Authors: Papa Modou Gueye
Source: SPIRE - Sciences Po Institutional REpository (preprint, 2022-03-15)
Field: Materials Science — Materials Chemistry

Etude microstructurale des aciers ferritiques-martensitiques sous irradiation : impact des conditions d'irradiation. Ferritic-martensitic (F-M) steels with high chromium content are envisaged as structural material (hexagonal tubes) for future reactors due to their good thermal conductivity and their excellent swelling resistance under irradiation. However, one of the key issues that limit the operating window of these steels is the low temperature embrittlement (<400 °C) due to hardening. This low temperature embrittlement is caused by the formation of NiSiPCr enriched clusters, alpha’ clusters (Cr rich clusters) and dislocation loops. Understanding of the mechanisms underlying the formation of NiSiPCr enriched clusters and alpha’ clusters is necessary to understand the microstructural evolution under irradiation. The aim of this work performed in the framework of the European H2020/M4F project (Multiscale modeling for fusion and fission materials) is to study the impacts of irradiation conditions on the microstructural evolution of ferritc-martensitic (F-M) steels, in particular FeCr model alloys. Samples characterization before and after irradiation were carried out by atom-probe tomography. The influence of irradiation dose and Ni, Si and P content on the microstructural evolution of Fe15CrX (X=Ni, Si, P or NiSiP) alloys revealed the existence of a synergetic effect of the impurities on clustering in the alloys. NiSiPCr enriched clusters first enriched in P and then enriched in Ni and Si. NiSiPCr enriched clusters were shown to be small dislocation loops or point defects clusters. alpha’ clusters characteristics evolve with the dose. The influence of the ion energy on NiSiPCr enriched clusters and alpha’ clusters formation shows that loops and NiSiPCr enriched clusters grow with the increase of the ion energy whereas there is no significant effect on alpha’ clusters formation. The influence of the damage rate on NiSiPCr enriched clusters formation in 15CrNiSiP alloy is very close to those observed in the case of the influence of ion energy. The influence of the damage rate on alpha’ clusters formation show that the density of Cr-rich zones increases when flux decrease. Rastered or defocused irradiation mode have no significant effect on NiSiPCr enriched clusters, alpha’ clusters formation and loops. Comparison between ion and neutron irradiations showed that there is no significant effect due to the difference in damage rate between these two irradiations on NiSiPCr enriched clusters formation in these alloys. We also noticed that it is preferable to use a higher ion energy to obtain a microstructure similar to the one observed under neutron irradiation. After neutron irradiation, NiSiPCr enriched clusters density increases with the concentration of impurities in the alloys. High impurity content in alloys seems to delay the ’ clusters formation. After ion irradiation of Fe9Cr alloy, ’ clusters do not appear whereas alpha' clustering is observed after neutron irradiation because of ballistic dissolution. Ballistic dissolution is also responsible for the fact that concentration fluctuations are less developed under ion irradiation in the Fe15CrNiSiP alloy. Irradiation ; Materials science ; Metallurgy ; Martensite ; Microstructure ; Nuclear physics ; Physics Fusion materials and technologies ; Microstructure and Mechanical Properties of Steels