Insights into the sources of irradiation hardening in a neutron irradiated 304L stainless steel following post-irradiation annealing

The 304L SS was irradiated in Barsebäck 1 BWR reactor to 5.9 dpa followed by post-irradiation annealing at 500 °C and 550 °C up to 20 h. Correlations of irradiation hardening with the main hardening features of dislocation loops and Ni/Si-rich clusters were made using the least squares regression. The dispersed-barrier hardening model tended to overestimate the contribution of Ni/Si-rich clusters in the as-irradiated condition. Contribution of Ni/Si-rich clusters to irradiation hardening appeared to be more appropriately accounted for by the short-range order strengthening mechanism and was dependent on the Ni and Si concentrations in the clusters. Irradiation hardening in the as-irradiated condition was predominantly from dislocation loops with minor contribution from the Ni/Si-rich clusters. This is consistent with recent molecular dynamics simulations which showed that Ni/Si-rich clusters were weak obstacles to dislocation glide. As dislocation loops were annihilated at a faster pace than the Ni/Si-rich solute clusters, the Ni/Si-rich clusters played a more important role in the remaining hardening after thermal annealing. Contribution of Ni/Si-rich clusters to remaining hardening outweighed that of dislocation loops after thermal annealing at 550 °C for 5 h and became predominant after annealing for 20 h.