Balden, M.

Jacob, W.

Manhard, A.

Schmid, K.

English

MPG.PuRe

Influence of the microstructure on the deuterium retention in tungsten

Influence of the Microstructure on theDeuterium Retention in TungstenA. Manhard ∗,a K. Schmid, a M. Balden a W. Jacob aaMax-Planck-Institut fu¨r Plasmaphysik, Boltzmannstraße 2, D-85748 Garching b.Mu¨nchen, GermanyAbstractThe microstructure of tungsten samples is systematically modified by recrystal-lization to investigate the structure dependence of deuterium (D) retention. After athorough characterization by scanning and transmission electron microscopy, non-recrystallized samples and samples recrystallized by different degrees are loadedwith deuterium in a low-temperature plasma device. The deuterium inventory ismeasured by nuclear reaction analysis and thermal desorption spectroscopy. Thepost implantation surface morphology is investigated by scanning electron, opticaland atomic force microscopy. The modification by recrystallization allows a widevariation in the crystallite size and has a strong impact on the measured D retention.Both the total amount and the binding state of the retained D are changed.Key words:PACS: 28.52.Fa, 52.40.Hf, 61.72.-y, 61.72.Ff1 IntroductionDue to its favorable physical properties, such as low erosion yield and highmelting temperature, tungsten (W) is foreseen as a plasma-facing material infusion devices, such as ITER [1]. In these machines the surface material isexposed to large fluxes and fluences of hydrogen isotope ions and energeticneutrals. The hydrogen isotope retention in tungsten is generally consideredto be small. Despite substantial efforts to quantify hydrogen retention in tung-sten the underlying mechanisms are not yet fully understood [2,3]. In partic-ular, the influence of the microstructure can be significant. Therefore, it is∗ corresponding authorEmail address: armin.manhard@ipp.mpg.de (A. Manhard).Preprint submitted to Elsevier 4 January 2012necessary to perform dedicated experiments on tungsten samples with a sys-tematic modification and characterization of the microstructure. For this pur-pose, rolled polycrystalline tungsten samples of the same base material, froma single manufacturing batch, were recrystallized to produce specimens withdifferent grain structures. The grain structure of the non-recrystallized andrecrystallized samples was analyzed by scanning electron microscopy (SEM).The dislocation density of the non-recrystallized and recrystallized materialwas investigated with scanning transmission electron microscopy (STEM). Af-ter characterization the samples were implanted with deuterium in a fullyquantified low-temperature deuterium plasma with D+3 as the dominant ionspecies. After implantation the D depth profiles in the near surface region (upto ≈ 7 µm) were measured by nuclear reaction analysis (NRA). After NRA,the binding states of the retained D were investigated by Thermal DesorptionSpectroscopy (TDS). Great care was taken during transfer and storage of thesample between implantation and the NRA and TDS experiments. Air expo-sure was avoided where possible to avoid excessive oxidation of the sample.Before and after TDS, the surface morphology of the samples was investi-gated by optical microscopy. The optical microscopy results were extended byatomic force microscopy and SEM together with focused ion beam techniquesfor selected details.This article will first describe the sample preparation and how the structurewas modified by recrystallization. Then the D implantation setup and the ap-plied analysis methods, NRA, TDS and microscopy will be described. In theresults section the paper will focus on the observed modifications of the Wmicrostructure and how they relate to the observed variations in the D depthprofiles from NRA and binding states from TDS. Also the formation of blistersduring D implantation and their disappearance during degassing of D in theTDS measurements will be discussed.2 Experimental procedureThe base material for all samples was polycrystalline tungsten 1 with a guar-anteed purity of 99.97 wt.% rolled to a thickness of 0.8 mm. All samples werecut from sheet metal (thickness 0.8 mm) from a single sintering batch to en-sure the same initial microstructure and chemical composition. The sampleswere mechanically polished to a mirror-like finish and cleaned in an ultrasonicbath. Then part of the samples were recrystallized at different temperaturesin a graphite furnace with a protective gas atmosphere (Ar for up to 1700 K,He for higher temperatures). They were wrapped in a tungsten foil envelopeto prevent carbide formation on the surface. The recrystallization tempera-tures ranged from 1700 to 2000 K. The holding time was of the order of 20 to1 manufactured by PLANSEE Metall GmbH, 6600 Reutte, Austria260 minutes and thus always at least twice the time necessary for recrystalliza-tion as reported in the literature [4]. Repeated baking at the same temperatureshowed that the influence of the holding time on the average grain structurewas small. Ostwald ripening was occasionally observed, but can be consid-ered a minor effect on these timescales. SEM analysis after recrystallizationshowed strong formation of small carbide grains on the outside of the envelope,but none on the sample itself. The non-recrystallized samples were degassedfor 1 hour at 1200 K which is well below the recrystallization threshold, butreduces stresses in the samples and removes hydrogen.After polishing and, for part of the samples, recrystallization, the grain struc-ture of the samples was analyzed by SEM. High grain orientation contrast wasachieved by using the annular semiconductor backscattered electron detector(BSE). In order to probe the dislocation density, TEM samples from the non-recrystallized and the recrystallized material were prepared by electrochemicalthinning. They were then analyzed with an electron energy of 30 keV in a highresolution SEM which is also equipped with a STEM detector.The samples were implanted with deuterium ions in a quantified plasma source[5] which delivers mostly D+3 ions. The implantation was performed at ≈ 300 Kwith a sample bias of -100 V DC. The sample bias together with the plasmapotential produces ions with an energy of about 115 eV, corresponding to amean energy of 38 eV per deuteron. The deuteron flux was 9×1019 D m−2 s−1.The non-recrystallized samples were exposed to different fluences between 3×1023 and 6 × 1024 D m−2. The samples recrystallized at 1700 and 2000 Kwere implanted together with the non-recrystallized samples during the sameplasma exposure at a fluence of 6× 1024 D m−2.Previous experiments [6] have shown that prolonged exposure of implantedtungsten samples to ambient air can lead to a large fraction of the deuteriumbeing released during TDS not as D2 molecules, but as heavy water (HDO,D2O). This is attributed to oxide formation on the sample surface during airexposure. The reaction of desorbing deuterium with the surface oxide duringTDS analysis then leads to the formation of heavy water. This heavy waterfraction is very hard to quantify. In order to suppress surface oxidation, airexposure of the samples was kept to a minimum. After implantation, thesamples were stored in a vacuum exsiccator whenever they were not neededfor analysis.To determine the D amount and depth profile in the near surface region (upto ≈ 7 µm) the D(3He,p)α nuclear reaction was used. By using energies of theprimary 3He beam of 690, 1500 and 4000 keV different depth regions can beprobed for D due to the resonant nature of the cross section for this particularnuclear reaction. Based on the shape and intensity of the proton spectrum foreach 3He beam energy the depth distribution of D in sample can be determined3as described in [7].The TDS measurements were performed in a dedicated experimental setup lo-cated in a low D-background UHV chamber. The sample is heated by electronbombardment and the temperature of the sample is measured by a K-typethermocouple attached to the target. The sample was heated with a linearramp of 1.4 K/s to 1200 K. During heating, HD (mass 3), D2 (mass 4), HDO(mass 19), and D2O/CD4 (mass 20) molecules were recorded by a quadruplemass spectrometer (Pfeiffer QMG700). The QMS signal was not calibratedto absolute desorption fluxes. Nonetheless, the desorption spectra and totaldesorbed D amount can still be compared between samples because identicalheating ramps were used.The surface morphology of all implanted samples was investigated by opticalmicroscopy with differential interference contrast (DIC) before and after TDS.For one of the non-recrystallized samples, markers were applied to the samplesurface with the focused ion beam (FIB) of a dual beam SEM/FIB, allowingto investigate exactly the same spot before and after TDS. The dual beammicroscope was also used to probe individual blisters that were found on thenon-recrystallized samples.3 Results and discussionThe analysis of the grain structure by SEM with BSE contrast showed that thegrains on the non-recrystallized material were small with typical dimensionsranging from less than 1 µm up to about 2 µm (Fig. 1a). Many grains alsoappeared distorted and were difficult to distinguish. The degassing at 1200 Kdid not cause a detectable change of the sample microstructure.After recrystallization at 1700 K the grain structure changed significantly. Nowmany small (approximately 1 to 5 µm) grains were present next to few larger(approximately 10 to 20 µm) grains (Fig. 1b). Note that even the smaller grainsare now very clearly visible in the SEM picture and do not appear distortedanymore. Therefore, the sample can be considered fully recrystallized. Afterrecrystallization at 2000 K the grain size distribution has changed further.Typical grain dimensions are now between 10 and 50 µm (see Fig. 1c), andthe regions with many small grains, which were present after recrystallizationat 1700 K, are no longer present. The sample now comprises much less, butsignificantly larger grains.The STEM image of the non-recrystallized sample shows a dense and clearlyvisible network of dislocations distributed throughout the whole grain (seeFig. 2a). The images of the recrystallized samples (Fig. 2b and 2c) show only4very few, faint and often blurred features inside the grains. These are likelydue to small dust flakes or chemical residue on the sample surface. Only thegrain boundaries are clearly visible.The deuterium depth distributions determined from NRA for the non-recrystallized material show a high deuterium concentration in a very narrowregion close to the sample surface and a small secondary peak at ≈1 µm (i.e.far beyond the implantation range of several nm). Deeper into the bulk of thesample, the deuterium content decays exponentially as can be seen in Fig. 3.NRA yields an increase of the total retained deuterium amount with fluenceas shown in Fig. 4. Comparison with the TDS results (see below) shows thesame relative increase of the deuterium inventory.NRA depth profiling of the recrystallized material revealed the same peakingof the deuterium concentration near the surface as the non-recrystallized ma-terial. The exponential decay of the D concentration at depths well beyond1 µm was also observed. However, in the intermediate region up to 1 µm,significant changes in the depth profile were observed as Fig. 3 illustrates. Inparticular the sample recrystallized at 2000 K showed a considerably lower Dconcentration in this range, but a very clear secondary peak at 1 µm. Thetotal amount of deuterium retained in the recrystallized samples is also shownin Fig. 4. The total deuterium inventory of the sample recrystallized at 1700 Kclosely matched that of the non-recrystallized sample implanted to the samefluence, while for the one recrystallized at 2000 K the D inventory was reducedby approximately a factor of 2. The same relative change was also found byTDS.TDS showed two D2 release peaks at 520 and 680 K which are typical for non-recrystallized tungsten [2]. These release peaks were only rudimentary for thelowest implantation fluence of 3×1023 D m−2. For increasing fluence, the 680 Kpeak developed first. At 6× 1024 D m−2, both peaks were clearly identifiableas can be seen in Fig. 5. For the sample recrystallized at 1700 K, only a singlebroad release peak at approximately 600 K was observed. For the materialrecrystallized at 2000 K, the release peak was roughly at the same position.However, it was considerably lower and narrower, so that the total deuteriumretention was strongly reduced compared to the other samples exposed to thesame fluence.Optical microscopy with DIC indicated strong blistering of the non-recrystallized material already after implantation with 3 × 1023 D m−2. Thenumber and average size of blisters increased with fluence. Most blistershave an roughly circular circumference. The diameter of the blisters is upto ≈ 20 µm. Atomic force microscopy revealed that the blisters have a heightof up to ≈ 200 nm. It was discovered that a large fraction of the blisters disap-peared after TDS. Fig. 6 shows this for the sample exposed at 3×1024 D m−2.5On this sample, T-shaped markers were cut with the FIB for orientation beforeTDS. This observation points towards an at least partial elastic deformationdue to a high pressure D2 gas filling of subsurface cavities. This assumption isfurther supported by observations with the dual beam SEM/FIB. Small holeswere drilled into the lid of several blisters by the FIB. In regular intervals,the drilling was interrupted in order to image the blister with the SEM. Af-ter sufficiently long drilling time, all investigated blisters flattened completely.Cross-sectioning of punctured and collapsed blisters with the FIB and subse-quent imaging with the SEM revealed a thin crack system running along grainboundaries at a depth of approximately 1 µm. On the surface of the recrystal-lized samples no significant morphology changes, in particular, no blisteringwas observed after plasma exposure.4 ConclusionsThis article reports the investigation of the dependence of the deuterium reten-tion in tungsten on the tungsten microstructure. The crystallite size was variedin a controlled fashion by high temperature recrystallization and subsequentlycharacterized by SEM and STEM microscopy. The STEM investigation alsorevealed that recrystallization essentially removed most of the dislocations. In-vestigating the binding states and depth distributions of D implanted into thesamples by TDS and NRA, respectively, revealed that recrystallization had astrong impact on both. In addition, the total fraction of retained D was re-duced in the sample recrystallized at 2000 K. In the non-recrystallized samplesstrong blistering was observed after implantation of low-energy deuterons at asample temperature of approximately 300 K. Optical microscopy investigationof these blistered samples before and after TDS showed that most of the blis-ters had disappeared hinting towards elastically deformed gas filled cavitiesas the dominant blister formation process in these samples. No blistering wasobserved on the recrystallized samples. In bulk tungsten, D is mainly trappedas an interstitial at crystal defects like dislocations and grain boundaries sincethe solubility of D in a perfect W lattice is extremely low. The comparisonof non-recrystallized and recrystallized W clearly shows a correlation betweenthe D inventory and the density of these defects. In large cavities, i.e. blisters,however, D is most likely trapped as D2 gas under high pressure, as the disap-pearance of the blisters after TDS or after puncturing the blister with the FIBsuggests. This conclusion is also supported by the by the results presented in[8].6References[1] R. Parker, G. Janeschitz, H. D. Pacher, D. Post, et al., Plasma–wall interactionsin ITER, J. Nucl. Mater. 241-243 (1997) 1.[2] O. V. Ogorodnikova, J. Roth, M. Mayer, Ion–driven deuterium retention intungsten, J. Appl. Phys. 103 (2008) 034902.[3] M. Poon, A. A. Haasz, J. W. Davis, Modelling deuterium release during thermaldesorption of D+–irradiated tungsten, J. Nucl. Mat. 374 (2008) 390.[4] W. Espe, Werkstoffkunde der Hochvakuumtechnik, Vol. 1, VEB Deutscher Verlagder Wissenschaften, Berlin, 1960.[5] A. Manhard, T. Schwarz-Selinger, W. Jacob, Quantifcation of the deuterium ionfluxes from a plasma source, Plasma Sources Sci. Technol. 20 (2011) 015010.[6] K. Moshkunov, K. Schmid, M. Mayer, V. Kurnaev, Y. Gasparyan, Air exposureand sample storage time influence on hydrogen release from tungsten, J. Nucl.Mater. 404 (3) (2010) 174–177.[7] M. Mayer, E. Gauthier, K. Sugiyama, U. von Toussaint, Quantitative depthprofiling of deuterium up to very large depths, Nucl. Inst. Meth. B 267 (2009)506.[8] V. K. Alimov, J. Roth, R. A. Causey, D. A. Komarov, C. Linsmeier, A. Wiltner,F. Kost, S. Lindig, Deuterium retention in tungsten exposed to low-energy, high-flux clean and carbon-seeded deuterium plasmas, J. Nucl. Mat. 375 (2008) 192.7Figure captionsFig. 1. Backscatter SEM images of W samples: a) non-recrystallized, b)1700 K, c) 2000 K. All images were taken at the same magnification (1000×).Fig. 2. Bright field STEM images of W samples: a) non-recrystallized, b)1700 K, c) 2000 K. All images were taken at the same magnification (35000×).Fig. 3. NRA depth profiles of D retained in non-recrystallized and recrystal-lized tungsten samples after bombardment with 38 eV/D at a sample temper-ature of 300 K.Fig. 4. Total amount of D retained in non-recrystallized and recrystallizedtungsten samples after exposure to a deuterium plasma with 38 eV/D at asample temperature of 300 K as determined by NRA.Fig. 5. D2 release spectra from non-recrystallized and recrystallized tungstensamples exposed to a fluence of 6 × 1024 D m−2 with 38 eV/D at a sampletemperature of 300 K. The heating rate was 1.4 K/s for all samples.Fig. 6. Surface of non-recrystallized tungsten sample after implantation (a)and after TDS (b). T-shaped markers were applied to the sample surface byFIB.Figures(a) (c)(b)20 mFig. 1. Backscatter SEM images of W samples: a) non-recrystallized, b) 1700 K, c)2000 K. All images were taken at the same magnification (1000×).8(a) (c)(b)0.5 mFig. 2. Bright field STEM images of W samples: a) non-recrystallized, b) 1700 K,c) 2000 K. All images were taken at the same magnification (35000×).0 1 2 3 4 5 61 0- 51 0 - 41 0 - 31 0 - 2 D concentration (at. fr.)	Fig. 3. NRA depth profiles of D retained in non-recrystallized and recrystallizedtungsten samples after bombardment with 38 eV/D at a sample temperature of 300K.1 0 2 3 1 0 2 4 1 0 2 501 02 03 0r e c r y s t a l l i z e d( 2 0 0 0  K )r e c r y s t a l l i z e d( 1 7 0 0  K )Total D amount (1019  D / m2 )F l u e n c e  ( D  /  m 2 )n o n - r e c r y s t a l l i z e dFig. 4. Total amount of D retained in non-recrystallized and recrystallized tungstensamples after exposure to a deuterium plasma with 38 eV/D at a sample tempera-ture of 300 K as determined by NRA.93 0 0 6 0 0 9 0 0 1 2 0 001234 5 2 0  K 6 8 0  KQMS D2 signal (arb. units)T e m p e r a t u r e  ( K ) n o n - r e c r y s t a l l i z e d r e c r y s t a l l i z e d  ( 1 7 0 0  K ) r e c r y s t a l l i z e d  ( 2 0 0 0  K )6 0 0  KFig. 5. D2 release spectra from non-recrystallized and recrystallized tungsten sam-ples exposed to a fluence of 6× 1024 D m−2 with 38 eV/D at a sample temperatureof 300 K. The heating rate was 1.4 K/s for all samples.Fig. 6. Surface of non-recrystallized tungsten sample after implantation (a) andafter TDS (b). T-shaped markers were applied to the sample surface by FIB.10

Influence of the microstructure on the deuterium retention in tungsten

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