Post-irradiation annealing study on low-alloy steels: Determining the contributions of matrix damage and clustering to hardening

In this work, post-irradiation annealing (PIA) at temperatures up to 425 °C was carried out on two low-Cu steels with varying Mn content that were irradiated at high flux to doses up to ∼0.1 dpa. Atom probe tomography (APT) was used to characterise the number, size, and composition of clusters in the as-irradiated material and select PIA samples, and the results compared alongside irradiation-induced change in hardness. For both steels it was found that as annealing temperature increased there was dissolution of Mn and Si from the clusters into the surrounding matrix. The number density and volume fraction of clusters were found to decrease with increasing annealing temperature. As expected from particle hardening models, the change in hardness due to irradiation was proportional to the square root of cluster volume fraction normalised by cluster size for the PIA material. The as-irradiated material, however, deviated from this trend. It is proposed that the observed increase above the expected hardening for the as-irradiated material is attributed with unstable matrix damage, which cannot be directly characterised by APT.