Cross-sectional examination of surface layers on electropolished thin specimens

This study examines the structure of surface layers produced during electropolishing for preparation of electron-transparent thin specimens of Al-3.80wt%Cu. The observation and microanalysis are first conducted in plan view of as-electropolished specimens with the AEM (analytical electron microscope) and SIMS secondary ion mass spectrometer. Then, ultramicrotomy is used to obtain thin cross sections of as-electropolished specimens. The cross-sectional observation and microanalysis are performed with the high-resolution CTEM (conventional transmission electron microscope) and AEM. Specimens were thinned for electron microscopy with the twin-jet electropolishing technique. Two different electrolytes were used: a mixture of 15% perchloric acid, 10% glycerol, and 75% ethanol (the PA specimen) and 30% nitric acid and 70% methanol (the NA specimen). Plan view analysis with the AEM and SIMS showed that a Cu-rich layer is present on both PA and NA specimens: the layer on the PA specimen is thicker than that on the NA specimen. This result was demonstrated more clearly by cross-sectional observations and microanalysis using high-resolution CTEM and AEM. The layer thickness was found to be approximately 50 nm for the PA specimen and 5 approximately 10 nm or the NA specimen.