Effect of Edges and Corners on Stresses in Thermally Grown Alumina Scales

Residual stress near edges and corners of thermally grown alumina scales were investigated. In this study, an edge is the intersection of two orthogonal flat surfaces and a corner is the intersection of three such surfaces. Microfluorescence measurements, performed on alloys with composition Fe-28Al-5Cr (at.%, bal. Fe) oxidized at 900°C, showed a large (>50%) reduction in hydrostatic stress in the vicinity of edges and corners. Surprisingly, significant stress reduction persists out to distances twenty to fifty times the scale thickness from the edge. Finite-element analysis calculations confirm the experimental results and provide a considerably more detailed picture of the stress distribution and its components and show that much of the observed stress reduction near an edge, is due to plastic deformation of the underlying metal.