Study of Scale Formation on Al2O3-Scale Forming Alloys by Time-of-Flight Secondary Ion Mass Spectrometry (TOF–SIMS)

Abstract

Abstract Al2O3-scale formation on high temperature Ni-base and Fe-base structural alloys is a subject of significant technological and scientific interest. Such scale formation involves either (or both) inward oxygen grain boundary diffusion and new oxide growth at the scale/metal interface, or outward aluminum grain boundary diffusion and new oxide growth at the scale/gas interface. The details of scale growth have been studied using a commercial Kanthal A1 alloy and so-called double oxidation experiments—growth of an Al2O3 scale in 16O2, followed by further oxidation for twice the 16O2 duration in 18O2—at temperatures between 1000 and 1400°C. A time-of-flight secondary ion mass spectrometer (TOF-SIMS) with isotopic imaging resolution of ≈100 nm was used for determination of the 16O and 18O distributions in the scale. A focused primary Ga+ ion beam was used for imaging the oxygen isotopes in the negative polarity mode, and a de-focused Xe+ beam was used for sputtering. One novel aspect of this work was the use of taper sections to compensate for the sub-micrometer scale thickness of this oxidation-resistant alloy.