Observations of native oxides on ion beam polished heterogeneous Cr–Mo weld TEM foil specimens

Abstract

Native oxide films formed on argon ion beam polished transmission electron microscopy (TEM) specimens have been characterised. TEM studies of heterogeneous 9Cr –1Mo (P91) and 2.25Cr –1Mo (P22) creep resistant ferritic steel welds reveal the existence of oxide layers formed on thin TEM specimen foils after short term exposure to laboratory air. These films which are grown on as-polished specimens form a rim of oxide a few nanometres wide around the perforation edges of both P91 parent alloy and P22 weld metal samples. Selected area diffraction patterns indicate an epitaxial relationship exists between oxide layers and the steel substrates which conform to the Nishiyama– Wassermann orientation relationship. As a result, diffraction and mass thickness contrast and moiré patterns are observed in transmission electron micrographs. The confusing and obstructive nature of the additional contrast can lead to the loss of, or potential misinterpretation of, important information in images. Efforts to minimise the occurrence of these oxide layers during sample preparation and transport between preparation equipment and the microscope have been made.