摘要:

Protective oxide scales on high temperature materials often fail in service leading to increased oxidation rates. Thus, a knowledge of oxide failure behaviour and failure strains is required. This paper reviews the measured failure strains of chromium oxide and aluminium oxide scales at room and oxide growth temperatures under applied strain. Data on iron and nickel oxide scales are also included for modelling purposes. Failure strains in tension show a power-law dependence on the measured void size within the layer, as expected from fracture mechanics models. Residual tensile or compressive growth and cooling strains prior to loading can increase or decrease the measured failure strains depending on the loading mode, i.e. tension or compression. At growth temperature, lateral oxide growth and creep may act to increase the failure strains, leading in some cases to measured failure strains in excess of 2%. However, the effective failure strains in tension, after taking account of the residual strains, oxide creep, etc., are in most cases less than 0.5% even for the thinnest scales. In compression, the effective values of the failure strains are usually higher than those in tension.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689482

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Experimental results on the failure of oxide scales on chromia-forming and alumina-forming alloys are reviewed and discussed in terms of the modes, onset and kinetics of spallation. Other factors affecting the extent of spallation are listed. It is concluded that despite there existing a large body of experimental results, the understanding of the mechanisms of spallation is still not sufficient to enable effective improvement of the service properties of high temperature alloys.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689483

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Reactive elements considerably improve the high-temperature oxidation resistance, especially the adherence, of chromia- and alumina-forming alloys. This paper is an updated review of existing experimental data, with emphasis on the possible effects of reactive elements on scale adhesion. The viability of the numerous proposed mechanisms is discussed, and suggestions are made regarding the way forward in this particularly important research subject.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689484

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Protective oxide scales are typically transition metal oxides grown onto rough, machined surfaces. The quality of the oxide, judged by the extent to which it reduces subsequent oxidation and provides certain standards of mechanical, chemical and electrical behaviour, is determined by processes at interfaces both at the boundaries of the oxide and within the oxide. Understanding these processes is the key to controlling oxide quality and oxidation. This understanding is usually approached at two levels: atomistic studies (and here the state of the art has developed enormously) and macroscopic studies (elasticity theory plus empirical deformation maps and basic fracture mechanics). Such studies can be of value, at least as a framework. However, the link between atomistic and macroscopic can only be made successfully via an intermediate level, mesoscopic, in which the grain structure and other microstructures are explicit. This paper outlines two aspects of mesoscopic modelling, covering the processes at the metal-oxide interface in the presence of a vacancy flux and stress generation within a grain boundary network. Also described are approaches to metal-oxide adhesion, especially the image charge formalism which recognizes that the major energy term is electrostatic, illustrated by the difference between non- reactive metals on MgO and NiO. For NiO there is evidence for a thin intermediate layer of a higher oxide.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689485

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

The nature of the adhesion of oxide to metal is a problem which we are only just beginning to understand from a fundamental point of view. What governs the choice of atomic sites where oxide and metal make contact? What is the chemical nature of the interatomic bonding and how strong is it? It has very recently become possible to solve the Schrödinger equation with sufficient accuracy to answer these questions for a simple system. We illustrate this with the niobium-sapphire interaction, for which the calculations provide an explanation of the structure observed in high resolution transmission electron microscopy. Some of the results of detailed quantum mechanical calculations can be interpreted with simple electrostatic concepts such as ionic bonding and the image interaction.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689486

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

The development of stresses in the scale during the high-temperature oxidation of metals can have important consequences for the long-term protection of components in service, particularly if this leads to scale failure, allowing access of the environment to the metal surface. Such stresses may result from externally applied deformation of the scale/substrate system, from thermal effects due to differential thermal contraction/expansion between the scale and the metal, from geometrical effects or from the intrinsic scale-growth process itself. In this paper, a review is presented of some of the models that have been published to account for scale-growth stresses and thermal stresses, with emphasis on quantitative estimation of such stresses and the resulting strains in the scale, in the metal and in the metal/scale interface. Although most models of intrinsic scale-growth stresses are based on the volume change as metal is converted to oxide in a confined location within the scale or at the scale/metal interface, there is little consensus on how these stresses develop. Several quantitative, but incompatible, models have been proposed for outward-growing scales in which oxide is assumed to form in the scale grain boundaries or at the base of these boundaries following inward transport of oxidant, although other qualitative models have stressed the importance of pores or cracks as paths for such species. Most models for the development of growth stresses use elastic analysis and neglect plasticity and creep effects, which may not be justified for a slow-growing scale. A qualitative model has been suggested that can account for the presence of a stress in such a scale without invoking formation of oxide within a constrained location. Rather, it results from climb of a fraction of the intrinsic misfit interfacial dislocations into the metal to annihilate vacancies at the scale/metal interface, followed by adjustment of the spacing of the remaining dislocations to maintain epitaxy. The importance of relaxation has been demonstrated in the model of stress generation during the oxidation of silicon. Reasonable quantitative models are now available to describe the development of geometrically induced stresses and thermal stresses. The latter are generally based on elastic analysis, which is justified for the high strain rates induced on rapid cooling or heating cycles.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689487

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

The importance of the mechanical stability of oxide scales in the corrosion resistance of metallic components operating at high temperature is discussed. The mechanisms of scale fracture due to stresses generated during growth, applied directly or by thermal cycling, are described. Methods of measuring these mechanical propertiesin situhave been described and measurements of their fracture toughness produced. The influence of defects in the scale on fracture behaviour is discussed, together with methods whereby the effect of small irregular-shaped defects can be quantitatively assessed. Finally, the influence of strain rate on scale fracture is summarized, and by comparing the yield and fracture behaviour of surface scales it is suggested that the mechanical behaviour of these scales over a wide range of operating conditions can be predicted.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689488

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Temperature changes pose a significant threat to the integrity of protective oxide layers because of the differential strains developed between oxide and substrate. These may induce either tensile or compressive in-plane stresses within the oxide layer. The former may produce cracks through the thickness of this layer but oxide spallation under such tensile conditions appears to be a difficult process. By contrast, spallation is relatively easy under compression, i.e. in most cases during cooling from the oxidation temperature. The two principal routes, buckling and wedging, leading to spallation under these conditions are described. It is shown that the wedging process is likely to be the one which occurs in protective oxide layers of industrial significance. The results of finite element analyses of this decohesion route are presented. The rate of propagation of the interfacial wedge crack is shown to be strongly influenced by creep relaxation within the metal substrate. Alloys which have low creep strength would then be expected to have relatively high resistance to spallation. These trends are summarized in a number of‘spallation maps’.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689489

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Recent advances in the understanding of the structure and dynamics of solid/solid interfaces has led to some new answers for traditional questions about the role of the metal/scale interface in the growth of reaction product scales on pure metals. In particular, the action of interfacial dislocations in the creation and annihilation of the point defects supporting the diffusional growth of scales is of interest, as is their role as growth sites, for scales grown either by anion or cation diffusion. New knowledge about the metal/scale interface has permitted the consideration of blocking the interfacial reaction step as a means to retard the scaling kinetics and to alter the fundamental scaling mode. Such considerations seem consistent with the well-known phenomena ascribed to the reactive element effect (REE), which traditionally has been discussed for alloys forming chromia and alumina scales but has recently been extended to other metals. An insight into the REE mechanism is provided through the consideration of pinning of interfacial dislocations by the segregation of large reactive element ions, an action which poisons the usual interfacial reaction step.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689490

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

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.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689491

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor