Understanding and removing FIB artifacts in metallic TEM samples using flash electropolishing

Abstract

Focused ion beam methods have often become a de facto choice for metallic materials that could also be easily electropolished. FIB preparation of TEM samples is widely used for radioactive materials, since the site specificity, coupled with the minuscule volume of material used for the TEM sample, can produce TEM samples with minimal to no detectable radioactivity. FIB preparation has problems however, as evidenced by artifacts such the subsurface black spots (clusters of vacancies and/or interstitials), dislocations, amorphous layers and phase changes, and must be accounted for when conducting TEM experiments. This study has two main objectives, first, presenting evidence on two types of surface artifacts (moiré fringes and surface dislocations) observed in FIB prepared Fe-Cr alloys and pure Fe. This evidence will include similar artifacts produced in ferritic based systems using two other ion sputtering techniques: conventional Ar⁺ ion milling with a Gatan PIPS®, and an Ar⁺ based Fischione NanoMill®. The second objective documents our method of removing all of the FIB artifacts using flash electropolishing (FEP) of FIB TEM lamella, demonstrating our success in producing electropolished FIB lamella from an FeCr HT-9 alloy free of both the subsurface and surface artifacts. With the proper choice of parameters, TEM samples from FeCr alloys are comparable to those prepared by jet polishing of bulk TEM samples. These comparisons between FIB prepared and FEP FIB samples are done using both TEM imaging and diffraction contrast imaging STEM (DCI-STEM) on samples from an unirradiated and neutron irradiated Fe-Cr alloy. The observed black spot damage, moiré fringes and surface dislocations in the ion beam prepared samples are discussed in terms of how they could impact the microstructural analysis of irradiated metals.