Ferritic steels are highly resistant to radiation-induced void swelling and are therefore good candidate materials for fast and fusion reactor components. Several mechanisms have been suggested to explain this behavior, but there is still insufficient experimental information available for any general conclusions to be drawn.
The work described in this paper forms part of a larger investigation into swelling resistance, some of which has already been published [1,2]. The experiments reported here were carried out in the Van de Graaff accelerator at Oak Ridge National Laboratory, and the aim was to study the evolution of the microstructure in three different materials, two ferritic and one austenitic, during the early stages of damage. Samples of iron-10% chromium, ferritic steel (FV448), and Type 316 stainless steel were mounted side by side on the targets so that they experienced the same irradiation environment and were bombarded with 4-MeV iron ions to four doses up to 10 dpa (displacements per atom) at three temperatures between 510 and 540¼C. The void and dislocation structures were then examined in a transmission electron microscope. The specimen preparation processes were based on those already developed at Oak Ridge, but some modifications were needed to adapt them to the facilities available, and these are briefly described. A computer model was used to determine dislocation bias from the measured loop and void growth rates in each alloy.
Swelling occurred in the iron-10% chromium and Type 316 steel, but not in the FV448. The peak swelling temperature in the iron-10% chromium was 525°C, and the voids were associated with dislocations. Dislocation loops were observed in all three materials; their growth in the ferritic alloys was rapid, particularly in the iron-10% chromium. The loop and void growth rates in each metal were compared, and the results indicated that ferritic swelling resistance is not an intrinsic property arising from the lattice structure, but is a consequence of several factors including impurity content, damage depth, and preirradiation microstructure.