STRESS EFFECTS IN HIGH-TEMPERATURE OXIDATION OF METALS

In this review the role of stress in the high temperature oxidation of metals is examined. In the introductory sections the Wagner theory of oxide growth and its modification to account for short circuit diffus ion are outlined. The direct influence of oxide growth stresses on def ect diffusion is then examined and shown to be a significant factor wh en constrained volume changes accompany the growth of the oxide layer. Methods for evaluating growth stresses are critically reviewed and th e possible origins of such stresses considered in detail. The response of the oxide layer to thermally and mechanically induced stresses is considered in the last part of the review. Under tensile conditions, t hrough-thickness cracks can readily be produced but may also heal with fresh oxide if imposed strain rates are relatively low. Spallation of the oxide is not usually found under tensile conditions but occurs fr equently under compressive stresses by either a 'buckling' or 'wedging ' process. Detailed descriptions of these are provided and their use a s the basis of spallation maps is described.