Study of the formation of solutes diffuse agglomerations under irradiation in ferritic standard alloys

The formation of diffuse agglomeration of solutes (Cu, Mn, Ni, P and Si) under neutronic irradiation induce the embrittlement and the hardening of the steels used in the manufacture of PWR vessels. The processes inducing the formation of these diffuse agglomeration of solutes are still badly understood. In order to settle between the different hypotheses able to explain their formation (displacement series, homogeneous precipitation or heterogeneous precipitation), ferritic standard alloys with or without copper, in presence of manganese, nickel and phosphorus (FeCuMnNiP and FeMnNiP) are irradiated with electrons and ions. Their microstructure is then characterized by tomographic atomic probe. In the same time, an agglomeration dynamic model allowing to estimate the evolution of the point defects population under irradiation is used. The comparison between the experimental results and and the model anticipations suggest that the formation of the solutes agglomerations is heterogeneous on the point defects agglomerations. The comparison between the experimental results obtained on these two alloys show that the lack of copper slows down the precipitation kinetics of the solutes agglomerations. In the same way, in comparing the results obtained on the FeCuMnNiP alloy and the previous results obtained on a FeCu binary alloy, it appears that the presence of manganese, nickel and/or phosphorus slows down the precipitation kinetics. (O.M.)