Fundamentals of Cavity Formation in α-Fe and Fe-Cr Alloys

Ferritic-martensitic steels are attractive candidate materials for fusion and advanced fission reactors primarily due to their low swelling characteristic, attractive thermo-mechanical properties, and the potential for development of nanostructured ferritic alloys. However, significant discrepancies exist regarding the effect of solutes and irradiation temperature on cavity swelling under ion versus neutron irradiation conditions. Several mechanisms have been proposed that may affect cavity swelling, but no general theory or model has received complete acceptance to explain these phenomena.

To better understand the formation of cavities in ferritic steels, we have performed multi-temperature (400-550°C) single-beam and simultaneous dual-beam irradiations (ex-situ and in-situ) on a series of ultra-high purity Fe and Fe-Cr alloys (3-14 wt% Cr), as well as nanostructured alloys. Helium production rates of 0.1, 10, and 50 appm He/dpa were selected to examine the He effects for fission- and fusion-relevant conditions. Transmission electron microscopy was used to characterize the microstructures in more than 50 different irradiated samples. As a complement to the ex-situ irradiated samples, real-time dynamic formation and evolution of defects were performed in an in-situ ion irradiation TEM facility. Atom probe tomography was used to examine the formation of Cr-rich alpha prime precipitates.

The relative importance of the solute content, irradiation temperature, sink strength and He/dpa on cavity swelling was investigated and discussed. Cavities were observed in all the irradiated samples between 400-550°C. We show that the narrow temperature range of observable cavities reported in prior ion irradiation studies is likely an artifact of the near-surface and implanted ion effects due to the use of insufficiently high ion energies. The peak swelling temperature was noticeably higher for the Fe-Cr alloys than pure Fe. A higher He/dpa level resulted in a higher peak swelling temperature for pure Fe. Bimodal size distribution of cavities was only observed in the 10 and 50 appm He/dpa irradiated samples. Peak cavity swelling occurs at intermediate He production rate ~10 appm He/dpa. Swelling as a function of Cr content is non-monotonic and depends on the ratio of biased to neutral sink strength, which is likely associated with solute trapping of defects and the formation of α’ precipitates. Helium concentrations up to ~1 at% were well managed in nanostructured alloys with a high density of nanoparticles.