a result of exterior energy and the barrier energy of the formation of the compound could be exceeded easily. Intermetallics of MoSi2, WSi2, Mo5Si3, Mo3Si and SiC/MoSi2 composite powders were synthesized by mechanically activated reactive synthesis (MARS). The mechanically induced self-sustaining reaction was observed in MoSi2 and MoSi2 + 10 wt%SiC stoichiometry system. It has concluded that mechanically activated reactive synthesis is an effective method for the preparation of high melting-point refractory compounds.
  •  
49.
  • Garcia, Jose, et al. (författare)
  • Controlling cobalt capping in sintering process of cermets
  • 2017
  • Ingår i: International journal of refractory metals & hard materials. - : Elsevier BV. - 0263-4368. ; 62, s. 126-133
  • Tidskriftsartikel (refereegranskat)abstract
    • Critical sintering parameters for the formation of cobalt capping on cermets were evaluated experimentally. The results were compared to existing theories regarding cobalt capping formation on cemented carbides. Four different cermets grades were selected. The influence of the carbon activity, the sintering atmosphere (vacuum, flow rate, gas mixture, pressure) and the cooling rate was investigated. The results are in accordance with the observations that the Co surface top-layer forms during a specific short temperature interval during the cooling step for each grade, but it is affected by the preceding conditions; e.g. enhanced by a preceding Co-enrichment in the surface zone in a decarburizing atmosphere. The formation can be inhibited by using a carburizing atmosphere, such as carbon monoxide gas, or a sinter-HIP (hot isostatic pressure) step during cooling. A phenomenological model for formation of Co-capping in cermets considering melting point of alloys, cobalt migration, carbon activities, vapor pressure of reactive elements as well as processing parameters (sintering atmosphere, cooling rate etc.) is discussed.
  •  
50.
  • Garcia, J., et al. (författare)
  • Kinetics of formation of graded layers on cemented carbides : Experimental investigations and DICTRA simulations
  • 2011
  • Ingår i: International Journal of Refractory Metals and Hard Materials. - : Elsevier BV. - 0263-4368. ; 29:2, s. 256-259
  • Tidskriftsartikel (refereegranskat)abstract
    • Kinetics of formation of fcc-free layers on Co-W-Ti-Ta-Nb-C-N cemented carbides was investigated by experimental methods and DICTRA simulations. The layer formation obeys a parabolic law, indicating a diffusion-controlled process. For DICTRA simulations, the influence of the mobilities for all diffusing elements in the liquid binder phase at the sintering temperature was investigated. The best agreement between experimental and simulations was obtained considering that the mobility of all metallic elements is two times slower compared with the mobility of the non-metallic elements. © 2010 Elsevier Ltd. All rights reserved.
  •  
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