Relations Between Oxidation Induced Microstructure and Mechanical Durability of Oxide Scales (original) (raw)
Related papers
Oxidation of Metals, 2010
The evolution of microstructure and growth stresses in oxide scales grown on a (110) iron single crystal surface at 650°C was studied by electron backscatter diffraction and in situ energy-dispersive diffraction with synchrotron radiation. Within this high temperature regime, the oxidation kinetics and scale microstructure were not significantly different from those encountered in the oxidation of ferrous polycrystals. Thus, epitaxial strains did not determine the stress state within the oxide scale. Relevant sources of growth stresses were inferred to be volumetric differences between the iron oxides in the early stages, and later, inner oxide formation, scale consumption as well as pore formation. These sources caused time-dependent stress cycles in magnetite and wüstite during oxidation. In the hematite layer stress cycles did not occur and creep appeared to be the predominant stress relieving mechanism. On cooling, the differences in thermal expansion caused residual stress gradients through the oxide scale.
Modeling of Residual Stress in Oxide Scales
1998
The magnitude of the residual stress in an oxide scale, and how this varies with temperature, is of major importance in understanding the failure mechanisms of oxide scales. This stress encompasses both growth stresses introduced at the oxidation temperature and thermal-expansion-mismatch stresses induced on heating and cooling, as well as any externally applied stresses or stress relaxation which takes place in the scale/substrate system. Although some of these components are reasonably well understood (e.g., thermal stresses), growth stresses and the relaxation of the total scale stress by creep or fracture processes are much less well understood. In this study a model has been developed to predict stress generation and relaxation in oxide scales as a function of time and temperature for both isothermal exposure and cooling to room temperature. The model determines growth stress and thermal-stress generation in the scale and how this is balanced by stresses in the substrate. The substrate stresses are then allowed to relax by creep and the scale stresses recalculated. This model accurately predicts the room-temperature scale stresses for a range of scale/alloy systems. The model can be used to show how the scale stress depends on oxidation temperature, cooling rate, substrate, and scale thickness. The model predictions are discussed in light of experimental observations for alumina scales on FeCrAlY.
Corrosion Science, 2006
This article focuses on the detailed microstructural investigation of the oxide scale formed on a 304 steel in the presence of oxygen and water vapour (40%) at 600°C. The work has been carried out using a combination of microanalytical techniques including FIB, TEM, EDX and electron diffraction. The local breakdown of the initially protective oxide scale and the growth of island/crater oxide morphology are described. Special consideration is given to the influence of the microstructure of the steel and the oxide scale on the breakdown behaviour.
Effect of reactive element oxide inclusions on the growth kinetics of protective oxide scales
Acta Materialia, 2007
A model is presented for describing the growth kinetics of a protective oxide scale containing reactive element (RE) oxide inclusions (pegs). The formation of RE oxide inclusions due to dissolution and diffusion of the RE from intermetallic precipitates along grain or phase boundaries in the alloy is considered. The average oxide scale growth kinetics depend on the RE content, the parabolic rate constant of the protective oxide scale, the alloy grain/phase size and the size of the RE containing precipitates. The specimen thickness determines the amount of RE available for oxidation. If the RE in the alloy has been consumed completely, then the RE oxide inclusions attain a maximum size. After this point, a decrease in the average oxidation kinetics occurs. Very good agreement between experiments and calculations was obtained for the oxidation of a free-standing NiCoCrAlY coating at 1373 K.
Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys
Oxidation of Metals, 1989
The oxidation behavior in air of two ODS alloys, VIA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an 180 tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.
Oxidation of Metals, 2010
The oxidation behavior of iron polycrystals and single crystals with (110) surface orientation was studied at 450°C. Energy-dispersive diffraction with synchrotron radiation provided in situ information regarding the evolution of stress gradients and fiber texture in the oxide scales. Within this low-temperature regime, grain boundaries caused the oxidation kinetics of polycrystalline iron to be more rapid than iron single crystals only during the first minutes of oxidation. Epitaxial growth of iron oxides occurred only on single crystal substrates during the initial oxidation. In situ stress analyses suggested that stress relief occurred invariably in the magnetite layer due to the formation of a fine-grained seam near the iron substrates. Above the magnetite and in the hematite layer, the growth stresses depend initially on volumetric strains and later on inner oxide formation and creep of the hematite.
Growth mechanisms of oxide scales on ODS alloys in the temperature range 1000-1100°C
Materials and Corrosion/Werkstoffe und Korrosion, 1990
After a short overview of the production, microstructure and mechanical properties of nickel-and iron-based Oxide Dispersion Strengthened (ODS) alloys, the oxidation properties of this class of materials is extensively discussed. The excellent oxidation resistance of ODS alloys is illustrated by comparing their behaviour with conventional chromia and alumina forming wrought alloys of the same base composition.
Microstructural investigation of the oxide scale on low carbon steel
There has been a long history of studies on the oxidation behaviour of carbon steel at high temperatures. The oxidation behaviour and the scale structure developed are complex. The microstructure and the ratio of the three classical layers of wüstite, magnetite and haematite may be difficult to identified by conventional techniques such as optical, standard Scanning Electron Microscopy (SEM) or X-ray diffraction. An unambiguous characterisation of the scale and the correct identification of the phases within the scale are difficult unless the crystallographic structure for each phase in the scale is considered and a microstructuretexture analysis is carried out. Throughout this work, Electron Backscatter Diffraction (EBSD) has proved to be a powerful technique for identifying the individual phases in the oxide scale accurately in order to obtain a better understanding of the mechanism of the oxide scale growth on steel and how scale microstructures develop. The results have shown how oxidation conditions such as temperature and time of exposure effect the grain size and microstructural development of the oxide phases.
Sub-stoichiometric oxides for wear resistance
WEAR, 2019
Sub-stoichiometric oxides formed by tribo-oxidation were often analyzed in tribofilms, like γ-Ti 3 O 5 , Ti 5 O 9 and Ti 9 O 17 and Mo 0975 Ti 0025 O 2 as well as double oxides, like β-NiMoO 4 or NiTiO 3. Thus, the contribution of suboxides with planar defects (Ti n O 2n-1, Ti n-2 Cr 2 O 2n-1) or as block-structures (Nb 3n+1 O 8n-2) to the tribological profile of carbides by using monolithic oxides is of scientific interest and has model character. The tribological profiles of these model oxides under dry unidirectional sliding have shown, that sub-oxides have a contribution to the tribological behavior of carbides and cermets, when they are tribo-oxidatively formed, because their tribological profiles as monolithic materials are homologuous in part or totally, or compete with hardmetals or cermets, depending from the operating conditions regarded.
Wear, 1995
An approach is proposed for describing the rate of loss from a metal surface subjected to the simultaneous action of high-temperature oxidation and mechanical erosion, in terms that involve the major parameters that determine the oxidation behavior of the alloy and the "erosion potential" of the environment. Different regimes are identified where oxidation or erosion is dominant, and where the two processes act together. It is shown that under conditions where exfoliation of the oxide scale (or of an adherent deposit) can occur, surface loss by erosion-oxidation is a strong function of the metal temperature and the erodent flux. Some simplifying assumptions are made to provide a workable framework for incorporating the parameters in a way that is consistent with the observed modes of material loss. The effects of erodent impacts on scale spallation are statistical in nature, and an initial attempt to address this issue is illustrated.