Alloy surface composition resulting from fabrication,as determined by surface secondary-ion mass spectrometry

Abstract

The metallurgical application of surface secondary-ion mass spectrometry (SSIMS), in which the primary-ion beam is defocused to permit analysis of the outermost surface layers, has been explored by analysing a variety ofalloy surfaces following fabrication procedures and comparisons made with Auger electron spectroscopy. The main advantages of SSIMS are its high sensitivity and its ability to detect surface chemical compounds directly, including hydrogen and its compounds. The composition of the solidified surface of a Ag–28·5Cu braze alloy depended markedly on its pretreatment; a surface enrichment of copper relative to silver was observed following melting processes in environments containing oxygen, while sawing and polishing enriched the surface in silver. The high sensitivity of SSIMS also enabled surface segregation of many impurities and additives to be detected on the solidified braze alloys and on free-machining brass swarf following ‘burning’ to remove machining oils. Surface compound formation, e.g. oxides, hydroxides, chlorides, as well as hydrocarbon contamination, could be identified from the complex SSIMS spectra from all air-exposed technical-alloy surfaces. The oxide layers on ground and hot-rolled 3·5Ni–Cr–Mo–V low-alloy steel were enriched in most of the alloying elements and in manganese, and also contained aluminium, sodium, and potassium, to which SSIMS is very sensitive. A composition–depth profile, obtained with auxiliary ion etching, through the 0·4 μm oxide layer formed at 1000°Cin air on a Ni–Cr–Al γ–γ′ Cr₃C₂ eutectic doped with 0· 5% yttrium showed that it was enriched in aluminium and chromium oxides which were strongly interlinked but depleted in nickel oxide, and that the yttrium was segregated to the oxide/alloy interface. Decarburization of the scale and the presence of the second-phase inclusions in the alloy were also shown by SSIMS.