STEM analysis of the transient oxidation of a Ni-20Cr alloy at high temperature

High-spatial scanning transmission electron microscopy (STEM) has been used to study the development of transient scales on a commercially available N i-20Cr alloy. The samples were examined using electron-transparent cross se ctions of the metal and oxide for oxidation times between 0 and 30 min in a furnace at 950 degrees C in laboratory air. The samples were polished to 1 mu m diamond before oxidation, producing a recrystallized grain structure within 100 nm of the surface. Upon oxidation, the initial scale consisted a lmost exclusively of chromia. However, at the metal-oxide interface, thin l ayers of silica and alumina were detected. At longer oxidation times, (>5 m in), localized thickening of the silica layer was observed. With increased oxidation time, (>25 min), these regions spread along the metal-oxide inter face until an almost continuous silica layer had formed The silica layer wa s present at much shorter oxidation times than reported by other workers, h owever, this may be because of the thin layer being undetectable using micr oprobe techniques. The scale formed was found to be adherent, although the alloy contained sulfur and did not contain reactive elements. However, sulf ur was not found to segregate to the metal-oxide interface possibly because of the presence of the amorphous silica layer.