Evolution mechanism of Ni3Si precipitate and anisotropic Frank loop variation in Ni-based alloy revealed by proton irradiation and MD simulations

Abstract

Proton irradiation with a low damage rate was conducted on the Ni-based alloy GH3535 to investigate the evolution behavior of dislocation loops and irradiation-induced segregation (RIS), both of which are known to degrade mechanical properties. This study identified that RIS induced the nucleation of Ni₃Si precipitates from Ni-Si clusters in the irradiated Ni-based alloys, which is considered a significant contributor to irradiation hardening in stainless steel. Atomic probe tomography (APT) analysis revealed Ni enrichment on both sides of the dislocation loop edges, forming a "W-shaped" profile. Meanwhile, Si accumulated along the edge, inhibiting loop growth and leading to the formation of shadows. Molecular dynamics (MD) simulations confirmed the mechanism of this enrichment distribution. With increasing irradiation dose, Ni₃Si nucleates at loop edges and coarsens by absorbing nearby Si atoms, ultimately resulting in the dissolution of the loop. This RIS at dislocation loops results in a Si-depleted matrix, stabilizing the nucleation of CrMo-enriched M₂C carbides and newly formed Frank loops. Meanwhile, four Frank loop (FL) variants were found to have anisotropic behavior. Scanning transmission electron microscopy (STEM) characterization across several crystal orientations revealed that the highest density of Frank loops occurred in the variant perpendicular to the crystal orientation. MD simulations indicated a distinct increase in the formation energy of the four variants under compressive stress, highlighting the influence of stress from ion penetration on Frank loop nucleation. These findings provide valuable insights into the evolution of precipitates and quantitative analysis of dislocation loops in Ni-based alloys.