Transmission Electron Microscopy Investigation on the Electron-stimulated Oxidation of Iron Nitrides by 2-MeV Electron Irradiation

Abstract

An iron nitride sample was irradiated by 2-MeV electrons intermittently for 2100 s with a dose rate of 6.3 × 1024 e.m.−2 s−1 inside a 3-MV high-voltage transmission electron microscope. The electron-stimulated oxidation of Fe4N and Fe2–3N was investigated in situ and ex situ using conventional transmission electron microscopy and high-resolution electron microscopy. It was found that both Fe4N and Fe2–3N nitrides were oxidized by the residual gas in the vacuum chamber to form Fe3O4 oxides. The orientation relationship between Fe4N (γ′) and Fe3O4 (o) was (110)γ′//(220)o, [001]γ′//[001]o, and that between Fe2–3N (ϵ) and Fe3O4 (o) was (110)ϵ//(−220)o, [1–11]ϵ//[001]o. Crystal lattice deformation from iron nitride to iron oxide took place during the dynamic oxidation process. Structural models were proposed to understand the oxide formation, and the models were confirmed by experimental observations. The irradiation effects of Fe4N and Fe2–3N crystals were compared. The results show that Fe4N is more sensitive than Fe2–3N to electron irradiation. These results are important not only for the fabrication of insulating iron oxide film, but also in the field of the surface modification of iron nitride to improve its mechanical properties.