WRINKLING OF THE OXIDE SCALE ON AN ALUMINUM-CONTAINING ALLOY AT HIGH-TEMPERATURES

To survive in an oxygen-bearing gas, an alloy grows an oxide scale to cover itself. The scale thickens slowly at a high temperature, until the compressive stress generated by oxidation and cooling causes it to spall off. A widely known model assumes that the scale-alloy interface has unbonded flaws, which allow the scale to buckle under the compression, followed by delamination and cracking. For the scale to buckle under a typical stress, this model requires flaws with radii exceeding ten times the scale thickness. In general, such flaws are too large to be produced during oxidation. This paper proposes a different model. It is commonly observed that wrinkles appear at the high temperature, caused by the oxidation-induced compressive stress. Initially the interface remains bonded, and the alloy conforms to the shape change by creep or diffusion. If the high temperature is maintained long enough, voids will grow on the interface, and the scale will slide, fold and crack. Cooling can intervene at any point; for example, even when the scale only wrinkles without extensive voiding, large thermal stress on the wavy interface may cause the scale to spall off. The initial wrinkling is analyzed as a time-dependent bifurcation by using a variational principle. The base alloy is assumed to conform by metal diffusion along the interface, motivated by the strain energy release due to wrinkling. The results are discussed in connection with experimental observations.