Formation and stability of the early stage solute and defect clusters in Cu-free ferritic steels during irradiation

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• The mechanism of the role of specific element species in the local stability of the Mn-Ni-Si quartet clusters are investigated by DFT. • The interaction of Mn with the dumbbell interstitials is stronger than other solutes. and this is favorable for formation of the cluster. • Si can stabilize solute clusters because it forms stable covalent bonds with other solutes at 1nn. Irradiation induced significant volume fractions of Mn-Ni-Si clusters have been characterized as severe cause for the matrix embrittlement in low-Cu reactor pressure vessel steels (RPV) at high flux. The formation mechanism of these clusters is still unclear, especially when the RPV does not contain Cu. In this work, we perform density functional theory (DFT) calculations to examine the mechanism of the role of specific element species in the local stability of clusters in Fe-Mn-Ni-Si system. The binding energy of different species or configurations for quartets including Mn, Ni, Si, vacancies, and interstitial atoms is presented. Based on the DFT data, we found that point defects are more likely to promote cluster formation, and when Mn is present near the dumbbell interstitials, is favorable for formation of the cluster. Si also plays an important role by forming stable covalent bonds at 1nn to other solutes, but Si atoms cannot be close to each other because this would interfere with the magnetic properties of the surrounding Fe atoms to the extent of reducing the stability of the clusters. These results are helpful for a better understanding of the cluster formation mechanism and can provide fundamental parameters for the solute-cluster evolution with larger scale simulations.