An approach to a global model of the mechanical behaviour of oxide scales

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Abstract
The protective effect of oxide scales is strongly linked with their behaviour under mechanical loading. Scale cracking may occur at relatively low strains and a decisive part is played by the process of scale crack healing due to ongoing oxidation. It can even be maintained that many technical high temperature materials only survive in service for a long time due to the good healing capability of the protective oxide scales formed on them. This illustrates the fact that the characterization of the mechanical behaviour of protective oxide scales also has to include oxidation-induced effects. In order to characterize the behaviour of protective scales under mechanical loading quantitatively a number of parameters can be used. On the one hand, there are the conventional parameters used for bulk materials as well, such as elastic and plastic or creep properties as well as fracture mechanical properties. On the other hand, for growing oxide scales parameters describing the oxidation-related mechanical properties should be used in addition. The latter describe effects like the superimposed build-up of oxide growth stresses, pseudo-plasticity due to crack-healing processes and final loss of the protective effect after a certain loading history due to subsurface zone depletion of protective scale forming elements. In the first part of this paper the conventional parameters are discussed with respect to their application to a quantitative description of the mechanical behaviour of oxide scales. The second part focuses on the oxidation-related parameters of scale deformation. In all cases where relevant model equations exist, they are discussed in some detail. In the third part an attempt is made to combine the model equations of the first two parts with a global representation in the form of a scale-failure diagram. Three principal types of scale-failure diagram are introduced and discussed, and in conclusion it is recommended to extend these diagrams by a third dimension in the form of the physical defect size in the scale. A comparison of a theoretical scale-failure diagram with the experimentally determined borderlines for the scale on Alloy 800 H shows that the theoretical models yield conservative values. The models can, therefore, be used to assess conditions for the safe operation of the materials.