Stress-free oxide scale growth

As indicated earlier, protective oxide scales typically have a PBR greater than unity and are, therefore, less dense than the metal from which they have formed. As a result, the formation of protective oxides invariably results in a local volume increase, or a stress-free oxidation strain" . If lateral growth occurs, then compressive stresses can build up, and these are intensified at convex and reduced at concave interfaces by the radial displacement of the scale due to outward cation diffusion (Fig. 7.7) . 

Ideal stress-free growth of oxide scales... 

Thus, if sufficiently accurate measurements can be performed on oxidized specimens, comparison between the theoretical and actual positions of these reference planes would provide a quantitative evaluation of the departure from ideal stress-free scale growth and would permit an evaluation of the effective role and the action of scale/substrate interface during scale growth and to interpret it in terms of local strains and stresses within both the oxide scale and its underlying substrate close to the scale/metal interface could become possible. 

The situation is more complex for non-ideal solid solutions because the partial molar volume of each constituent varies as a function of composition. Selective oxidation of one constituent of a solid solution results in concentration gradients for all constituents in the alloy underlying the oxide scale. Therefore, local variations of partial molar volumes result in local volume changes that must be accommodated by an additional displacement field parallel to the growth and diffusion direction to maintain the system in a stress-free state. An accurate evaluation of such a volume change and the related displacement field requires many data, often not available, to determine or calculate concentration profiles and partial molar volumes. However, the assumption of ideal solution behaviour would often provide estimated values of a sufficient accuracy. 

As shown in the previous section, stress-free growth of an oxide scale on a metallic substrate can occur only if there is no obstacle to the free migration... 

The discoimection translation is a conservative movement but the misfit dislocations must be maintained along the scale/substrate interface to avoid the build-up of large interfacial stresses. Because of PBR values higher than 1, the growth of one molecular layer of semi-coherent oxide at the scale/ substrate interface involves fewer cations than the corresponding number of metal atoms in the substrate reticular plane in contact with the scale. Therefore, the translation of disconnections caimot be the only scale growth process as it would not permit the minimization of interface stresses and the maintainance of the interface in a stress-free state. Discoimection translation needs to be assisted by the climb of oxide misorientation dislocations and/or by the climb of misfit and misorientation dislocations on the metal side. Climb of oxide misorientation dislocations would result in an expansion of the oxide lattice normal to the interface, while the climb of metal dislocations would result in a recession of the metal lattice. 

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