Characterisation of the surface over-layer of welded uranium by FIB, SIMS and Auger electron spectroscopy

Secondary ion mass spectrometry (SIMS), focused ion beam (FIB) milling and Auger electron spectroscopy have been used to examine the composition and surface structure of uranium welded by an electron beam. Four characteristic areas of the metal surface were designated, these being: body (unaffected by the welding process), heat affected zone (HAZ), weld edge and weld. Surface composition to a depth of a few microns was determined using SIMS profiling, and direct thickness measurements of surface over-layers on the metal were made using FIB. Marked variations in both the thickness and composition of the over-layers were found when comparing the body/HAZ, weld edge and weld material. Additionally, numerous prismatic inclusions, approximately 5 μm 2, were observed at the surface of the weld material. Auger and SIMS analyses indicated these inclusions were nitro-carbides. An increased concentration of these inclusions and other elemental impurities at the margins of the weld and within the HAZ indicated that during welding a segregation of impurities from the molten metal had occurred. The results are discussed in terms of processes considered to be occurring during welding and their implications in regard to corrosion in hydrogen atmospheres.