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Helium and deuterium irradiation effects in W-Ta composites produced by pulse plasma compaction
Journal of Nuclear Materials 492 (2017) 105e112
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Helium and deuterium irradiation effects in W-Ta composites produced by pulse plasma compaction M. Dias a, *, N. Catarino a, D. Nunes b, E. Fortunato b, I. Nogueira c, M. Rosinki d, J.B. Correia e, P.A. Carvalho a, f, E. Alves a ~o Nuclear, Instituto Superior T Instituto de Plasmas e Fusa ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal CENIMAT-I3N, Departamento de Ci^ encia dos Materiais, Faculdade de Ci^ encias e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c CEFEMA, Instituto Superior T ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal d GeniCore, Warsaw, Poland e rio Nacional de Energia e Geologia, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal LNEG, Laborato f SINTEF Materials and Chemistry, Forskningsveien 1, 0314 Oslo, Norway a
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a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 3 May 2017 Accepted 8 May 2017 Available online 10 May 2017
Tungsten-tantalum composites have been envisaged for first-wall components of nuclear fusion reactors; however, changes in their microstructure are expected from severe irradiation with helium and hydrogenic plasma species. In this study, composites were produced from ball milled W powder mixed with 10 at.% Ta fibers through consolidation by pulse plasma compaction. Implantation was carried out at room temperature with Heþ (30 keV) or Dþ (15 keV) or sequentially with Heþ and Dþ using ion beams with fluences of 5 1021 at/m2. Microstructural changes and deuterium retention in the implanted composites were investigated by scanning electron microscopy, coupled with focused ion beam and energy dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, Rutherford backscattering spectrometry and nuclear reaction analysis. The composite materials consisted of Ta fibers dispersed in a nanostructured W matrix, with Ta2O5 layers at the interfacial regions. The Ta and Ta2O5 surfaces exhibited blisters after Heþ implantation and subsequent Dþ implantation worsened the blistering behavior of Ta2O5. Swelling was also pronounced in Ta2O5 where large blisters exhibited an internal nanometer-sized fuzz structure. Transmission electron microscopy revealed an extensive presence of dislocations in the metallic phases after the sequential implantation, while a relatively low density of defects was detected in Ta2O5. This behavior may be partially justified by a shielding effect from the blisters and fuzz structure developed progressively during implantation. The tungsten peaks in the X-ray diffractograms were markedly shifted after Heþ implantation, and even more so after the sequential implantation, which is in agreement with the increased D retention inferred from nuclear reaction analysis. © 2017 Published by Elsevier B.V.
Keywords: Tungsten-tantalum fiber composites Mechanical alloying Helium implantation Deuterium implantation Blistering Fuzz
1. Introduction Refractory metals are currently under intense investigation for applications in nuclear devices due to their resistance to plasma erosion and moderate tritium inventory [1]. The highest melting point and highest sputtering threshold, together with adequate corrosion resistance and highest tensile strength of all metals at elevated temperatures, render tungsten a potentially suitable
* Corresponding author. E-mail address: [email protected] (M. Dias). http://dx.doi.org/10.1016/j.jnucmat.2017.05.007 0022-3115/© 2017 Published by Elsevier B.V.
material for plasma facing and structural components in nuclear fusion reactors [2e5]. However, in this context, the major issue associated with the presently available tungsten grades is their brittleness at low temperatures [6], which is worsened by irradiation [7]. Tantalum, another refractory metal, evidences low neutron activation and high radiation resistance as well as high fracture toughness. However, Ta is a rare commodity and cannot be envisaged for bulk applications. Alloying with Re, Mo or Ta can be used to modify the mechanical behavior of tungsten but limited property variation and precipitation of brittle secondary phases restrict this approach [8e10]. Reinforcement of tungsten matrices with tungsten fibers has also been investigated [11] and the behavior of these
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at room temperature with either a 30 keV Heþ beam and a 15 keV Dþ beam or with a sequence of Heþ and Dþ ion beams with the same energies, employing a setup described elsewhere [18]. The Heþ and Dþ irradiation energies were defined with the SRIM software package [19] in order to obtain similar implantation-depth ranges for the two ionic species. The simulated implantation profile was approximately Gaussian with a maximum buried at a depth of 87 nm for H at 15 kev and of 80 nm for He at 35 keV. Constant Heþ and Dþ fluences of 5 1021 at/m2 were used in all implantations.
composite systems seems, to a large extent, determined by the properties of the drawn tungsten fibers. For instance, potassiumdoped tungsten fibers enhance the strength and ductility of the material up to 2200 K [12]. However, the fabrication process by layer deposition of the tungsten matrices with tungsten fibers revealed weak bonding between the layers which leads to a composite cracking [11]. An alternative strategy to increase the toughness of W-based materials lies in dispersing ductile Ta particles or fibers in a W matrix [13e16]. However, previous results on W-Ta composites indicated that Ta acts as getter for residual oxygen, leading to the formation of Ta oxide [14], where blisters were prone to form upon Heþ implantation [10]. Therefore, the response of these systems to irradiation requires elucidation. In the present study tungsten-tantalum fiber composites were produced by pulse plasma compaction (PPC) at 1773 K. The materials were subsequently implanted, at room temperature, with Heþ (30 keV with a fluence of 5 1021 at/m2) or Dþ (15 keV with a fluence of 5 1021 at/m2), as well as with sequential Heþ and Dþ beams using the same conditions. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and focused ion beam (FIB), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and X-ray diffraction (XRD) were used to characterize the W-Ta composites.
Microstructural observations were performed with a JEOL JSM7001F scanning electron microscope equipped for EDS. The metallographic preparation involved grinding with SiC paper and polishing with 6, 3 and 1 mm diamond suspensions. Polished surfaces were investigated after implantation with secondary electrons (SE) and backscattered electrons (BSE) signals. The FIB experiments were carried out with a Zeiss Auriga Cross Beam dual beam microscope using 30 kV of charged Gaþ ions. For improved visibility of topographic effects the samples were tilted 70 . Sample preparation for TEM involved cutting lamellas in regions of interest and thinning down to a thickness of 50 nm. TEM observations were performed with a Hitachi H8100 microscope.
2. Experimental
2.4. Ion beam analysis
2.1. Composite production W-Ta composites with an atomic proportion of 90/10 were produced by PPC from W powder of 99.9% nominal purity with average particle size of 1 mm (AlfaAesar) and Ta fibers of 99.99% nominal purity with 100 mm of diameter (MaTecK) cut to a length of about 1 cm. Preliminary experiments showed that nanostructuring W results in higher densification, therefore, the W powder was ball milled using a Retsch PM400MA planetary mill operated at 170 rpm for 1 h and subsequently mixed with Ta fibers in a turbula for 1 h. Pulse plasma compaction is based on applying electric current discharges whilst heating and pressing the powders/fibers. In essence, the energy resulting from the discharge of a condenser bank is applied in one shot to the material, which reaches almost instantly temperatures in the sintering range. In contrast to processes like spark plasma sintering or field assisted sintering, the rapid heating rate in PPC allows sintering at relatively low temperatures, avoiding extensive particle boundary melting. Pulse plasma compaction uses pulse current but its distinctive feature is the greater energy delivered in very short pulses achieving power values up to 600 MW [17]. The W-Ta powder mixture was consolidated using a battery of capacitors discharge of PPC unit of GeniCore company - model GC_A_V2L200HV with a graphite dies of 10 mm of diameter and 5 mm of height. A preliminary degassing step was conducted at 873 K for 2 min under a pressure of 15 MPa. After the degassing, the sample was further heated with rate of 200 K/min to reach the required sintering temperature of 1773 K. Consolidation was carried under 5 103 mbar vacuum with a pressure of 100 MPa while a 300 mF capacitor, charged to a voltage of 5.2 kV, was used to discharge 7.5 kJ with a pulse frequency of 2.5 Hz and a pulse period duration of 150 ms for 300 s. After this cycle, the samples were cooled with rate of 300 K/min to room temperature in the same level of vacuum. 2.2. Implantation Polished surfaces of sintered W-Ta composites were implanted
2.3. Microstructural observations
RBS and NRA were carried out with 3Heþ ion beams at 2100,
Fig. 1. SEM/BSE images showing the consolidated microstructure of the W-Ta composites. (a) Ta fibers dispersed in the W matrix. (b) Magnified detail of a fiber crosssection displaying Ta2O5 interlayers at the interfacial regions as well as bordering Ta regions. The images were taken using the backscattered electrons detector (BSE) which is sensible to the atomic number of the elements. Therefore, elements with high Z appear as bright while the others with low Z are dark.
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Fig. 2. SEM/SE images showing Ta2O5/Ta/W interfacial regions implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions.
Fig. 3. SEM/SE images showing microstructures of Ta regions in the W-Ta composites implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions.
1500 and 750 keV to evaluate the distribution profile and amount of D retained in the implanted samples. The D(3He,p)4He nuclear reaction was used to calculate the deuterium content by RBS and NRA [20] and the data was subsequently analyzed with NDF code [21]. 2.5. X-ray diffraction XRD measurements were carried out with grazing geometry [22] using a Bruker D8 AXS diffractometer with Cu Ka1 and Ka2 lines € bel mirror. The ICDD Database [23] was used for phase and a Go identification. The Powder Cell software package [24] was employed to simulate diffractograms for comparison with experimental data. The peak broadening was measure for (110) of tungsten and the values of full width at half maximum (FWHM) are discounted of the instrumental broadening (0.08). 3. Results and discussion Fig. 1 shows the microstructure of the W-Ta composite and the typical longitudinal cross-section of the Ta fibers. The interfacial regions show Ta2O5 interlayers and bordering metallic Ta (see Fig. 1 (b)). The oxide interlayers developed during the thermomechanical treatment due to the high affinity of O towards Ta, which acted as getter for the residual oxygen in the initial W powder. Due to interdiffusion, the interfacial oxide migrated towards the center of the fiber while metallic Ta migrated in the opposite direction. Figs. 2 and 3 present surface images of the W-Ta composites implanted with Heþ and sequentially with Heþ and Dþ ions. After
single Dþ implantation, no changes could be detected in the surface of any of the constituents of the composite materials. Moreover, the W matrix remained essentially unaltered both after single Heþ implantation and after sequential Heþ and Dþ implantations. The height difference between the W matrix and the other phases observed resulted from preferential polishing of the Ta-rich regions during metallographic preparation prior to implantation (see Fig. 1 (b)) and does not represent swelling. In addition, the Ta surfaces presented small blisters of similar size after Heþ implantation and after sequential Heþ and Dþ implantations (Fig. 3 (a) and (b), respectively). No blistering preference has been detected for grain boundaries or other defects. The implantations induced dramatic changes in the Ta2O5 surfaces; blistering and swelling were severe (Figs. 2 and 4 (a) and (b)). These effects were more pronounced after the sequential Heþ and Dþ implantation (Fig. 4 (b)), indicating that Heþ implantation promoted Dþ trapping. Helium implantation is known to induce blistering and plastic deformation in refractory metals [25]. The phenomenon is associated with the high trapping energy of helium atoms in lattice defects of the matrix [26] and is believed to create additional traps for D [24]. A nanometer-sized fuzz was formed inside large blisters at the surface of Ta2O5 both with Heþ and with sequential Heþ and Dþ implantations (see Fig. 4 (c) and (d)). The fuzz structure exhibited a composition similar to the underlying oxide as demonstrated by the X-ray maps shown in Fig. 5. Blistering in Ta2O5 after sequential Heþ and Dþ implantation has been observed previously in W-Ta powder composites consolidated by spark plasma sintering (SPS) [27,28]. Moreover, blistering after
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Heþ implantation was reported for oxides such as Al2O3, MgAl2O4 [29], SrTiO3 and LiNbO3 [30] and LiTaO3 [31]. These studies showed that in a so-called ‘high-temperature regime’ blistering is controlled by diffusion of atomic He, whereas in a ‘low-temperature regime’ blistering is originated when lateral compressive stresses associated with implantation-produced cavities exceed the yield strength [32] which is consistent with a “gas driven” mechanism [33]. The exposure of W to energetic Heþ beams/plasmas is known to induce a multitude of effects [34e43]. In general, blistering occurs for temperatures below 1600 K and beam energy above 20 keV [33,34,44,45], while irradiation at higher temperatures and lower energy result in pits, holes and bubbles [35e37]. Nevertheless, the outcome varies with the fluence employed [33e37]. Namely, helium implantation in tungsten at room temperature, moderate energies (8 keV) and moderate fluences (2 1021 He/m2) has been reported to induce blistering [46,47]. The blistering mechanism at low temperatures [32] is similar to the one described for oxides [30]. Kajita et al. [48] presented a diagram for He-induced bubbles and fuzz as a function of temperature and incident ion energy for fluences in the 1 1026e6 1027 m2 range (see Fig. 6). Under these conditions, fuzz forms for energy between 20 and 100 eV and temperature between 1000 and 2000 K [32]. The formation of fuzz on irradiated surfaces at high temperatures has been reported for several metals [42,47,49e51] in particular for pure Ta irradiated with He at high temperature as low energy [52]. Although the mechanisms behind this behavior are not fully elucidated. Nevertheless, the formation of fuzz at room temperature has not been described before for metals or oxides. Fig. 6 summarizes the known Heþ irradiation effects for W with fluences of 2 1021 and 1.0 1026e6 1027 He/m2 as reported by Kajita et al. [26,44,45] and for Mo with fluence of 1.1 1027 He/m2 [45], together with the results of the current study for W, Ta and Ta2O5 obtained with a fluence of 5 1021 He/m2. The absence of surface effects for W in the present conditions (room temperature and 30 keV) is in agreement with Kajita's diagram. The blistering observed in Ta and Ta2O5 surfaces after Heþ implantation at room temperature and 30 keV is similar, respectively, to the ones observed for W implanted at 8 keV and room temperature [46] and for oxides [31]. Due to the impaired diffusion at room temperature blistering in Ta and Ta2O5 is probably associated with an increasing concentration of interstitial He concentration that led to high He gas pressure in the bubbles. Fuzz formation in W [43] has been reported for higher Heþ fluence and temperature compared to the conditions applied to the W-Ta composites. The fuzz observed inside the blisters in Ta2O5 is suggested to originate from bubbles created by helium implantation, which develop in depth by coalescence. The structure is further extended due to swelling, forming a rod-like fine structure [46]. The fact that blistering was also observed for Ta is believed to be associated with the presence of native oxide. Tantalum oxidation studies showed that the thickness of native Ta2O5 formed on pure tantalum assumes the value of approximately 5 nm [53], which is compatible with the smaller blisters observed. Fig. 7 presents TEM images of regions immediately below the surface for the W matrix, Ta fiber and Ta2O5 phase observed after sequential implantation with Heþ and Dþ in order to study the irradiation defects. The images show distributed dislocations loops in W matrix as well as in Ta regions. However, the Ta2O5 phase exhibits a low defects density when compared to metal phases, only a light and dark layers characteristic of a martensitic transformation [9] was observed. Studies reported in literature for oxides [28,29] show that generally the blistering effect is correlated with high density of defects observed by TEM. The present work
Fig. 4. Ta2O5 blisters observed in W-Ta implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions. (c) Cross section of blister shown in (b) and (d) fuzz structure inside the blister.
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Fig. 5. (a) SEM/SE image of W-Ta implanted with Heþ and Dþ showing the fuzz nanostructure inside a Ta2O5 blister and the corresponding X-ray maps for (b) O and (c) Ta.
Fig. 6. Effects of Heþ irradiation as a function of temperature and incident ion energy for W [44e46], for Mo [45] and the present results for W, Ta and Ta2O5.
Fig. 7. Bright-field TEM images of (a) W matrix, (b) Ta2O5 and (c) Ta fiber observed in W-Ta sample implanted sequentially with Heþ and Dþ ions.
evidences a different behavior for Ta2O5 phase. Moreover the thickness of Ta2O5 layer is higher than the one observed for the published oxides [Fig. 6], which allows inferring that helium atoms after implantation are retained in the Ta2O5 surface causing
blistering, swelling and consequently low density of defects. Fig. 8 (a) shows the 3Heþ RBS spectra at 1500 keV for the implanted W-Ta composites. After Heþ implantation a decrease in the backscattered signal was observed for the material indicating
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Fig. 8. RBS spectra of (a) W-Ta with implanted with single Dþ and Heþ ions and sequential Heþ þ Dþ ions, (b) NRA spectra for W-Ta composites implanted with single Dþ ions, Heþ ions and sequential Heþ þ Dþ ions.
surface modifications (see the inset in Fig. 8 (a)). However, this behavior was absent after single Dþ implantation. The fact that the decrease in the superficial backscattered yield is evident in W-Ta composites implanted with sequential Heþ and Dþ ions and not in the same material implanted with single Dþ ions, allows inferring that this behavior is associated with Heþ implantation, which seems also responsible for the blister formation (Figs. 2 and 3) as was reported before for W-Ta composites consolidated by SPS [10]. Fig. 8 (b) presents also the NRA spectra obtained from the implanted W-Ta composite. The integrated peak intensity shows that Dþ retention in the W-Ta composites is higher after sequential Heþ and Dþ implantation than for single Dþ implantation and must be associated with the defects created by the He implantation. Moreover, D retention in these materials seems to be similar
(24 1019 at/m2) to the one in W-Ta produced by SPS [27] (14 1019 at/m2) both implanted sequently with He and D. However, the D retention in W-Ta implanted with D in the same conditions than those in pure W [27] is two magnitude order higher (for W-Ta is 15 1019 at/m2 and for pure W was 0.6 1019 at/m2) which seems to be associated with the formation of the oxides and seems to increase the retention. Fig. 9 shows the diffractogram of the W-Ta composite implanted with (a) single Dþ ions, (b) single Heþ ions and (c) sequentially Heþ and Dþ ions together with the simulations of pure Ta (d) and W (e) elements. The diffractogram of W-Ta composite implanted with single Dþ ions evidenced W peak broadening, and this effect is more pronounced for the composite implanted with single Heþ ions and with sequentially Heþ and Dþ ions. A small peak of Ta was
Fig. 9. Experimental X-ray diffratogram of the W-Ta composite implanted with (a) single Dþ ions, (b) single Heþ ions, (c) sequential Heþ and Dþ ions, and simulations for (d) Ta with the Wetype structure, (e) W elements. The inset evidences the deconvolution of the major peak for W-Ta composite implanted with sequential Heþ and Dþ ions.
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observed at 2q ¼ 38.5 and no more peaks were detected probably due their small volume fraction of the fibers dispersed in the W matrix. The peak broadening is associated with the defects created by the implanted ions: (i) deuterium implantation creates additional vacancies which can trap D and create additional conditions for deuterium agglomeration in clusters [54], and (ii) He atoms stay in solid solution in W matrix as stable He filled vacancies preventing recombination of SIA (self-interstitial atoms) thus originating considerable defect density and strain in the W lattice as self-interstitial atoms and dislocations [55e57]. The position of the original peak presented in the W simulation and W-Ta implanted with Dþ ions is the same at 2q ¼ 40.3 . Moreover, the full width at half maximum (FWHM) for the W-Ta implanted with Dþ ions corresponds to a 0.19 which seems to be consistent with a W matrix with small deformation. A software fitting with a Lorentz function for the most intense peak for W-Ta implanted with sequential Heþ and Dþ ions is presented in the inset of Fig. 9. The results indicate the existence of two peaks: 2q ¼ 39.8 with FWHM of 0.53 and another the original one at 2q ¼ 40.3 with a FWHM of 0.22. The peak at 2q ¼ 39.8 albeit having a smaller maximum evidences larger integrated intensity which means higher volume fraction of this phase. Moreover, the high value of FWHM (0.53 vs 0.22) indicates a higher lattice strain. The shift in position to 2q ¼ 39.8 of this broader W peak indicates a lattice parameter swelling of 1.1% from the original one. Therefore, the Heþ and Dþ implantation originates considerable defects resulting in lattice distortion manifested as a broadening of the Bragg peaks and lattice expansion/swelling of a significant portion of the material manifested as a partial peak shift. 4. Conclusions W-Ta composites with 10 at.% Ta consolidated using pulse plasma compaction technique were implanted with of Heþ or/with Dþ ion beams and sequentially with Heþ (with a energy of 30 keV) and Dþ (with an energy of 15 keV) using ion beams with fluences of 1021 at/m2. The investigations revealed that the W-Ta composite implanted with single Heþ or sequentially with Heþ and Dþ ions presented blistering in the Ta2O5 and Ta-rich regions. Moreover, the blister cavities in Ta2O5 evidence the formation of a nanometersized fuzz structure which can be associated with the high oxide thinness. Transmission electron microscope observations of a region containing W matrix and one Ta fiber implanted sequentially with Heþ and Dþ indicate the presence of dislocations. Low density of defects was observed in Ta2O5 which indicates that the blistering seems to act as a shield of the surface. The results indicated that Dþ retention increased with Heþ pre-implantation. X-ray diffraction results evidence an increase of lattice strain in W matrix after implantation with single Heþ ions and with sequentially Heþ and Dþ ions, as well as significant lattice swelling with the latter. These effects are caused by self interstitial atoms and He retention in vacancies in the W matrix. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement number 633053. IST activities also received financial ^ncia e a Tecnologia through support from Fundaç~ ao para a Cie projects Pest-OE/SADG/LA0010/2013 and UID/CTM/50025/2013, and FEDER funds through the COMPETE 2020 Programme under the project number POCI-01-0145-FEDER-007688. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support was also received from the Portuguese Science and Technology Foundation under the
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Helium and deuterium irradiation effects in W-Ta composites produced by pulse plasma compaction M. Dias a, *, N. Catarino a, D. Nunes b, E. Fortunato b, I. Nogueira c, M. Rosinki d, J.B. Correia e, P.A. Carvalho a, f, E. Alves a ~o Nuclear, Instituto Superior T Instituto de Plasmas e Fusa ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa, Portugal CENIMAT-I3N, Departamento de Ci^ encia dos Materiais, Faculdade de Ci^ encias e Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal c CEFEMA, Instituto Superior T ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal d GeniCore, Warsaw, Poland e rio Nacional de Energia e Geologia, Estrada do Paço do Lumiar, 1649-038 Lisboa, Portugal LNEG, Laborato f SINTEF Materials and Chemistry, Forskningsveien 1, 0314 Oslo, Norway a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2017 Received in revised form 3 May 2017 Accepted 8 May 2017 Available online 10 May 2017
Tungsten-tantalum composites have been envisaged for first-wall components of nuclear fusion reactors; however, changes in their microstructure are expected from severe irradiation with helium and hydrogenic plasma species. In this study, composites were produced from ball milled W powder mixed with 10 at.% Ta fibers through consolidation by pulse plasma compaction. Implantation was carried out at room temperature with Heþ (30 keV) or Dþ (15 keV) or sequentially with Heþ and Dþ using ion beams with fluences of 5 1021 at/m2. Microstructural changes and deuterium retention in the implanted composites were investigated by scanning electron microscopy, coupled with focused ion beam and energy dispersive X-ray spectroscopy, transmission electron microscopy, X-ray diffraction, Rutherford backscattering spectrometry and nuclear reaction analysis. The composite materials consisted of Ta fibers dispersed in a nanostructured W matrix, with Ta2O5 layers at the interfacial regions. The Ta and Ta2O5 surfaces exhibited blisters after Heþ implantation and subsequent Dþ implantation worsened the blistering behavior of Ta2O5. Swelling was also pronounced in Ta2O5 where large blisters exhibited an internal nanometer-sized fuzz structure. Transmission electron microscopy revealed an extensive presence of dislocations in the metallic phases after the sequential implantation, while a relatively low density of defects was detected in Ta2O5. This behavior may be partially justified by a shielding effect from the blisters and fuzz structure developed progressively during implantation. The tungsten peaks in the X-ray diffractograms were markedly shifted after Heþ implantation, and even more so after the sequential implantation, which is in agreement with the increased D retention inferred from nuclear reaction analysis. © 2017 Published by Elsevier B.V.
Keywords: Tungsten-tantalum fiber composites Mechanical alloying Helium implantation Deuterium implantation Blistering Fuzz
1. Introduction Refractory metals are currently under intense investigation for applications in nuclear devices due to their resistance to plasma erosion and moderate tritium inventory [1]. The highest melting point and highest sputtering threshold, together with adequate corrosion resistance and highest tensile strength of all metals at elevated temperatures, render tungsten a potentially suitable
* Corresponding author. E-mail address: [email protected] (M. Dias). http://dx.doi.org/10.1016/j.jnucmat.2017.05.007 0022-3115/© 2017 Published by Elsevier B.V.
material for plasma facing and structural components in nuclear fusion reactors [2e5]. However, in this context, the major issue associated with the presently available tungsten grades is their brittleness at low temperatures [6], which is worsened by irradiation [7]. Tantalum, another refractory metal, evidences low neutron activation and high radiation resistance as well as high fracture toughness. However, Ta is a rare commodity and cannot be envisaged for bulk applications. Alloying with Re, Mo or Ta can be used to modify the mechanical behavior of tungsten but limited property variation and precipitation of brittle secondary phases restrict this approach [8e10]. Reinforcement of tungsten matrices with tungsten fibers has also been investigated [11] and the behavior of these
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at room temperature with either a 30 keV Heþ beam and a 15 keV Dþ beam or with a sequence of Heþ and Dþ ion beams with the same energies, employing a setup described elsewhere [18]. The Heþ and Dþ irradiation energies were defined with the SRIM software package [19] in order to obtain similar implantation-depth ranges for the two ionic species. The simulated implantation profile was approximately Gaussian with a maximum buried at a depth of 87 nm for H at 15 kev and of 80 nm for He at 35 keV. Constant Heþ and Dþ fluences of 5 1021 at/m2 were used in all implantations.
composite systems seems, to a large extent, determined by the properties of the drawn tungsten fibers. For instance, potassiumdoped tungsten fibers enhance the strength and ductility of the material up to 2200 K [12]. However, the fabrication process by layer deposition of the tungsten matrices with tungsten fibers revealed weak bonding between the layers which leads to a composite cracking [11]. An alternative strategy to increase the toughness of W-based materials lies in dispersing ductile Ta particles or fibers in a W matrix [13e16]. However, previous results on W-Ta composites indicated that Ta acts as getter for residual oxygen, leading to the formation of Ta oxide [14], where blisters were prone to form upon Heþ implantation [10]. Therefore, the response of these systems to irradiation requires elucidation. In the present study tungsten-tantalum fiber composites were produced by pulse plasma compaction (PPC) at 1773 K. The materials were subsequently implanted, at room temperature, with Heþ (30 keV with a fluence of 5 1021 at/m2) or Dþ (15 keV with a fluence of 5 1021 at/m2), as well as with sequential Heþ and Dþ beams using the same conditions. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and focused ion beam (FIB), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and X-ray diffraction (XRD) were used to characterize the W-Ta composites.
Microstructural observations were performed with a JEOL JSM7001F scanning electron microscope equipped for EDS. The metallographic preparation involved grinding with SiC paper and polishing with 6, 3 and 1 mm diamond suspensions. Polished surfaces were investigated after implantation with secondary electrons (SE) and backscattered electrons (BSE) signals. The FIB experiments were carried out with a Zeiss Auriga Cross Beam dual beam microscope using 30 kV of charged Gaþ ions. For improved visibility of topographic effects the samples were tilted 70 . Sample preparation for TEM involved cutting lamellas in regions of interest and thinning down to a thickness of 50 nm. TEM observations were performed with a Hitachi H8100 microscope.
2. Experimental
2.4. Ion beam analysis
2.1. Composite production W-Ta composites with an atomic proportion of 90/10 were produced by PPC from W powder of 99.9% nominal purity with average particle size of 1 mm (AlfaAesar) and Ta fibers of 99.99% nominal purity with 100 mm of diameter (MaTecK) cut to a length of about 1 cm. Preliminary experiments showed that nanostructuring W results in higher densification, therefore, the W powder was ball milled using a Retsch PM400MA planetary mill operated at 170 rpm for 1 h and subsequently mixed with Ta fibers in a turbula for 1 h. Pulse plasma compaction is based on applying electric current discharges whilst heating and pressing the powders/fibers. In essence, the energy resulting from the discharge of a condenser bank is applied in one shot to the material, which reaches almost instantly temperatures in the sintering range. In contrast to processes like spark plasma sintering or field assisted sintering, the rapid heating rate in PPC allows sintering at relatively low temperatures, avoiding extensive particle boundary melting. Pulse plasma compaction uses pulse current but its distinctive feature is the greater energy delivered in very short pulses achieving power values up to 600 MW [17]. The W-Ta powder mixture was consolidated using a battery of capacitors discharge of PPC unit of GeniCore company - model GC_A_V2L200HV with a graphite dies of 10 mm of diameter and 5 mm of height. A preliminary degassing step was conducted at 873 K for 2 min under a pressure of 15 MPa. After the degassing, the sample was further heated with rate of 200 K/min to reach the required sintering temperature of 1773 K. Consolidation was carried under 5 103 mbar vacuum with a pressure of 100 MPa while a 300 mF capacitor, charged to a voltage of 5.2 kV, was used to discharge 7.5 kJ with a pulse frequency of 2.5 Hz and a pulse period duration of 150 ms for 300 s. After this cycle, the samples were cooled with rate of 300 K/min to room temperature in the same level of vacuum. 2.2. Implantation Polished surfaces of sintered W-Ta composites were implanted
2.3. Microstructural observations
RBS and NRA were carried out with 3Heþ ion beams at 2100,
Fig. 1. SEM/BSE images showing the consolidated microstructure of the W-Ta composites. (a) Ta fibers dispersed in the W matrix. (b) Magnified detail of a fiber crosssection displaying Ta2O5 interlayers at the interfacial regions as well as bordering Ta regions. The images were taken using the backscattered electrons detector (BSE) which is sensible to the atomic number of the elements. Therefore, elements with high Z appear as bright while the others with low Z are dark.
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Fig. 2. SEM/SE images showing Ta2O5/Ta/W interfacial regions implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions.
Fig. 3. SEM/SE images showing microstructures of Ta regions in the W-Ta composites implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions.
1500 and 750 keV to evaluate the distribution profile and amount of D retained in the implanted samples. The D(3He,p)4He nuclear reaction was used to calculate the deuterium content by RBS and NRA [20] and the data was subsequently analyzed with NDF code [21]. 2.5. X-ray diffraction XRD measurements were carried out with grazing geometry [22] using a Bruker D8 AXS diffractometer with Cu Ka1 and Ka2 lines € bel mirror. The ICDD Database [23] was used for phase and a Go identification. The Powder Cell software package [24] was employed to simulate diffractograms for comparison with experimental data. The peak broadening was measure for (110) of tungsten and the values of full width at half maximum (FWHM) are discounted of the instrumental broadening (0.08). 3. Results and discussion Fig. 1 shows the microstructure of the W-Ta composite and the typical longitudinal cross-section of the Ta fibers. The interfacial regions show Ta2O5 interlayers and bordering metallic Ta (see Fig. 1 (b)). The oxide interlayers developed during the thermomechanical treatment due to the high affinity of O towards Ta, which acted as getter for the residual oxygen in the initial W powder. Due to interdiffusion, the interfacial oxide migrated towards the center of the fiber while metallic Ta migrated in the opposite direction. Figs. 2 and 3 present surface images of the W-Ta composites implanted with Heþ and sequentially with Heþ and Dþ ions. After
single Dþ implantation, no changes could be detected in the surface of any of the constituents of the composite materials. Moreover, the W matrix remained essentially unaltered both after single Heþ implantation and after sequential Heþ and Dþ implantations. The height difference between the W matrix and the other phases observed resulted from preferential polishing of the Ta-rich regions during metallographic preparation prior to implantation (see Fig. 1 (b)) and does not represent swelling. In addition, the Ta surfaces presented small blisters of similar size after Heþ implantation and after sequential Heþ and Dþ implantations (Fig. 3 (a) and (b), respectively). No blistering preference has been detected for grain boundaries or other defects. The implantations induced dramatic changes in the Ta2O5 surfaces; blistering and swelling were severe (Figs. 2 and 4 (a) and (b)). These effects were more pronounced after the sequential Heþ and Dþ implantation (Fig. 4 (b)), indicating that Heþ implantation promoted Dþ trapping. Helium implantation is known to induce blistering and plastic deformation in refractory metals [25]. The phenomenon is associated with the high trapping energy of helium atoms in lattice defects of the matrix [26] and is believed to create additional traps for D [24]. A nanometer-sized fuzz was formed inside large blisters at the surface of Ta2O5 both with Heþ and with sequential Heþ and Dþ implantations (see Fig. 4 (c) and (d)). The fuzz structure exhibited a composition similar to the underlying oxide as demonstrated by the X-ray maps shown in Fig. 5. Blistering in Ta2O5 after sequential Heþ and Dþ implantation has been observed previously in W-Ta powder composites consolidated by spark plasma sintering (SPS) [27,28]. Moreover, blistering after
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Heþ implantation was reported for oxides such as Al2O3, MgAl2O4 [29], SrTiO3 and LiNbO3 [30] and LiTaO3 [31]. These studies showed that in a so-called ‘high-temperature regime’ blistering is controlled by diffusion of atomic He, whereas in a ‘low-temperature regime’ blistering is originated when lateral compressive stresses associated with implantation-produced cavities exceed the yield strength [32] which is consistent with a “gas driven” mechanism [33]. The exposure of W to energetic Heþ beams/plasmas is known to induce a multitude of effects [34e43]. In general, blistering occurs for temperatures below 1600 K and beam energy above 20 keV [33,34,44,45], while irradiation at higher temperatures and lower energy result in pits, holes and bubbles [35e37]. Nevertheless, the outcome varies with the fluence employed [33e37]. Namely, helium implantation in tungsten at room temperature, moderate energies (8 keV) and moderate fluences (2 1021 He/m2) has been reported to induce blistering [46,47]. The blistering mechanism at low temperatures [32] is similar to the one described for oxides [30]. Kajita et al. [48] presented a diagram for He-induced bubbles and fuzz as a function of temperature and incident ion energy for fluences in the 1 1026e6 1027 m2 range (see Fig. 6). Under these conditions, fuzz forms for energy between 20 and 100 eV and temperature between 1000 and 2000 K [32]. The formation of fuzz on irradiated surfaces at high temperatures has been reported for several metals [42,47,49e51] in particular for pure Ta irradiated with He at high temperature as low energy [52]. Although the mechanisms behind this behavior are not fully elucidated. Nevertheless, the formation of fuzz at room temperature has not been described before for metals or oxides. Fig. 6 summarizes the known Heþ irradiation effects for W with fluences of 2 1021 and 1.0 1026e6 1027 He/m2 as reported by Kajita et al. [26,44,45] and for Mo with fluence of 1.1 1027 He/m2 [45], together with the results of the current study for W, Ta and Ta2O5 obtained with a fluence of 5 1021 He/m2. The absence of surface effects for W in the present conditions (room temperature and 30 keV) is in agreement with Kajita's diagram. The blistering observed in Ta and Ta2O5 surfaces after Heþ implantation at room temperature and 30 keV is similar, respectively, to the ones observed for W implanted at 8 keV and room temperature [46] and for oxides [31]. Due to the impaired diffusion at room temperature blistering in Ta and Ta2O5 is probably associated with an increasing concentration of interstitial He concentration that led to high He gas pressure in the bubbles. Fuzz formation in W [43] has been reported for higher Heþ fluence and temperature compared to the conditions applied to the W-Ta composites. The fuzz observed inside the blisters in Ta2O5 is suggested to originate from bubbles created by helium implantation, which develop in depth by coalescence. The structure is further extended due to swelling, forming a rod-like fine structure [46]. The fact that blistering was also observed for Ta is believed to be associated with the presence of native oxide. Tantalum oxidation studies showed that the thickness of native Ta2O5 formed on pure tantalum assumes the value of approximately 5 nm [53], which is compatible with the smaller blisters observed. Fig. 7 presents TEM images of regions immediately below the surface for the W matrix, Ta fiber and Ta2O5 phase observed after sequential implantation with Heþ and Dþ in order to study the irradiation defects. The images show distributed dislocations loops in W matrix as well as in Ta regions. However, the Ta2O5 phase exhibits a low defects density when compared to metal phases, only a light and dark layers characteristic of a martensitic transformation [9] was observed. Studies reported in literature for oxides [28,29] show that generally the blistering effect is correlated with high density of defects observed by TEM. The present work
Fig. 4. Ta2O5 blisters observed in W-Ta implanted (a) with Heþ and (b) sequentially with Heþ and Dþ ions. (c) Cross section of blister shown in (b) and (d) fuzz structure inside the blister.
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Fig. 5. (a) SEM/SE image of W-Ta implanted with Heþ and Dþ showing the fuzz nanostructure inside a Ta2O5 blister and the corresponding X-ray maps for (b) O and (c) Ta.
Fig. 6. Effects of Heþ irradiation as a function of temperature and incident ion energy for W [44e46], for Mo [45] and the present results for W, Ta and Ta2O5.
Fig. 7. Bright-field TEM images of (a) W matrix, (b) Ta2O5 and (c) Ta fiber observed in W-Ta sample implanted sequentially with Heþ and Dþ ions.
evidences a different behavior for Ta2O5 phase. Moreover the thickness of Ta2O5 layer is higher than the one observed for the published oxides [Fig. 6], which allows inferring that helium atoms after implantation are retained in the Ta2O5 surface causing
blistering, swelling and consequently low density of defects. Fig. 8 (a) shows the 3Heþ RBS spectra at 1500 keV for the implanted W-Ta composites. After Heþ implantation a decrease in the backscattered signal was observed for the material indicating
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Fig. 8. RBS spectra of (a) W-Ta with implanted with single Dþ and Heþ ions and sequential Heþ þ Dþ ions, (b) NRA spectra for W-Ta composites implanted with single Dþ ions, Heþ ions and sequential Heþ þ Dþ ions.
surface modifications (see the inset in Fig. 8 (a)). However, this behavior was absent after single Dþ implantation. The fact that the decrease in the superficial backscattered yield is evident in W-Ta composites implanted with sequential Heþ and Dþ ions and not in the same material implanted with single Dþ ions, allows inferring that this behavior is associated with Heþ implantation, which seems also responsible for the blister formation (Figs. 2 and 3) as was reported before for W-Ta composites consolidated by SPS [10]. Fig. 8 (b) presents also the NRA spectra obtained from the implanted W-Ta composite. The integrated peak intensity shows that Dþ retention in the W-Ta composites is higher after sequential Heþ and Dþ implantation than for single Dþ implantation and must be associated with the defects created by the He implantation. Moreover, D retention in these materials seems to be similar
(24 1019 at/m2) to the one in W-Ta produced by SPS [27] (14 1019 at/m2) both implanted sequently with He and D. However, the D retention in W-Ta implanted with D in the same conditions than those in pure W [27] is two magnitude order higher (for W-Ta is 15 1019 at/m2 and for pure W was 0.6 1019 at/m2) which seems to be associated with the formation of the oxides and seems to increase the retention. Fig. 9 shows the diffractogram of the W-Ta composite implanted with (a) single Dþ ions, (b) single Heþ ions and (c) sequentially Heþ and Dþ ions together with the simulations of pure Ta (d) and W (e) elements. The diffractogram of W-Ta composite implanted with single Dþ ions evidenced W peak broadening, and this effect is more pronounced for the composite implanted with single Heþ ions and with sequentially Heþ and Dþ ions. A small peak of Ta was
Fig. 9. Experimental X-ray diffratogram of the W-Ta composite implanted with (a) single Dþ ions, (b) single Heþ ions, (c) sequential Heþ and Dþ ions, and simulations for (d) Ta with the Wetype structure, (e) W elements. The inset evidences the deconvolution of the major peak for W-Ta composite implanted with sequential Heþ and Dþ ions.
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observed at 2q ¼ 38.5 and no more peaks were detected probably due their small volume fraction of the fibers dispersed in the W matrix. The peak broadening is associated with the defects created by the implanted ions: (i) deuterium implantation creates additional vacancies which can trap D and create additional conditions for deuterium agglomeration in clusters [54], and (ii) He atoms stay in solid solution in W matrix as stable He filled vacancies preventing recombination of SIA (self-interstitial atoms) thus originating considerable defect density and strain in the W lattice as self-interstitial atoms and dislocations [55e57]. The position of the original peak presented in the W simulation and W-Ta implanted with Dþ ions is the same at 2q ¼ 40.3 . Moreover, the full width at half maximum (FWHM) for the W-Ta implanted with Dþ ions corresponds to a 0.19 which seems to be consistent with a W matrix with small deformation. A software fitting with a Lorentz function for the most intense peak for W-Ta implanted with sequential Heþ and Dþ ions is presented in the inset of Fig. 9. The results indicate the existence of two peaks: 2q ¼ 39.8 with FWHM of 0.53 and another the original one at 2q ¼ 40.3 with a FWHM of 0.22. The peak at 2q ¼ 39.8 albeit having a smaller maximum evidences larger integrated intensity which means higher volume fraction of this phase. Moreover, the high value of FWHM (0.53 vs 0.22) indicates a higher lattice strain. The shift in position to 2q ¼ 39.8 of this broader W peak indicates a lattice parameter swelling of 1.1% from the original one. Therefore, the Heþ and Dþ implantation originates considerable defects resulting in lattice distortion manifested as a broadening of the Bragg peaks and lattice expansion/swelling of a significant portion of the material manifested as a partial peak shift. 4. Conclusions W-Ta composites with 10 at.% Ta consolidated using pulse plasma compaction technique were implanted with of Heþ or/with Dþ ion beams and sequentially with Heþ (with a energy of 30 keV) and Dþ (with an energy of 15 keV) using ion beams with fluences of 1021 at/m2. The investigations revealed that the W-Ta composite implanted with single Heþ or sequentially with Heþ and Dþ ions presented blistering in the Ta2O5 and Ta-rich regions. Moreover, the blister cavities in Ta2O5 evidence the formation of a nanometersized fuzz structure which can be associated with the high oxide thinness. Transmission electron microscope observations of a region containing W matrix and one Ta fiber implanted sequentially with Heþ and Dþ indicate the presence of dislocations. Low density of defects was observed in Ta2O5 which indicates that the blistering seems to act as a shield of the surface. The results indicated that Dþ retention increased with Heþ pre-implantation. X-ray diffraction results evidence an increase of lattice strain in W matrix after implantation with single Heþ ions and with sequentially Heþ and Dþ ions, as well as significant lattice swelling with the latter. These effects are caused by self interstitial atoms and He retention in vacancies in the W matrix. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement number 633053. IST activities also received financial ^ncia e a Tecnologia through support from Fundaç~ ao para a Cie projects Pest-OE/SADG/LA0010/2013 and UID/CTM/50025/2013, and FEDER funds through the COMPETE 2020 Programme under the project number POCI-01-0145-FEDER-007688. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Financial support was also received from the Portuguese Science and Technology Foundation under the
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