We have seen that RIS was first observed in austenitic steels on the voids that are formed at large irradiation doses and lead to radiation swelling. The depletion of Cr at grain boundaries is suspected to play a role in irradiation-assisted stress corrosion cracking (IASCC); this is one of the many technological con­cerns related to RIS. The enrichment of Ni and the depletion of Cr can also stabilize the austenite near the sinks, and favor the austenite ! ferrite transition in the matrix.29 The segregation of minor elements can lead to the formation of y’-precipitates (as in Ni—Si alloys), or various M23C6 carbides and other phases.1,29

The segregation of major elements always involves an enrichment of Ni and a depletion of Cr at sinks over a length scale that depends on the alloy compo­sition and irradiation conditions.5 The contribution of various RIS mechanisms is still debated. It is not clear whether it is the IK effect driven by vacancy fluxes, as suggested by the thermal diffusion coefficients DNi < DFe < Dcr,30 or the migration of interstitial — solute complexes, resulting in the segregation of undersized atoms,29 that is dominant. Some models of RIS take into account only the first mechanism,5 while others predict a significant contribution of interstitials.12 For the segregation of minor elements, the size effect seems dominant, with an enrichment of undersized atoms (e. g., Si27) and a depletion of oversized atoms (e. g., Mo53) (Figure 7).

The effect of minor elements on the segregation behavior ofmajor ones has been pointed out since the first experimental studies29; the effect of Si and Mo additions has been interpreted as a means of increas­ing the recombination rate by vacancy trapping. As previously mentioned, oversized impurity atoms, such as Hf and Zr, could decrease the RIS.46

RIS in ferritic steels has recently drawn much attention, because ferritic and ferrite martensitic steels are frequently considered as candidates for the future Generation IV and fusion reactors.54

Experimental studies are more difficult in these steels than in austenitic steels, especially because of the complex microstructure of these alloys. Identification of the general trends of RIS behavior in these alloys

appears to be very difficult.55 Nevertheless, in some highly concentrated alloys, a depletion of Cr and an enrichment of Ni have been observed, reminding us of the general trends in austenitic steels54 (Figure 8). The RIS mechanisms are still poorly understood. The segregation of P at grain boundaries has been observed and, as in austenitic steels, the addition of Hf has been found to reduce the Cr segregation.5