Substructures in Oxide Scales on Nickel, Cobalt, and Nickel-Cobalt Alloys

Abstract

Scales produced on nickel, cobalt, and nickel‐cobalt alloys in pure oxygen at 1000° and 1200 °C have been studied, after different cooling procedures, in plan and section by optical and electron microscopy. Very large columnar grains are separated from the metal core by a thin layer of fine equiaxed grains. Subgrain boundaries are revealed on the surface of nickel‐rich scales by a form of growth or by thermal etching and of cobalt‐rich scales by precipitation. In section, the visibility of decorating subboundaries can be accentuated by preferential chemical etching. Growth facets and steps and thermal etch pits produced by surface processes are clearly evident on scale surfaces, certain of the etch pits probably being associated with emergent dislocations. Control of the alloy composition and cooling conditions leads to precipitation of either randomly, or along subgrain boundaries, or as very fine particles along preferred directions in the grains.