APFIM studies of the phase transformations in thermally aged ferritic FeCuNi alloys: Comparison with aging under neutron irradiation

An atom probe field ion microscopy characterization has been performed on a ternary Fe-1.28wt%Cu-1.43wt%Ni model alloy. Two sets of heat treatments at 400 and 500 degrees C were selected to follow the decomposition of the solid solution. The atom probe results suggest that the precipitation of copper from the body-centered cubic matrix involves classical nucleation, growth and coarsening processes. Nickel does not appear to be involved in these processes. The nickel and iron detected in the small particles in the early stage is rejected from the particles during subsequent growth and coarsening. The comparison of these data with those observed in neutron-irradiated pressure Vessel steels and in Fe-0.28wt%Cu-0.7wt47%Ni model steel indicates that thermal aging of ternary FeCuNi model alloys is not an appropriate process to elucidate the phase transformations in neutron-irradiated pressure vessel steels, and that the high nickel level detected in the clusters in the irradiated pressure vessel steels may be due to a ''radiation-induced'' rather than a ''radiation-enhanced'' precipitation process.