Dommelen, JAW Hans  van

Geers, MGD Marc

Shah, Varun

van Dommelen, J.A.W. (Hans)

Geers, Marc G.D.

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 Experimental characterization of tungsten monoblocksexposed to pulsed high heat loadsCitation for published version (APA):Shah, V., van Dommelen, J. A. W., & Geers, M. G. D. (2019). Experimental characterization of tungstenmonoblocks exposed to pulsed high heat loads. Poster session presented at Mate Poster Award 2019: 24thannual poster contest, Eindhoven, Netherlands.Document status and date:Published: 01/01/2019Document Version:Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the version of the article upon submission and before peer-review. There can beimportant differences between the submitted version and the official published version of record. Peopleinterested in the research are advised to contact the author for the final version of the publication, or visit theDOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and pagenumbers.Link to publicationGeneral rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            • Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            • You may not further distribute the material or use it for any profit-making activity or commercial gain            • You may freely distribute the URL identifying the publication in the public portal.If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, pleasefollow below link for the End User Agreement:www.tue.nl/taverneTake down policyIf you believe that this document breaches copyright please contact us at:openaccess@tue.nlproviding details and we will investigate your claim.Download date: 08. Jul. 2024DEPARTMENT OF MECHANICAL ENGINEERING – MECHANICS OF MATERIALSIn future tokamak based nuclear reactors (such as ITER), the divertor performs the vital function of extracting heat and ions from the plasma. Furthermore, the divertor designfor ITER consists of tungsten (W) monoblocks, equipped with a CuCrZr heat sink. Under normal operating conditions, the presence of pulsed high heat flux (HHF) loads of 10 to20 MWm-2 will result in a steep temperature gradient along the monoblock depth, ultimately evolving the bulk microstructure due to recrystallization and grain growth, inaddition to degradation of the surface via melting, roughening, erosion and crack nucleation. These microstructural changes will adversely influence the thermal fatigue lifetimeof the monoblocks, and are a major concern for the reactor lifetime. Thus, an in depth understanding of the damage mechanisms under HHF loading is necessary. As a first step,a thorough macro to micro scale characterization of the W monoblocks exposed to several thousand cycles of pulsed HHF loads is performed in this work.1. IntroductionSpatially dependent temperature profileFigure 1: Tokamak based concept of a fusion reactor along with the divertor component (a and b), designed based on the W monoblock geometry with a CuCrZr heat sink. The images in (c) and (d) respectively show the isometric andthe front view of the monoblock geometry. The temperature gradient along the monoblock depth due to the HHF load of 20 MWm-2 (highlighted in d) is depicted in (e).(a) JET reactor (Source: EUROfusion) 2. MethodPost exposure characterization:• Roughness: Interferometry.• Microstructure:1) Light microscopy.2) EBSD.• Hardness: Micro-indentation.HHF exposure:• Electron beam exposure at ITER divertor testing facility (IDTF), Russia.• Loading scheme: 10 MWm-2 (5000 cycles) +  20 MWm-2 (1000 cycles). Figure 2: The investigated cross-sections (W-H and T-H)following HHF exposure of W monoblock.Temperaturedependentpartitioning 3.2 Results: Microstructure characterization• Recrystallized grain structure up to depth levels of 5.5 mm.• Depth greater than 5.5 mm: Initial grain structure (deformed state).• Abnormal grain growth near surface, due to high temperature. • Surface to bulk: Transition from low angle to high angle (≥ 15o) type grain boundaries(b)<1010° 2020° 3030° 4040° 5050° 60>60Misorientation angle [±]010203040506070Frequency[%]Random> 2000±C1750±C° 2000±C1600±C° 1750±C1450±C° 1600±C1100±C° 1450±CFigure 5: Microstructural characterization of the HHF exposed monoblock (T-H plane), (a) optical micrograph (b)Height direction IPF map of the recrystallized region (c) Temperature/depth dependent grain boundary characterdistribution corresponding to the map shown in (b).(c)(a)5.5 mmEBSD mapping0 1.5 3 4.5 6 7.5 9X° distance [mm]01.534.567.59Y°distance[mm]Surface roughness (Ra) ! 41.86 µm°150°100°50050100150200250Height[µm](b) Divertor3.1 Results: Macrostructure characterization• Transition in surface reflectivity (red lines) indicatingthe extent of the HHF assisted microstructuralchanges due to recrystallization and graingrowth (Figure 3).• Substantial barreling of the monoblockgeometry due to high temperature andinternal stresses (Figure 4a).• No macro/micro-crack formation.Figure 3: The HHF assisted change in the reflectivityof the top 5–5.5 mm of the monoblock.Figure 4: Macrostructural changes in the monoblock geometry following HHF exposure with (a) a significant barrelingof the geometry, and (b) the surface topography with a mean roughness of 41.8 µm as compared to 0.72 µmin the unexposed state (not shown here).(a) (b)3.3 Results: Structure – Property• The hardness varies over the depth due to temperature variations. • Significant loss of hardness in top regionfollowing HHF exposure.• Two stage hardness profile:1) Recovery and partial recrystallization (T: 920-1100 ℃).2) Full recrystallization (T > 1100 ℃).(c) W monoblocksFigure 6: Temperature/depth dependent hardnessprofile in the HHF exposed W monoblock.4. ConclusionSubstantial roughening of the monoblock surface was observed along with barreling ofthe monoblock geometry following the HHF exposure. Additionally, significant changes inthe grain structure occurred up to 5.5 mm along the monoblock depth due torecrystallization. However, no recrystallization assisted macro-crack formation wasobserved. The inhibition of the macro-crack formation can be attributed to the higherfraction of low angle grain boundaries near the surface, thereby enhancing the straincompatibility between the grains and aiding in ductile behavior of the recrystallized W.(d) (e)Experimental characterization of tungsten monoblocks exposedto pulsed high heat loadsV.D. Shah, J.A.W. van Dommelen and M.G.D. GeersV.Shah1@tue.nl

Experimental characterization of tungsten monoblocks exposed to pulsed high heat loads

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