Effective Removal Method for Layer of FIB-Induced Surface Damage in Austenitic Stainless Steel

There is substantial concern about the phase change to artificial ferrite or martensite in commercial austenitic stainless steels during intense focused-ion-beam milling. In this work, severe Ga ion milling to prepare microscale samples was found to produce thin transformed layers on the surface of austenitic stainless steel. The surface BCC phase transformation layer was clearly distinguished on the specimen processed with the focused ion beam using cross-sectional analysis. It was determined that Ga ions had accumulated in the surface BCC layer. However, it was also confirmed that the Ga ion accumulation was localized at the top of the surface. This is expected to be caused by the decrease in the formation energy of the body-centered crystalline phase, which may result from the considerable changes in chemical composition and stress state by the accumulation of implanted Ga atoms. We propose an effective method to overcome the phase change problem caused by FIB, by using low-energy Ar ion milling. It was determined that low-energy FIB milling was insufficient by itself to clear the phase transformation layer. High quality micro-scale experimental samples of austenitic stainless steels can be achieved by additional low-energy Ar ion milling at an energy of 1 keV or less after high-energy FIB milling.