Stresses generated during oxidation sequences and high temperature fracture

Abstract

This paper deals with the stresses generated during high temperature oxidation and their relationship to scale fracture. In the first part, the objective is to differentiate growth stresses from thermal stresses and give evidence for relaxation phenomena. The results obtained for materials which develop NiO, Cr 2 0 3 or Ah0 3 scales indicate that, in most cases, growth stresses in the oxide films are negligible compared to thermal stresses, probably on account of stress relaxation at high temperature. Moreover, the stress sign in the oxide scale is inconsistent with conventional views based on the Pilling-Bedworth ratio, but closely related to the growth mechanism of the oxide: a preponderantly cationic diffusion leads to tensile stresses, whereas a preponderantly anionic diffusion leads to compressive stresses. Thermal stresses are closely related to the differences between the expansion coefficients of the scale and the substrate, but can be modified by parameters which can promote stress relaxation such as the cooling rate, the thickness of the film or the thickness of the substrate. In the second part, the objective is to relate growth stress to scale fracture. Growth stresses in oxide scales may be geometrically induced, of intrinsic nature or caused by epitaxial constraints or phase changes. The first two types in particular may lead to local scale failure whereby the number and size of physical defects determine the failure stresses or strains. While more experimental data on intrinsic growth stresses can now be found in the literature, no data exist for geometrically induced stresses. These can, however, be estimated from scale growth kinetics. The level of the kinetics shows their significance for scale failure, and thus it is recommended that more experimental work be done in this field.

Growth Stresses in Oxidized Tubes Under Uni- and Multi-Axial Oxidation Strain

“…In the literature on growth stresses it is often distinguished between geometrically-induced growth stresses and intrinsic-growth stresses (see for example Ref. 6. In the light of our model we can state, that the geometrically-induced contribution is caused by the oxidation-strain component which acts orthogonal to the surface and the intrinsic contribution is caused by the lateral-strain components.…”

“…6. Huntz and Schu¨tze distinguished between geometrically-induced stresses and intrinsic growth stresses.…”

“…But it seems that these authors did not recognize the importance of the oxidation mode (surface/interface oxidation). There are indications from the literature, 6 that the oxidation-strain components may depend on the distance from the interface. But for this work we make the simplifying assumption that they are constant.…”

“…Axial elongations were also reported in 11 for samples of Fe-Cr-Al-Y alloys with Al 2 O 3 scales. Lateral oxide growth on 18Cr steel was also determined by Huntz and Schu¨tze 6 with values increasing with time up to about 3%. The formation of wrinkles on oxide scales on steels, for example, is attributed to lateral growth.…”

Stress Determination in Thermally Grown Alumina Scales Using Ruby Luminescence

“…The intensity of the fluorescence signal increases very rapidly with reaction temperature. The rapidly increasing intensity results, in part, from increasing scale thickness, but, more likely from (1) an increased growth rate of a-Al2O3 relative to other aluminum oxides and (2) from the conversion of some metastable oxide phase (possibly y-Al2O3 ) to a-Al2O3-. .…”