Irradiation behaviour of Precipitation Hardened Ni-base Super-alloys with EHP Grade under Multi-ion Irradiation
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Irradiation behaviour of Precipitation Hardened Ni-base Super-alloys with EHP Grade under Multi-ion Irradiation Gwang-Ho Kim, Kiyoyuki Shiba, Tomotsugu Sawai, Ikuo Ioka, Kiyoshi Kiuchi Japan Atomic Energy Agency, Tokai, Naka, Ibaraki 319-1195, Japan ABSTRACT The irradiation behaviour in two different precipitation hardening types of Ni-base alloys with the ultra high purity grade (EHP), namely, the γ’ type and G phase type was investigated by multi-ion beam techniques simulated to the irradiation conditions in fuel cladding tubes used in sodium cooled FBRs. Single ion-beam irradiation tests were conducted up to 90 dpa (by Fe3+ or Ni3+ ) at 673 K. Triple ion-beam irradiation tests were conducted up to 90 dpa (by Ni3+, 90 appmHe and 1350 appmH) at 823K. The irradiation behaviour was examined by and the microscopic observation by TEM to the distribution of dislocations, cavities and voids. The behaviour was compared with those of PNC316. The dominating irradiation defects in EHP(γ’) alloy at 673 K by single ion-beam are Frank loops, perfect unfaulted loops and line dislocations. Whereas, those of EHP(WSi) alloy are the irradiation-induced Ȗ’ (Ni3Si) precipitates along {111} planes. Those dominating defect structures at 823 K by triple ion-beam are classified as followings, bimodal distributions in EHP(γ’), bubbles in EHP(WSi) and voids in PNC316. From those results, the excellent irradiation properties of EHP(WSi) alloy is clarified as the inhibition effects of secondary irradiation defects. INTRODUCTION The γ’-precipitation-strengthened Nimonic PE16 alloy has been studied extensively for fast-breeder reactor core application [1-6]. Because it was shown an excellent performance with respect to swelling during high-dose neutron irradiation and in ion irradiation test. However, in the Ni-base alloys which are mostly the ordered γ’ strengthened alloys, such as Nimonic PE-16, it was found that the Ni-base alloys exhibit the ductility loss due to irradiation [2,3]. The reason of the ductility loss was believed to be attributed to matrix hardening by radiation-induced point defect clusters [2] and radiation-induced solute segregation[2,5,6]. Generally, the result of radiation-induced solute segregation will not only cause the re-distribution of the matrix strengthener but also cause the formation of unexpected precipitates at defect sinks. In the present study, therefore, we approached with two points of view to improve the
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irradiation resistance of Ni-base alloys. The first is removing the M6C former elements and other impurities by the ultra high purity technology (EHP) to suppress the formation of unexpected precipitates. In EHP Ni-base alloy, impurities, such as C, O, N, P, S were reduced to less than 100 ppm in total. The second is the phase stability of matrix strengthener at high temperature during irradiation. Generally, in the fast-reactor, the cladding materials used in such high dose irradiations will experience more than 200 dpa of exposure, and have a temperature distribution from 673 to 973 K. In the prev
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