Cavity evolution and void swelling in dual ion irradiated tempered martensitic steels

Abstract

Microstructural evolutions in two variants of the F82H 8 wt%Cr tempered martensitic steel under dual (He+ plus Fe3+) ion irradiations at 500 °C have been characterized by transmission electron microscopy. Emphasis is on understanding the combined effects of displacements per atom (dpa) and the helium/dpa ratio (He/dpa) on cavity evolution and void swelling (fv). The dual ion database includes 231 alloy-dpa-He/dpa conditions for irradiations up to ∼ 82 dpa and 3700 appm helium. We hypothesize that the large “scatter” observed in the fv data is primarily due to local microstructural variations, especially those that shield regions at shallower depths from the effects of injected interstitial atoms, which would otherwise reduce fv. More generally, our hypothesis is that the highest fv data, at a specified dpa and He/dpa, provides the most appropriate estimate of swelling for fusion neutron irradiation conditions. The dose dependence of fv can be described by an incubation dpai, marking the onset of significant void growth, and a post-incubation swelling rate, fv’ = Δfv/Δdpa. Both dpai and fv’ decrease with increasing He/dpa. The fv’ data are quantitatively consistent with a simple model for defect partitioning between evolving bubble, void, and dislocation sinks strengths and biases. Most notably, the model correctly predicts both the incubation dpai, and the subsequent fv’, in spite of the fact that they reflect somewhat different physics. Our analysis also suggests that defect recombination is modest at 500 °C. Along with other considerations, this supports the hypothesis that, even at high dpa rates, dual ion irradiations can be used to reasonably emulate void nucleation and growth for a range of fusion-relevant neutron irradiation service conditions. A companion paper incorporates the results of this study in a reduced-order, data-driven void swelling model.