Identification of Defects In Ferritic/Martensitic Steels Induced by Low Dose Irradiation
- PDF / 349,257 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 100 Downloads / 178 Views
Identification of Defects In Ferritic/Martensitic Steels Induced by Low Dose Irradiation R. Schäublin and M. Victoria CRPP, Fusion technology, Materials Group, Ecole Polytechnique Fédérale de Lausanne, 5232 Villigen PSI, Switzerland ABSTRACT The ferritic/martensitic steels which are candidates for the first wall of the future fusion reactor are investigated in TEM. While the irradiation doses expected in this reactor are in the range of 100 dpa per year, there is still a lack of knowledge on the nature of the irradiation induced defects for the low doses at which hardening is already occuring. This hardening depends strongly on the type of interaction between the moving dislocations and the defects. The early defects, which start to appear as black dots in TEM, are expected to be either three dimensional clusters of interstitials or vacancies, or dislocation loops. The nature and size of these defects is carefully studied in the F82H steel for doses ranging from about 0.5 dpa to 9 dpa and irradiation temperatures ranging from 40°C to 330°C. For that purpose, various weak beam techniques are explored at the limit of resolution of a TEM used in diffraction mode. Results are presented here for the cases of a 1.7 dpa and a 8.8 dpa irradiation. INTRODUCTION Martensitic steels provide excellent radiation damage resistance for fission and fusion reactor applications. Significant hardening results from irradiation at temperatures of 400°C and below, but total elongation remains in the 7% range even in simple Fe-Cr binary alloys [1]. The He production rate for fusion 14 MeV neutrons is 13 appm/dpa, 1 appm/dpa for fission neutrons and about 130 appm/dpa for 590 MeV protons. The effect of the He on the mechanical properties however is believed to be small [2,3]. H production is about 800 appm/dpa for 590 MeV protons [4] but rapidly escapes the material. The ferritic/martensitic steels present a good resistance to swelling with a rate of about 1 % for 100 dpa [5]. This is understood by the fact that the irradiation induced vacancies are impeded to form voids. The trapping of the vacancies is still hardly understood, and various models were proposed [5,6] that are based on a capture by impurities or by dislocations. In the ferritic/martensitic steels irradiation is known to promote a dislocation structure with both a/2 and a/2 Burgers vectors, the latter being predominant when the Cr content is below that of Fe-6Cr [7]. The dislocation with a/2 Burgers vector are believed to arise from the growth of irradiation induced a/2 interstitial loops [6]. Hardening is understood to arise due to the formation of voids, precipitates, and dislocation loops with both a/2 and a/2 Burgers vectors. In this study the focus is given to the ferritic/martensitic steel F82H, the Japanese candidate for the future fusion reactor material [8]. Irradiation doses are varied from 0.5 to 9.2 dpa, for temperatures ranging from 40°C to 330°C. The lowest doses are intended for the investigation of the early defects, while the highest doses are useful for the
