Oxide scale formation and microstructural changes during high temperature exposure of mechanically alloyed ODS alloys studied by AFM, TEM and SIMS/SNMS

The growth phenomena of protective scales formed on mechanically alloyed ox ide dispersion strengthened alloys were studied using quantitative structur al analyses (atomic force microscopy as well as transmission and scanning e lectron microscopy) in combination with secondary ion and secondary neutral mass spectrometry. The main emphasis was placed on the early stages of the oxidation process when the oxide film is just a few nanometres thick. The scale morphology, composition and growth during short-time exposure (up to 180 min) of the ferritic oxide dispersion strengthened alloy INCOLOY MA956 at 800 and 1000 degrees C in air were found to be dependent on oxidation co nditions (time and temperature) as well as the substrate crystallographic o rientation. The growth mechanism of the alumina scales in the early stages of oxidation seemed to be dominated by outward cation diffusion, contrary t o the later stages (1000 degrees C/180 min) when an inward grain boundary o xygen diffusion becomes dominant. Oxide grain boundary density and conseque ntly oxide growth by grain boundary diffusion appeared to depend on the cry stallographic orientation of the alloy.