H. Kurishita

M. Akiba

N. Noda

N. Yoshida

T. Tanabe

National Institute for Fusion Science (NIFS-Repository)

§ 8. Development and Synthetic Evaluationof High-Z Plasma Facing MaterialsKurishita, H. (IMR, Tohoku Univ.),Noda, N., Yoshida, N. (RIAM, Kyushu Univ.),Tanabe, T. (CIRSE, Nagoya Univ.),Akiba, M. (JAERI)High Z materials, especially tungsten (W) and itsalloys, are very promising for use as PPM and PFC.However, it is known that they exhibit significantembrittlment (low temperature embrittlement,recrystallization embrittlement, radiation embrittlementand helium embrittlement) and the data on a variety ofproperties required for PMCIPFC are very limited.This project is aiming at developing advanced (W) alloyswith improved resistance to such embrittlement and atevaluating the properties, together with baselineproperties, for several commercially available W alloysto construct their data base.i) W alloy developmentIn order to improve the resistance to embrittlement,tungsten alloys ofW-O.3wt%TiC with the microstructureof fme grains (,....,1 Jll11 grain size) and fmely dispersedparticles of transition metal carbide with several nmdiameter were fabricated by mechanical alloying (MA)with planetary ball mill and vacuum hot pressing (VHP)and HIP, followed by hot forging and rolling. Thedeveloped alloys were subjected to impact three-pointbending and static tensile tests, X ray diffraction (XRD)analysis and TEM observations. As a result, the alloysshowed much improved ductility compared withcommercially available pure tungsten, but considerabledifference in ductility was recognized between thedeveloped alloys. It was considered that suchdifference may be attributed to three issues. The first isthat brittle W2C precipitates are formed during VHP byreaction ofW with WC impurity contaminated from potsand balls made of WC/Co used for MA and have thedetrimental effect of promoting the embrittlement, evenwhen they are very fme and undetectable by XRD. Thesecond is that there often occurs heterogeneity in grainsize and particle distribution. This heterogeneity maycome from the heterogeneity in MA treated powder,which is closely related to the use of planetary ball mill.The third is the occurrence of decarburization during212VHP due to reaction of carbon with oxygen impuritycontained in power, resulting in significant loss ofcarbon content necessary for carbide formation. Inorder to solve these issues, the followings are beingcarried out. To produce MA treated powder withoutWC/Co impurity and any microstructural heterogeneity,a newly developed 3MPDA (three mutuallyperpendicular directions agitative ball mill) with potsand balls of TZM was used and. the optimum MAcondition was established. To eliminatedecarburization, HIP with a metal capsule was applied to3MPDA-ball milled powder using two-step sintering at1350 and 1950 OC for 3 h. As a result, it wasrecognized that there is no appreciable microstructuralheterogeneity, no appreciable loss of carbon content. Avery high density of transition metal carbide precipitatesof 7 nm in average diameter.was observed.In addition to the MA-HIP method, another methodof the recrystallization control by multi-step internalnitriding treatments is being applied to achieve themicrostructures of fme grains and fmely dispersedparticles ofTiN.ii) Synthetic EvaluationIn order to clarify how the properties needed forPFMlPFC depend on the microstructure of tungstenmaterial, two kinds ofpure-tungsten (99.95%) specimenswith different microstructures, i.e., stress relieved andrecrystallized specimens, were subjected to syntheticevaluation of hydrogen and helium retention, blistering,high heat load, .erosion, irradiation damage, reaction ofcarbon with tungsten, fracture toughness and hightemperature creep, etc.It was found that the effect of the microstructure isdifferent, depending on each of the properties. Forexample, the hydrogen and helium retention, blistering,erosion and reaction of carbon layer with tungsten weresignificantly suppressed in the recrystallized specimens,indicating that dislocations may give the detrimentaleffect. On the other hand, high heat load properties, theresistance to irradiation damage and mechanicalproperties such as fracture toughness were better in thestress relieved specimens than in the recrystallized ones.These results tell us that one should choose the differentmicrostructure, depending on the location, surface layerand interior of PFMlPFC. A systematic study on therelationship between the microstructure and each of theproperties needed for PFMlPFC should be performedand are now in progress.

Development and Synthetic Evaluation of High-Z Plasma Facing Materials

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