STEM Analysis of the Transient Oxidation of a Ni-20Cr Alloy at High Temperature

High-spatial scanning transmission electronmicroscopy (STEM) has been used to study the developmentof transient scales on a commercially available Ni-20Cralloy. The samples were examined usingelectron-transparent cross sections of the metal and oxide foroxidation times between 0 and 30 min in a furnace at950°C in laboratory air. The samples were polishedto 1 μm diamond before oxidation, producing a recrystallized grain structure within 100 nm ofthe surface. Upon oxidation, the initial scale consistedalmost exclusively of chromia. However, at themetal-oxide interface, thin layers of silica and alumina were detected. At longer oxidation times,(>5 min), localized thickening of the silica layerwas observed. With increased oxidation time, (>25min), these regions spread along the metal-oxideinterface until an almost continuous silica layer hadformed. The silica layer was present at much shorteroxidation times than reported by other workers, however,this may be because of the thin layer being undetectable using microprobe techniques. The scale formedwas found to be adherent, although the alloy containedsulfur and did not contain reactive elements. However,sulfur was not found to segregate to the metal-oxide interface possibly because of the presence ofthe amorphous silica layer.