PLASTIC-DEFORMATION OF OXIDE SCALES AT ELEVATED-TEMPERATURES

The atomic force microscope (AFM) has been used to observe and charact erize for the first time surface steps and grooves on the faces of Cr2 O3 grains formed as an oxide scale on Ni-30Cr and Ni-30Cr-0.5Y alloys during high temperature oxidation. The very high spatial resolution of the AFM is required to characterize these features. We propose that t hese surface features, whose dimensions are in the range of nanometers and tens of nanometers, may be interpreted as evidence of highly loca lized plastic deformation of the oxide scale. The size and spacing of the steps and grooves are consistent with models of plastic deformatio n based on slip bands derived from dislocation climb or dislocation gl ide. Mechanical twinning and the models for stress-driven surface inst ability are also possibly responsible for some surface features. The a ddition of yttrium to the alloy seemed to enable enhanced plastic defo rmation of the scale. The strain corresponding to the observed feature s, estimated by simple models, could relax a significant part of oxide growth and thermal stresses.