Chromium redistribution in thermally aged and irradiated ferritic-martensitic steels

Ferritic-martensitic steels containing 8-12 at.% chromium are considered as structural materials for spallation sources and fusion reactors. Materials will be subjected to intense damage rates, e.g. 50-100 dpa per year at full power. Therefore, the behavior under irradiation of these steels must be investigated. Our earlier dual-beam irradiation results on the DIN 1.4914 steel showed a decomposition into chromium-enriched and chromium-depleted regions. The mean concentration of the chromium-depleted regions was found to be 5.19 +/- 0.32 at.% after irradiation at 500 degrees C to a fluence of 50 dpa, as measured by atom probe field-ion microscopy. The chromium distribution in the matrix of the DIN 1.4914 steel after thermal ageing at temperatures between 400 and 600 degrees C has been investigated for times up to 17000 h. The carbides were characterized by means of transmission electron microscopy and extraction replicas. The concentrations of the constituents of the matrix were measured by means of atom probe. The mean chromium concentrations in the matrix are found to be 8.66 +/- 0.32, 4.5 +/- 0.3, and 7.2 +/- 0.4 at.%, after ageing at 400. 500, and 600 degrees C, respectively. The matrix contains virtually no carbon. The results are discussed in terms of phase decomposition and species segregation. (C) 1998 Elsevier Science S.A. All rights reserved.