Flash electropolishing for TEM: Reducing FIB-induced defects in tungsten with protocols for new materials.

Focused ion beam (FIB) milling has become the dominant approach for site-specific transmission electron microscopy (TEM) specimen preparation; however, FIB damage remains a critical limitation for reliable microstructural characterisation, particularly in radiation effects studies. Tungsten is especially susceptible to FIB damage due to its high nuclear stopping power, which promotes the formation and strong diffraction contrast of FIB-induced 'black spot' defects that are indistinguishable from very fine irradiation-induced loops/defects resulting from low to intermediate temperature neutron irradiation. In this work, flash electropolishing is systematically evaluated as a post-FIB treatment for minimising preparation-induced artefacts for TEM analysis of tungsten-based alloys. Using a range of non-, ion-, and neutron-irradiated tungsten materials, the effectiveness of flash electropolishing has been assessed through direct comparison with conventional FIB and plasma-FIB preparation including low-energy Ga, Ar, Xe ion cleaning. The results demonstrate that flash electropolishing effectively removes FIB-damaged layers and 'black spot' defects, thereby enabling reliable observation of irradiation-induced dislocation structures. Key processing parameters governing flash electropolishing quality - including lamella thickness, applied voltage, polishing duration, electrolyte chemistry, and cathode geometry - have been systematically evaluated, and clear criteria were established for determining when flash electropolishing is required to ensure reliable microstructural analysis. This work also provides practical guidance for implementing flash electropolishing as an artefact-controlled specimen-preparation approach for TEM characterisation of FIB-produced specimens. The systematic protocol can be extended to other, non-tungsten materials. LAY DESCRIPTION: Flash electropolishing is demonstrated as an effective post-FIB treatment for removing preparation-induced damage in tungsten TEM specimens, including FIB-induced 'black spot' defects that can obscure irradiation microstructures. Through systematic comparison with conventional and plasma-FIB methods across irradiated and non-irradiated tungsten alloys, key processing parameters and reliability criteria are established. The work provides a practical, artefact-controlled preparation protocol for accurate radiation-damage characterisation and offers a broadly adaptable strategy for other material systems.