The role of secondary oxide inclusions ("pegs") on the spalling resistanceof oxide films

The resistance to spalling of a thermally grown oxide from a nickel-based s uperalloy (PWA 1484) is shown to be related to the spatial distribution and areal density of secondary oxide inclusions ("pegs") formed at the alumina /alloy interface. Spalling, which develops with time after cooling to room temperature, initiates from the edges of polished samples. On surfaces cut parallel to the solidification direction in the underlying alloy, the spall s then propagate as strips across the material in the solidification direct ion, along the primary dendrites. The secondary oxide inclusions, which con sist of a Ta-rich oxide core and an outer alumina sheath, appear to form by preferential oxidation of the inter-dendritic precipitates. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd All rights reserved.