A solution to FIB induced artefact hydrides in Zr alloys

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 delta 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 mu g/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 gamma 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 delta with a larger volume fraction. After TEM lamella were cooled with liquid nitrogen during FIB or broad ion beam final thinning only gamma hydrides were present, which was confirmed with diffraction analysis by observing gamma hydride super-lattice 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. (C) 2018 Elsevier B.V. All rights reserved.