Nanoscale defect formation in fcc Ni and low-Fe content Ni _x Fe _{1− x} single crystals induced by self-ion irradiation

Fcc Ni_xFe_1-x single crystal alloys serve as model systems to study defect evolution under self-ion irradiation at room temperature across fluences from 4 × 10¹³ to 2 × 10¹⁵ cm^-2. Defect characterization was performed using Rutherford backscattering spectrometry/channeling (RBS/C) and transmission electron microscopy (TEM) to distinguish defect types and calculate damage kinetics via a Multi-Step Damage Accumulation Model. Complementary molecular dynamics simulations of overlapping collision cascades revealed defect formation mechanisms at low irradiation doses. The simulations indicated that an A15 Frank-Kasper phase forms within the {111} plane in pure Ni, Ni_0.88Fe_0.12, and Ni_0.77Fe_0.23 up to a fluence of 2 × 10¹⁴ cm^-2, which underlies Frank loop nucleation. This provided insight into the role of Fe atoms in influencing dislocation nucleation and evolution. The combination of TEM observations, and RBS/C analysis using Monte Carlo-based McChasy simulations revealed that point defects - interstitials, dumbbells, vacancies, and A15 phase - dominate in all compositions up to 2 × 10¹⁴ cm^-2. At higher fluences of 2 × 10¹⁵ cm^-2, these defects transform into complex structures such as dislocation loops and stacking fault tetrahedra. Notably, in Ni_0.77Fe_0.23, compact 3D precipitates form but remain too small to evolve into Frank loops, despite significant Fe decoration.