A solution to FIB induced artefact hydrides in Zr alloys

Abstract

The preparation of TEM samples using focused-ion beam (FIB) techniques allows for characterization of electron-transparent site-specific areas, such as oxide-metal interfaces and crack tips, leading to improved understanding of material degradation mechanisms. Using conventional FIB techniques to prepare zirconium alloy TEM samples results in the production of artefact δ hydrides during the final thinning stage, which can confound analyses and mechanistic interpretation, especially when understanding hydride morphology and crystallography is critical. A comparison of various preparation methods was performed to determine their influence on artefact hydrides in TEM samples. As-received Zircaloy-4 material with <10 μg/g hydrogen was quenched, fabricated into electropolished (EP) foils, TEM lamella were extracted using FIB, then the lamella were final thinned using different ion beam preparation techniques. Low-loss electron energy loss spectroscopy (EELS) was performed on the EP and FIB-prepared samples before and after final thinning. The expected post-quench distribution of γ hydrides was present in the EP and FIB samples before final thinning, however, after FIB or broad ion beam final thinning at room temperature the hydride phase was predominately δ with a larger volume fraction. After TEM lamella were cooled with liquid nitrogen during FIB or broad ion beam final thinning only γ hydrides were present, which was confirmed with diffraction analysis by observing γ hydride superlattice reflections. The large surface area to volume ratio, reactive surface, and beam heating during conventional FIB final thinning is believed to cause hydrogen-containing impurities in the vacuum chamber to dissociate and allow hydrogen to enter the specimen. Specimen cooling and the presence of a nearby cold finger reduce the hydrogen partial pressure near the sample surface and significantly reduce the hydrogen adsorption, absorption, and diffusion kinetics. Future investigations using FIB-prepared TEM samples of zirconium should apply cryo-techniques to prevent artefact hydrides.