Formation of Ni–Si clusters and their relationship with dislocation loops in irradiated stainless steels

Ni–Si cluster is a major contributor to the irradiation hardening in irradiated stainless steels. The heterogeneous nucleation of Ni–Si clusters is associated with radiation-induced segregation at sinks (e.g., dislocation loops), and the possible formation of Ni–Si clusters on dislocation loops may influence the evolution process of the two types of defects. Besides, the nature of Ni–Si clusters is unclear yet, reflected by the discrepancies in the reported cluster composition. In this study, model alloys of 316L stainless steel with different Si contents were irradiated to 5 dpa at 450°C. Ni–Si clusters were analyzed by atom probe tomography and were compared with the loops observed by transmission electron microscopy. Results showed that higher Si content promoted cluster number density and size but did not change cluster composition. Discontinuous torus-shaped, continuous torus-shaped, and disk-shaped clusters were discovered. The cluster shape, size, and crystallographic orientation supported the formation of torus-shaped clusters on Frank loops and perfect loops. In large torus-shaped clusters, Si and Ni tended to concentrate in the outer and inner regions of the torus, respectively, and it may be attributed to the stress field of the interstitial-type Frank loops on which the torus-shaped clusters were formed. The constant composition of large Ni–Si clusters indicated the possible existence of a preferential Ni–Si phase. Combining the results of the 1D concentration profile and the MSM using Si atoms and Ni atoms, the Ni/Si ratio of the large clusters was between 1.6–3.0.