Solute cluster formation in austenitic and ferritic alloys under ion irradiation: a three-dimensional atom probe characterization

Vessel and internal core steels of nuclear power reactors are subjected to energetic neutron irradiation. The production of point defects in the material results from elastic collisions in displacement cascades. On a microscopic scale, changes in the microstructure and local microchemistry are observed in both steels. On a global scale, a shift in the transient ductile to brittle temperature of vessel steels and an increase in the irradiation-assisted stress-corrosion-cracking susceptibility of internal core stainless steels are observed. Also, on a nanometre scale, radiation-induced phase transformation occurs in both steels. A systematic approach has proved its efficiency here to study the specific role of displacement cascades on cluster formation in two different materials: ferritic steels (bcc structure) and austenitic steels (fcc structure). In order to produce the primary knock atom, model ferritic steels containing a small amount of copper as well as a commercial austenitic steel (316) were irradiated using Fe and Ni ions, respectively. The different resulting microstructures are then studied on a nanoscale using tomographic atom probe three-dimensional reconstruction.