APFIM STUDIES OF THE PHASE-TRANSFORMATIONS IN THERMALLY AGED FERRITICFECUNI 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 tre atments at 400 and 500 degrees C were selected to follow the decomposi tion of the solid solution. The atom probe results suggest that the pr ecipitation of copper from the body-centered cubic matrix involves cla ssical nucleation, growth and coarsening processes. Nickel does not ap pear to be involved in these processes. The nickel and iron detected i n the small particles in the early stage is rejected from the particle s during subsequent growth and coarsening. The comparison of these dat a with those observed in neutron-irradiated pressure Vessel steels and in Fe-0.28wt%Cu-0.7wt47%Ni model steel indicates that thermal aging o f ternary FeCuNi model alloys is not an appropriate process to elucida te the phase transformations in neutron-irradiated pressure vessel ste els, and that the high nickel level detected in the clusters in the ir radiated pressure vessel steels may be due to a ''radiation-induced'' rather than a ''radiation-enhanced'' precipitation process.