Radiation-induced modifications in ZrN: effect of micro vs. nano crystallites

Abstract

ZrN is a model system used to understand the irradiation response of isostructural advanced nuclear fuel, UN. UN possesses desirable material properties, such as improved thermal conductivity and fissile uranium density compared to oxides, but understanding damage accumulation and recovery processes in its rocksalt structure remains limited. Micro- and nanocrystalline ZrN were studied under ion irradiation using 600 keV Ar+ ions with fluences varying from 1 × 1015 to 1 × 1017 ions/cm2 at room temperature. Structural parameters, crystallite size, and microstrain of crystalline phases were determined via a unique Rietveld refinement of glancing incidence XRD data. Transmission electron microscopy was used to investigate microstructural modifications, and nanoindentation was used to explore the evolution of mechanical properties. The results show a pronounced increase in the crystallite size of nanocrystalline ZrN, while induced damage degrades the coherence of scattering domains in both ZrN systems post-irradiation. Microcrystalline ZrN displays swelling with increasing fluence, whereas nanocrystalline ZrN contracts before showing a continued unit cell expansion at higher fluences. AFM was used to confirm swelling in mi-crocrystalline ZrN, while oxidation in nanocrystalline ZrN is explored using ToF-ERDA and is attributed to a radiation-induced REDOX reaction. Nanoindentation results confirmed increases in the post-irradiated surface hardness and elastic modulus. The TEM investigation provides evidence of periodic and incommensurate ordering in irradiated regions of microcrystalline ZrN. The formation of a distinct epitaxial layer of cubic ZrO2 on the surface of microcrystalline ZrN has been attributed to the effects of incident ion interactions.