- Email: [email protected]
WC-Cu thermal barriers for fusion applications
Accepted Manuscript WC-Cu thermal barriers for fusion applications
M. Dias, F. Guerreiro, E. Tejado, J.B. Correia, U.V. Mardolcar, M. Coelho, T. Palacios, J.Y. Pastor, P.A. Carvalho, E. Alves PII: DOI: Reference:
S0257-8972(18)30212-3 doi:10.1016/j.surfcoat.2018.02.086 SCT 23157
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 October 2017 20 February 2018 21 February 2018
Please cite this article as: M. Dias, F. Guerreiro, E. Tejado, J.B. Correia, U.V. Mardolcar, M. Coelho, T. Palacios, J.Y. Pastor, P.A. Carvalho, E. Alves , WC-Cu thermal barriers for fusion applications. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.02.086
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT WC-Cu thermal barriers for fusion applications M. Dias 1, F. Guerreiro 1, E. Tejado 2, J.B. Correia 3, U.V. Mardolcar 4, M. Coelho 5, T. Palacios 2, J.Y. Pastor 2, P.A. Carvalho 1,6, E. Alves 1 1
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
Departamento de Ciencia de Materiales-CIME, Universidad Politécnica de Madrid,
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2
c/Professor Aranguren 3, E28040-Madrid, Spain. 3
LNEG, Laboratório Nacional de Energia e Geologia, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal
4
Departamento de Física, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001
Lisboa Portugal e Centro de Ciência Moleculares e Materiais, Faculdade de Ciências 5
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da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Diapor ferramentas diamantadas, Zona Industrial de Rio Meão, Apartado 412,
SINTEF Materials and Chemistry, Forskningsveien 1, NO-0314 Oslo, Norway
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6
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4524-907 Rio Meão, Portugal
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Marta Dias
PT
CORRESPONDING AUTHOR:
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Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal Telephone: +351 21 9946000 Fax: +351 21 9550117 Email: [email protected]
ACCEPTED MANUSCRIPT Abstract
WC-Cu cermets have been devised for thermal barriers between the plasma facing tungsten tiles and the copper-based heat sink in the first wall of nuclear fusion reactors. Composite materials with 50 and 75 v/v % WC have been prepared by hot
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pressing at 1333 and 1423 K with pressures of 37 and 47 MPa, respectively. Microstructural changes have been investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. The materials consolidated have also been evaluated in terms of Archimedes’ density,
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thermal diffusivity, Vickers hardness and elastic modulus. Implantation was carried out at room temperature with Ar+ at 100 keV ion beam with a fluence of 4
1020
MA
at/m2. The materials consisted of homogeneous dispersions of WC particles in a Cu matrix and presented densifications of about 90 %. Incipient swelling in copper-rich
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regions have been observed on the implanted surfaces, however no significant changes have been detected by X-ray diffraction. Higher WC content in the cermet
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materials increased hardness and the elastic modulus. The cermets’ thermal diffusivity
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barrier.
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was significantly lower than that of pure copper or tungsten, as desirable for a thermal
Keywords: WC-Cu cermet, hot pressing, thermal diffusivity, densification, mechanical properties, implantation.
ACCEPTED MANUSCRIPT 1. Introduction
The high melting point, high sputtering threshold and low tritium inventory turn tungsten into a suitable material for plasma facing and structural components in the first wall of nuclear fusion reactors. However, a major disadvantage of current
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tungsten-grades is their relatively high ductile-to-brittle transition temperature [1]. Therefore, operation at high temperatures is desirable to preserve the integrity of W components and also meets the purpose of increasing reactor efficiency. A CuCrZr alloy has been selected as heat sink material to remove heat from the plasma facing components due to its high conductivity, strength and microstructural stability [2].
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However, the service temperature of this material is relatively low and operation at
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higher temperatures demands thermal barriers between the plasma-facing W and the CuCrZr heat sink.
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A WC-Cu cermet interlayer serving as thermal barrier for fusion applications is proposed in this work. Tungsten carbide (WC) combines favorable properties, such
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as lower thermal conductivity than pure tungsten [3] and similar thermal expansion behavior [4], with sufficient electrical conductivity [5] and high melting point [5]. In
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addition, WC shows good wettability by molten metals, like copper [5]. Both carbon
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and tungsten have very low solubility in liquid copper and the high hardness of WC [5] is expected to act as mechanical reinforcement of the composite material. Therefore, WC-Cu cermets are promising as thermal barrier interlayers between W tiles and the Cu-based heat sink, allowing for graded transitions. A previous study revealed that 25 vol. % Cu can effectively reduce the large difference in thermal expansion between tungsten and the copper alloy [6] and the same can be expected for WC-Cu cermets with even lower thermal conductivity. High-energy neutrons resulting from fusion reactions induce atomic displacement cascades in first wall
ACCEPTED MANUSCRIPT materials and the response of the proposed thermal barriers to irradiation requires elucidation. The present work is focused on the characterization and testing of WC-Cu cermet materials sintered by hot pressing. The materials were implanted at room temperature with Ar+ at 100 keV ion beam with a fluence of 4 1020 at/m2. Scanning
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electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction were used to characterize the microstructure of the WCCu cermet materials. The geometric density, thermal diffusivity, (nano- and micro-)
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hardness and elastic modulus were evaluated at room temperature.
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2. Experimental
WC-Cu powder mixtures with 50 and 75 vol. % WC were mixed in a turbula
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for 1 h in N2 atmosphere using commercially pure WC powder (spherical particles with size < 1 µm) with 99.9% nominal purity and Cu powder (irregular particles with
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size < 37 µm) with 99.99% nominal purity. The WC-Cu powders were consolidated by hot pressing with an Idea Vulcan 70 VP press at temperatures between 1333 and
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1423 K using loads of 37 and 47 MPa (Table 1) with a heating rate of ~2 K/s and
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graphite dies of 10 x 55 x 5 mm3. The thermomechanical cycle was optimized for each cermet composition to minimize flow of liquid metal. Figure 1 shows schematically the thermal cycles employed. For the 50% Cu sample a temperature just below the melting point of Cu was used to prevent liquid extrusion out of the mold cavity. Therefore, 50% Cu sample extrusion has not been observed enabling a higher processing temperature.
The microstructures were investigated with secondary and backscattered
ACCEPTED MANUSCRIPT electron signals (SE and BSE, respectively) on polished as-sintered and implanted surfaces, using a JEOL JSM-7001F instrument operated at an accelerating voltage of 20 kV. The instrument is equipped with an Oxford EDS system used for point analysis and X-ray map collection. The samples were etched with a solution of Fe2Cl3 in ethanol. After implantation the samples were observed in SE mode at glancing
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angle (70º).
Density was measured using the Archimedes method. Thermal diffusivity measurements were performed with a flash instrument Flash Line 5000 Anter Corporation in the 373 – 673 K temperature range. Vickers microhardness (HV) was
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determined with an AKASHI MVK-EIII (Japan) tester using loads of 2.94, 4.90 and 9.81 N for 15 s. Instrumented nanoindentation to a maximum load of 0.49 N was
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performed on the same samples using a standard Berkovich tip, calibrated with fused silica. The average values of hardness and elastic modulus were taken from the
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unloading curve according to Oliver and Pharr method [7]. These tests allowed to obtain both the nano-hardness (nH) and elastic modulus (nE) at room temperature.
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The results were compared with those obtained via Vickers tests for the hardness and
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via impulse excitation technique (IET) [8] for the elastic modulus. The IET method measures the flexural vibration resonance of the materials and was applied using a
AC
Grindosonic MK4i equipment. Polished surfaces of sintered samples were implanted at room temperature with 100 keV Ar+ ion beams with a fluence of 4
1020 at/m2. The SRIM 2013
program [7] was used to compute the distribution of the tracks of argon ions and defects caused by argon. For the radiation dose equal to 3 1020 at/m2, the mean computed degree of damage over the entire region of deceleration of ions is ~ 100 dpa and the maximum concentration of argon at a depth of 600 nm is ~ 4 at.%.
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3. Results and discussion
Figure 2 shows the microstructures of the as-sintered 75WC-25Cu and 50WC50Cu cermets. The consolidated materials consisted of dispersions of WC particles in
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a Cu matrix without any evidence of oxide formation as attested by the maps presented in Figure 3. The configurations observed in Figure 2 (a) and (b) are typical of processing conditions where solid grains coexist with a wetting liquid and the packed particles bond together by liquid phase sintering [9]. In general, the WC
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particles were homogeneously and finely dispersed in the Cu matrix, although larger aggregates with medium size of ~30 m and ~20 m could be detected, respectively,
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in 75WC-25Cu and 50WC-50Cu (Figure 2 (c) and (d)). These probably represent uninfiltrated agglomerates of WC powder or Cu particles loosely lumped which were
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incompletely wetted during hot pressing [10]. The application of higher temperature
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resulted in denser configurations for the agglomerates in 75WC-25Cu. Figure 4 presents the microstructure of the 50WC-50Cu material before and
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after implantation observed at glancing angle. The implanted sample revealed swelling in Cu-rich regions, however the WC grain remain apparently unaltered. A
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similar behaviour was observed for the sample with 75WC-25Cu composition. Swelling by Ar implantion was previously reported for Be [11] and Si [12] and has been proposed to be associated with dislocations loops created inside the material. Nevertheless, the absence of peak shifts and/or widening in the glancing angle diffractograms demonstrates that major structural changes did not occur with implantation (Figure 5).
ACCEPTED MANUSCRIPT Similar densifications have been achieved for both materials as presented in Table 2. Figure 6 presents the thermal diffusivity of the as-sintered WC-Cu cermet as a function of temperature together with literature values for pure Cu [5], W [13] and WC [14].The thermal behavior of 75WC-25Cu is closer to that of pure WC, and cannot be adjusted to the rule of mixtures, since the thermal diffusivity values in
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agreement this rule (composition proportion) should be higher. This is probably due to lack of infiltration in WC agglomerates (as observed in the microstructure in Figure 2 (c)), rendering heat percolation through copper more difficult.
The average elastic modulus and hardness for each composition are presented
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in Tables 3 and 4, respectively. The modulus values obtained by IET and nE are similar for each material and increase with WC content. Hardness is higher for lower
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indentation loads, which is a normal behavior for materials with porosity. In addition, the area affected by the indentation increases with increasing peak load. Therefore,
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the affected area of each component is also higher, which also explains the increase of
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4. Conclusions
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the standard error in the nanoindentation (nH) results.
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The WC-Cu cermet material with 50 and 75 % volume fraction of copper have been devised for thermal barriers. The materials have been prepared by hot pressing at temperatures ranging from 1333 to 1423 K with pressures of 37 to 47 MPa. The consolidated materials consisted of dispersions of WC particles in a Cu matrix without any evidence of oxide formation and with densifications of around 88%. The implanted materials revealed swelling in copper-rich regions however no significant modifications were observed by X-ray diffraction. The thermal diffusivity of the new cermet materials is much lower than that of copper or tungsten, as desirable for
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Acknowledgements
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This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 20142018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support was also
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received from the Portuguese Science and Technology Foundation (FCT) under the PTDC/CTM/100163/2008 grant and the PEST-OE/CTM-UI0084/2011 contracts. M.
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Dias acknowledges the FCT grant SFRH/BPD/68663/2010. The authors would also like to acknowledge to Ministerio de Economía y
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Competitividad of Spain (research projects MAT2012-38541-C02-02 and MAT201570780-C4-4-P), and Comunidad de Madrid (research project S2013/MIT-2862-
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CE
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MULTIMATCHALLENGE) for funding for this research.
ACCEPTED MANUSCRIPT References: [1] G.M.Wright, E. Alves, L.C. Alves, N.P. Barradas, P.A. Carvalho, R. Mateus, J. Rapp, “Hydrogenic retention of high-Z refractory metals exposed to ITER divertor relevant plasma conditions”, Nucl. Fusion 50 (2010) 055004. [2] V. Barasbash et.al., The ITER International Team, J. Nucl. Mater. 367-370 (2007) 21.
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[3] M. Akoshima et al. ECTP2014 - 20th European Conference on Thermophysical Properties (2014).
[4] http://www.wesltd.com/divisions/hardmetal/html/Tungsten-carbide.html [5] R.C. Gassmann, Mater. Sci. Tech., 12 (1996), 691–696.
[6] E. Tejado, M. Dias, J.B. Correia, T. Palacios, P.A. Carvalho, E. Alves, J.Y. Pastor, New WC-Cu thermal barriers for fusion applications: High temperature mechanical
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behavior, Journal of Nuclear Materials 498 (2018) 351-361.
[7] W.C. Oliver, G.M. Pharr, An improve technique for determining hardness and
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elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992).
[8] Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio for
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Advanced Ceramics by Impulse Excitation of Vibration, ASTM C1259-98, Annual Book of ASTM Standards, American Society for Testing and Materials, West
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Conshohocken, Pennsylvania, 1999, pp. 386–400 (15.01). [9] Z. Z. Fang, O. O. Eso, “Liquid phase sintering of functionally graded WC–Co
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composites”, Script. Mater. 52 (2005) 785-791. [10] A. R. Kennedy, J. D. Wood, and B. M. Weager, “Wetting and spontaneous
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infiltration of ceramics by molten copper,” J. Mater. Sci., 35, 12 (2000) 2909–2912. [11] P. Liu, Q. Zhan, W. Hu, Y. Jia, W. Han , X. Yi, F. Wan, Microstructure evolution of beryllium with argon ion irradiation, Nuclear Materials and Energy 000 (2017) 1-5. [ 12 ] S. Momota, J. Zhang, T. Toyonaga, H. Terauchi, K. Maeda, J. Taniguchi, T. Hirao, M. Furuta, T. Kawaharamura, Control of swelling height of Si crystal by irradiating Ar beam. J Nanosci Nanotechnol 12, 1 (2012) 552-556. [ 13 ] M. Fujitsuka et al., Effect of neutron irradiation on thermal difusivity of tungsten-rhenium alloys, Journal of Nuclear Materials 238-287 (2000) 1148-1151.
ACCEPTED MANUSCRIPT [ 14 ] M. Akoshima, Y. Yamashita, Y. Hishinuma, T. Tanaka, and T. Muroga, “Thermal Diffusivity Measurements of Candidate Ceramic Materials for Shielding Blankets,” in ECTP2014 - 20th European Conference on Thermophysical Properties,
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2014.
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Table 1 – Consolidation parameters (temperature, pressure and time) for each cermet composition. Composition %(v/v) 50WC-50Cu
75WC-25Cu
Temperature (K)
1333
1423
Pressure (MPa)
37
Time (min)
5
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Conditions
47 6
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Table 2 – Densification values of the cermet for both the compositions. Composition %(V/V) Property 50WC-50Cu
@1333 K, 37Mpa, 5 min
@1423 K, 47Mpa, 6 min
10.8 ± 0.001
12.3 ± 0.004
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Density (g/cm )
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88
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Densification (%)
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75WC-25Cu
89
ACCEPTED MANUSCRIPT Table 3 – Elastic modulus mean values and standard error of the cermets measured with impulse excitation technique (IET) and nanoindentation (nE) for all the compositions. Elastic modulus
Composition %(V/V)
(GPa)
75WC-25Cu
IET
278 ± 10
376 ± 10
nE
265 ± 6
394 ± 17
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50WC-50Cu
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Table 4 – Hardness mean values and standard error of the cermets measured with Vickers hardness (HV) and instrumented indentation hardness (nH) for all the compositions .
Composition %(V/V)
Max.
(GPa)
load (gf)
50WC-50Cu
75WC-25Cu
nH
50
5.2 ± 0.3
10.9 ± 0.8
HV
300
—
—
500
4.3 ± 0.2
9.3 ± 0.7
1000
3.50 ± 0.06
9.0 ± 0.7
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Hardness
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List of captions Figure 1 – Hot pressing consolidation cycles for each sample. Figure 2 – SEM/BSE images showing the microstructure of WC-Cu cermets with different compositions (a) and (c) 75WC-25Cu and (b) and (d) 50WC-50Cu consolidated by hot pressing.
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Figure 3 – (a) Microstructure of the 50WC-50Cu cermets and the EDS maps of (b) Cu, (c) W and (d) O elements. Corresponding X-ray maps for Cu–K, W– M and O– K.
Figure 4 – SE images showing the microstructure of 50WC-50Cu cermet tilted 70º (a) before and (b) after implantation.
Figure 5 – Diffractogram of 75WC-25Cu sample (a) implanted and (b) as sintered. The symbols: *
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indicate the peaks of WC and o indicate the peaks of Cu in both cases.
Figure 6 – Thermal diffusivity of samples: 75WC-25Cu and 50WC-50Cu cermets. For comparison the
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curves for pure Cu [6], W [16] WC [17] are also presented.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
ACCEPTED MANUSCRIPT
Highlights
The composite materials consisted of homogeneous dispersions of WC particles in a Cu matrix and presented densifications around 88%;
Implantation induced incipient swelling in copper-rich regions;
Higher WC content in the cermet materials increased hardness and the elastic modulus;
The cermets’ thermal diffusivity was significantly lower than that of pure copper or tungsten.
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CE
PT
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M. Dias, F. Guerreiro, E. Tejado, J.B. Correia, U.V. Mardolcar, M. Coelho, T. Palacios, J.Y. Pastor, P.A. Carvalho, E. Alves PII: DOI: Reference:
S0257-8972(18)30212-3 doi:10.1016/j.surfcoat.2018.02.086 SCT 23157
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
27 October 2017 20 February 2018 21 February 2018
Please cite this article as: M. Dias, F. Guerreiro, E. Tejado, J.B. Correia, U.V. Mardolcar, M. Coelho, T. Palacios, J.Y. Pastor, P.A. Carvalho, E. Alves , WC-Cu thermal barriers for fusion applications. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.02.086
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT WC-Cu thermal barriers for fusion applications M. Dias 1, F. Guerreiro 1, E. Tejado 2, J.B. Correia 3, U.V. Mardolcar 4, M. Coelho 5, T. Palacios 2, J.Y. Pastor 2, P.A. Carvalho 1,6, E. Alves 1 1
Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
Departamento de Ciencia de Materiales-CIME, Universidad Politécnica de Madrid,
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2
c/Professor Aranguren 3, E28040-Madrid, Spain. 3
LNEG, Laboratório Nacional de Energia e Geologia, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal
4
Departamento de Física, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001
Lisboa Portugal e Centro de Ciência Moleculares e Materiais, Faculdade de Ciências 5
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da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal Diapor ferramentas diamantadas, Zona Industrial de Rio Meão, Apartado 412,
SINTEF Materials and Chemistry, Forskningsveien 1, NO-0314 Oslo, Norway
ED
6
MA
4524-907 Rio Meão, Portugal
CE
Marta Dias
PT
CORRESPONDING AUTHOR:
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Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal Telephone: +351 21 9946000 Fax: +351 21 9550117 Email: [email protected]
ACCEPTED MANUSCRIPT Abstract
WC-Cu cermets have been devised for thermal barriers between the plasma facing tungsten tiles and the copper-based heat sink in the first wall of nuclear fusion reactors. Composite materials with 50 and 75 v/v % WC have been prepared by hot
SC RI PT
pressing at 1333 and 1423 K with pressures of 37 and 47 MPa, respectively. Microstructural changes have been investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction. The materials consolidated have also been evaluated in terms of Archimedes’ density,
NU
thermal diffusivity, Vickers hardness and elastic modulus. Implantation was carried out at room temperature with Ar+ at 100 keV ion beam with a fluence of 4
1020
MA
at/m2. The materials consisted of homogeneous dispersions of WC particles in a Cu matrix and presented densifications of about 90 %. Incipient swelling in copper-rich
ED
regions have been observed on the implanted surfaces, however no significant changes have been detected by X-ray diffraction. Higher WC content in the cermet
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materials increased hardness and the elastic modulus. The cermets’ thermal diffusivity
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barrier.
CE
was significantly lower than that of pure copper or tungsten, as desirable for a thermal
Keywords: WC-Cu cermet, hot pressing, thermal diffusivity, densification, mechanical properties, implantation.
ACCEPTED MANUSCRIPT 1. Introduction
The high melting point, high sputtering threshold and low tritium inventory turn tungsten into a suitable material for plasma facing and structural components in the first wall of nuclear fusion reactors. However, a major disadvantage of current
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tungsten-grades is their relatively high ductile-to-brittle transition temperature [1]. Therefore, operation at high temperatures is desirable to preserve the integrity of W components and also meets the purpose of increasing reactor efficiency. A CuCrZr alloy has been selected as heat sink material to remove heat from the plasma facing components due to its high conductivity, strength and microstructural stability [2].
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However, the service temperature of this material is relatively low and operation at
MA
higher temperatures demands thermal barriers between the plasma-facing W and the CuCrZr heat sink.
ED
A WC-Cu cermet interlayer serving as thermal barrier for fusion applications is proposed in this work. Tungsten carbide (WC) combines favorable properties, such
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as lower thermal conductivity than pure tungsten [3] and similar thermal expansion behavior [4], with sufficient electrical conductivity [5] and high melting point [5]. In
CE
addition, WC shows good wettability by molten metals, like copper [5]. Both carbon
AC
and tungsten have very low solubility in liquid copper and the high hardness of WC [5] is expected to act as mechanical reinforcement of the composite material. Therefore, WC-Cu cermets are promising as thermal barrier interlayers between W tiles and the Cu-based heat sink, allowing for graded transitions. A previous study revealed that 25 vol. % Cu can effectively reduce the large difference in thermal expansion between tungsten and the copper alloy [6] and the same can be expected for WC-Cu cermets with even lower thermal conductivity. High-energy neutrons resulting from fusion reactions induce atomic displacement cascades in first wall
ACCEPTED MANUSCRIPT materials and the response of the proposed thermal barriers to irradiation requires elucidation. The present work is focused on the characterization and testing of WC-Cu cermet materials sintered by hot pressing. The materials were implanted at room temperature with Ar+ at 100 keV ion beam with a fluence of 4 1020 at/m2. Scanning
SC RI PT
electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction were used to characterize the microstructure of the WCCu cermet materials. The geometric density, thermal diffusivity, (nano- and micro-)
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hardness and elastic modulus were evaluated at room temperature.
MA
2. Experimental
WC-Cu powder mixtures with 50 and 75 vol. % WC were mixed in a turbula
ED
for 1 h in N2 atmosphere using commercially pure WC powder (spherical particles with size < 1 µm) with 99.9% nominal purity and Cu powder (irregular particles with
PT
size < 37 µm) with 99.99% nominal purity. The WC-Cu powders were consolidated by hot pressing with an Idea Vulcan 70 VP press at temperatures between 1333 and
CE
1423 K using loads of 37 and 47 MPa (Table 1) with a heating rate of ~2 K/s and
AC
graphite dies of 10 x 55 x 5 mm3. The thermomechanical cycle was optimized for each cermet composition to minimize flow of liquid metal. Figure 1 shows schematically the thermal cycles employed. For the 50% Cu sample a temperature just below the melting point of Cu was used to prevent liquid extrusion out of the mold cavity. Therefore, 50% Cu sample extrusion has not been observed enabling a higher processing temperature.
The microstructures were investigated with secondary and backscattered
ACCEPTED MANUSCRIPT electron signals (SE and BSE, respectively) on polished as-sintered and implanted surfaces, using a JEOL JSM-7001F instrument operated at an accelerating voltage of 20 kV. The instrument is equipped with an Oxford EDS system used for point analysis and X-ray map collection. The samples were etched with a solution of Fe2Cl3 in ethanol. After implantation the samples were observed in SE mode at glancing
SC RI PT
angle (70º).
Density was measured using the Archimedes method. Thermal diffusivity measurements were performed with a flash instrument Flash Line 5000 Anter Corporation in the 373 – 673 K temperature range. Vickers microhardness (HV) was
NU
determined with an AKASHI MVK-EIII (Japan) tester using loads of 2.94, 4.90 and 9.81 N for 15 s. Instrumented nanoindentation to a maximum load of 0.49 N was
MA
performed on the same samples using a standard Berkovich tip, calibrated with fused silica. The average values of hardness and elastic modulus were taken from the
ED
unloading curve according to Oliver and Pharr method [7]. These tests allowed to obtain both the nano-hardness (nH) and elastic modulus (nE) at room temperature.
PT
The results were compared with those obtained via Vickers tests for the hardness and
CE
via impulse excitation technique (IET) [8] for the elastic modulus. The IET method measures the flexural vibration resonance of the materials and was applied using a
AC
Grindosonic MK4i equipment. Polished surfaces of sintered samples were implanted at room temperature with 100 keV Ar+ ion beams with a fluence of 4
1020 at/m2. The SRIM 2013
program [7] was used to compute the distribution of the tracks of argon ions and defects caused by argon. For the radiation dose equal to 3 1020 at/m2, the mean computed degree of damage over the entire region of deceleration of ions is ~ 100 dpa and the maximum concentration of argon at a depth of 600 nm is ~ 4 at.%.
ACCEPTED MANUSCRIPT
3. Results and discussion
Figure 2 shows the microstructures of the as-sintered 75WC-25Cu and 50WC50Cu cermets. The consolidated materials consisted of dispersions of WC particles in
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a Cu matrix without any evidence of oxide formation as attested by the maps presented in Figure 3. The configurations observed in Figure 2 (a) and (b) are typical of processing conditions where solid grains coexist with a wetting liquid and the packed particles bond together by liquid phase sintering [9]. In general, the WC
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particles were homogeneously and finely dispersed in the Cu matrix, although larger aggregates with medium size of ~30 m and ~20 m could be detected, respectively,
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in 75WC-25Cu and 50WC-50Cu (Figure 2 (c) and (d)). These probably represent uninfiltrated agglomerates of WC powder or Cu particles loosely lumped which were
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incompletely wetted during hot pressing [10]. The application of higher temperature
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resulted in denser configurations for the agglomerates in 75WC-25Cu. Figure 4 presents the microstructure of the 50WC-50Cu material before and
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after implantation observed at glancing angle. The implanted sample revealed swelling in Cu-rich regions, however the WC grain remain apparently unaltered. A
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similar behaviour was observed for the sample with 75WC-25Cu composition. Swelling by Ar implantion was previously reported for Be [11] and Si [12] and has been proposed to be associated with dislocations loops created inside the material. Nevertheless, the absence of peak shifts and/or widening in the glancing angle diffractograms demonstrates that major structural changes did not occur with implantation (Figure 5).
ACCEPTED MANUSCRIPT Similar densifications have been achieved for both materials as presented in Table 2. Figure 6 presents the thermal diffusivity of the as-sintered WC-Cu cermet as a function of temperature together with literature values for pure Cu [5], W [13] and WC [14].The thermal behavior of 75WC-25Cu is closer to that of pure WC, and cannot be adjusted to the rule of mixtures, since the thermal diffusivity values in
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agreement this rule (composition proportion) should be higher. This is probably due to lack of infiltration in WC agglomerates (as observed in the microstructure in Figure 2 (c)), rendering heat percolation through copper more difficult.
The average elastic modulus and hardness for each composition are presented
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in Tables 3 and 4, respectively. The modulus values obtained by IET and nE are similar for each material and increase with WC content. Hardness is higher for lower
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indentation loads, which is a normal behavior for materials with porosity. In addition, the area affected by the indentation increases with increasing peak load. Therefore,
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the affected area of each component is also higher, which also explains the increase of
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4. Conclusions
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the standard error in the nanoindentation (nH) results.
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The WC-Cu cermet material with 50 and 75 % volume fraction of copper have been devised for thermal barriers. The materials have been prepared by hot pressing at temperatures ranging from 1333 to 1423 K with pressures of 37 to 47 MPa. The consolidated materials consisted of dispersions of WC particles in a Cu matrix without any evidence of oxide formation and with densifications of around 88%. The implanted materials revealed swelling in copper-rich regions however no significant modifications were observed by X-ray diffraction. The thermal diffusivity of the new cermet materials is much lower than that of copper or tungsten, as desirable for
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Acknowledgements
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This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 20142018 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support was also
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received from the Portuguese Science and Technology Foundation (FCT) under the PTDC/CTM/100163/2008 grant and the PEST-OE/CTM-UI0084/2011 contracts. M.
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Dias acknowledges the FCT grant SFRH/BPD/68663/2010. The authors would also like to acknowledge to Ministerio de Economía y
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Competitividad of Spain (research projects MAT2012-38541-C02-02 and MAT201570780-C4-4-P), and Comunidad de Madrid (research project S2013/MIT-2862-
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MULTIMATCHALLENGE) for funding for this research.
ACCEPTED MANUSCRIPT References: [1] G.M.Wright, E. Alves, L.C. Alves, N.P. Barradas, P.A. Carvalho, R. Mateus, J. Rapp, “Hydrogenic retention of high-Z refractory metals exposed to ITER divertor relevant plasma conditions”, Nucl. Fusion 50 (2010) 055004. [2] V. Barasbash et.al., The ITER International Team, J. Nucl. Mater. 367-370 (2007) 21.
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[3] M. Akoshima et al. ECTP2014 - 20th European Conference on Thermophysical Properties (2014).
[4] http://www.wesltd.com/divisions/hardmetal/html/Tungsten-carbide.html [5] R.C. Gassmann, Mater. Sci. Tech., 12 (1996), 691–696.
[6] E. Tejado, M. Dias, J.B. Correia, T. Palacios, P.A. Carvalho, E. Alves, J.Y. Pastor, New WC-Cu thermal barriers for fusion applications: High temperature mechanical
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behavior, Journal of Nuclear Materials 498 (2018) 351-361.
[7] W.C. Oliver, G.M. Pharr, An improve technique for determining hardness and
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elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (6) (1992).
[8] Method for Dynamic Young’s Modulus, Shear Modulus, and Poisson’s Ratio for
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Advanced Ceramics by Impulse Excitation of Vibration, ASTM C1259-98, Annual Book of ASTM Standards, American Society for Testing and Materials, West
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Conshohocken, Pennsylvania, 1999, pp. 386–400 (15.01). [9] Z. Z. Fang, O. O. Eso, “Liquid phase sintering of functionally graded WC–Co
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composites”, Script. Mater. 52 (2005) 785-791. [10] A. R. Kennedy, J. D. Wood, and B. M. Weager, “Wetting and spontaneous
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infiltration of ceramics by molten copper,” J. Mater. Sci., 35, 12 (2000) 2909–2912. [11] P. Liu, Q. Zhan, W. Hu, Y. Jia, W. Han , X. Yi, F. Wan, Microstructure evolution of beryllium with argon ion irradiation, Nuclear Materials and Energy 000 (2017) 1-5. [ 12 ] S. Momota, J. Zhang, T. Toyonaga, H. Terauchi, K. Maeda, J. Taniguchi, T. Hirao, M. Furuta, T. Kawaharamura, Control of swelling height of Si crystal by irradiating Ar beam. J Nanosci Nanotechnol 12, 1 (2012) 552-556. [ 13 ] M. Fujitsuka et al., Effect of neutron irradiation on thermal difusivity of tungsten-rhenium alloys, Journal of Nuclear Materials 238-287 (2000) 1148-1151.
ACCEPTED MANUSCRIPT [ 14 ] M. Akoshima, Y. Yamashita, Y. Hishinuma, T. Tanaka, and T. Muroga, “Thermal Diffusivity Measurements of Candidate Ceramic Materials for Shielding Blankets,” in ECTP2014 - 20th European Conference on Thermophysical Properties,
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Table 1 – Consolidation parameters (temperature, pressure and time) for each cermet composition. Composition %(v/v) 50WC-50Cu
75WC-25Cu
Temperature (K)
1333
1423
Pressure (MPa)
37
Time (min)
5
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Conditions
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Table 2 – Densification values of the cermet for both the compositions. Composition %(V/V) Property 50WC-50Cu
@1333 K, 37Mpa, 5 min
@1423 K, 47Mpa, 6 min
10.8 ± 0.001
12.3 ± 0.004
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Density (g/cm )
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88
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Densification (%)
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75WC-25Cu
89
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Composition %(V/V)
(GPa)
75WC-25Cu
IET
278 ± 10
376 ± 10
nE
265 ± 6
394 ± 17
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Table 4 – Hardness mean values and standard error of the cermets measured with Vickers hardness (HV) and instrumented indentation hardness (nH) for all the compositions .
Composition %(V/V)
Max.
(GPa)
load (gf)
50WC-50Cu
75WC-25Cu
nH
50
5.2 ± 0.3
10.9 ± 0.8
HV
300
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500
4.3 ± 0.2
9.3 ± 0.7
1000
3.50 ± 0.06
9.0 ± 0.7
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Hardness
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List of captions Figure 1 – Hot pressing consolidation cycles for each sample. Figure 2 – SEM/BSE images showing the microstructure of WC-Cu cermets with different compositions (a) and (c) 75WC-25Cu and (b) and (d) 50WC-50Cu consolidated by hot pressing.
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Figure 3 – (a) Microstructure of the 50WC-50Cu cermets and the EDS maps of (b) Cu, (c) W and (d) O elements. Corresponding X-ray maps for Cu–K, W– M and O– K.
Figure 4 – SE images showing the microstructure of 50WC-50Cu cermet tilted 70º (a) before and (b) after implantation.
Figure 5 – Diffractogram of 75WC-25Cu sample (a) implanted and (b) as sintered. The symbols: *
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indicate the peaks of WC and o indicate the peaks of Cu in both cases.
Figure 6 – Thermal diffusivity of samples: 75WC-25Cu and 50WC-50Cu cermets. For comparison the
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curves for pure Cu [6], W [16] WC [17] are also presented.
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Figure 3
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Figure 5
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Highlights
The composite materials consisted of homogeneous dispersions of WC particles in a Cu matrix and presented densifications around 88%;
Implantation induced incipient swelling in copper-rich regions;
Higher WC content in the cermet materials increased hardness and the elastic modulus;
The cermets’ thermal diffusivity was significantly lower than that of pure copper or tungsten.
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