Isothermal and Cyclic Oxidation Behavior of Fe–B Based Alloys at 927 K

The high temperature oxidation behavior of an Fe–16Cr binary alloy, oxidized under different compressive stresses in air at 900 °C, was investigated. Surface and cross-sectional micrographs, observed by scanning electron microscopy, indicated that the resulting morphology of the thermally grown oxide scale depended on the compressive stress. Results showed that oxide scales were infact below 5 MPa stress after 10 h of oxidation. Delamination developed at the outer/inner oxide scale interface in the case of compressive stress above 5 MPa. Growth kinetics measurements revealed that the rate of oxide-scale growth increased by the compressive stress.     

微晶化对Ni20Cr合金热生长氧化膜应力的影响

利用氧化膜应力原位测量装置对Ni20Cr微晶涂层/合金体系在1000℃空气中氧化过程中发生的弯曲挠度变化进行测量.结果表明,微晶涂层1000℃氧化所形成的Cr_2O_3氧化膜中存在压应力,应力绝对值高于相同厚度的合金表面的热生长氧化膜,其原因在于涂层/Cr_2O_3界面结合好,应力释放较小.降温冷却过程中,通过微晶涂层...     

Determination of Oxide Growth Stress by a Novel Deflection Method

A new technique has been developed to monitorthe growth stresses in oxide scales by measuringspecimen deflection. Compared to the conventionaldeflection method, the new technique does not require acoating for protecting one side of the specimen fromoxidation. This new method does not have temperaturelimits and has improved precision for stress measurementin alumina scales. Two alumina-forming alloys, Fe-22Cr-5Al-0.3Y and Fe-22.6Cr4.42Al-0.21Ti,were studied with this method. During oxidation at1000°C in air, the average compressive stresses inthe alumina scales were found to decrease from ~1000 to ~100 MPa with increasing scale thickness on thetwo alloys. The growth stress in alumina scales wasdirectly proportional to the oxide growth rates. Thestresses in the substrate alloys were much lower, on the order of 1 MPa. Creep occurred in thesubstrate alloys and was taken into consideration in thestress analysis. It is believed that creep in the alloysubstrates allowed stresses in the oxide scales to relax. Annealing tests showed that thestresses relaxed much easier in thin than in thickscales.     

Effect of Oxide Growth Strain in Residual Stresses for the Deflection Test of Single Surface Oxidation of Alloys

Residual stresses developed in FeCrAlY and Ni₈₀Cr₂₀ alloys have been predicted considering growth strain and creep strain in oxide layer and creep strain in alloy or metal. Such stresses, a net compressive stress developed in oxide scales and a net tensile stress developed in alloy strip, produce deflection of a single surface oxidized specimen during high temperature isothermal oxidation. Stresses generated in these alloys and oxide scales were compared with creep deflections. Introducing oxide growth strain in the oxide scales increase the oxide stress value during the initial oxidation stage, during which creep analysis lacks prediction. Oxide stress reaches maximum value at certain oxidation time in the initial oxidation stage. After that oxidation time relaxation of oxide stress occurs considerably in later oxidation stage.     

Scale stresses during protective and breakaway corrosion of iron and rimming steel in CO2

Foil extension experiments have been used to determine the stresses arising in oxide scales growing on iron and rimming steel in CO₂ during the protective and breakaway stages of oxidation. There is no definite association of either tensile or compressive stresses with protective oxidation, and scale stresses cannot be invoked to explain the mode of protective growth or the nucleation of breakaway oxide. Breakaway oxidation generates compressive stresses in the scale which cause tensile deformation of thin foils. Metallographic observations suggest that significant factors in the transition from protective to breakaway oxidation are prior formation of a duplex protective scale and carburization of the underlying metal.     

Comparative study of oxidation kinetics for pure nickel oxidized under tensile and compressive stress

The growth of oxide scales on pure nickel oxidized under tensile and compressive stress at 973 K was investigated. The objective was to understand the effect of mechanical loading on the oxidation kinetics. The oxidation kinetics curves gained by thermogravimetric analysis (TGA) indicate that the oxidation rate of pure nickel was accelerated by the external stress. Compared with the tensile stress, the effect of compressive stress on the oxidation kinetics was more pronounced. Surface morphologies examined by scanning electron microscopy (SEM) reveal that granular oxides formed on the stressed specimen, which increase the short-circuit diffusion.     

Oxidation at High Temperature Under Three-Point Bending Considering Stress-Diffusion Coupling Effects

Stress-diffusion coupling effects during oxidation have attracted extensive attention. In this work, an analytical model is developed for oxidation kinetics and stress evolution considering stress-diffusion coupling effects for three-point bending tests under high temperature. With the oxide thickening, growth strain can be generated and induces compressive stress in the oxide. The total stress in the oxide is estimated by superimposing the stress caused by oxidation and the applied load. The oxidation kinetics and stress evolution at typical locations where maximum tensile or compressive stress occurs are discussed. The theoretical analysis reveals that tensile stress accelerates the oxidation rate, while compressive stress would decelerate it. The model prediction for the parabolic rate constant of the oxidation kinetics is consistent with experimental observation, which validates this analysis. In addition, the influence of the applied load on oxidation kinetics is investigated.     

Oxide growth stress measurements and relaxation mechanisms for alumina scales grown on FeCrAlY

Early‐stage tensile stress evolution in α‐Al₂O₃ scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200 °C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200 °C is dominated by creep in the α‐Al₂O₃. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide‐gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)₂O₃ to α‐Al₂O₃ produces an initial tensile stress in the alumina scale.     

Spallation Behavior of Thermally Grown Nickel Oxide on Nickel

Thermally grown oxide scales are often under compressive residual stresses, especially upon cooling, which can lead to spontaneous spallation and a loss of their protectiveness. This study uses nickel oxide formed on different purity of nickel as an example to investigate the spallation process. Samples oxidized after different times between 800 and 1100 °C were observed during cooling. The oxide scale buckled and spalled after reaching a critical thickness that depended on oxidation temperatures, substrate thickness, and metal purity. The observed buckling was only a secondary process that followed scale delamination under local tensile stresses at sample edges or corners. When the delamination eventually extended over a large enough area on the face of the specimen, the scale above it buckled, driven by the residual compressive stress in the oxide. Growth of the buckles took place by crack extension along regions of high pore densities in the oxide scale. The development of pores in the oxide layer, which depended strongly on substrate impurity levels, was found to be the most important factor controlling failures of the NiO scales. Sulfur segregated on pore surfaces, whereas TEM studies showed no S at NiO/Ni interfaces.     

Transformations and cracks in zirconia films leading to breakaway oxidation of Zircaloy

Using combined Raman spectroscopy, atomic force microscopy and optical microscopy, this paper suggests that breakaway oxidation of Zircaloy is caused by the change of circumferential stress sign from compressive to tensile, which triggers catastrophic cracks to propagate from the oxide free surface toward the oxide–metal interface. The stress sign changes at a critical oxide thickness, which depends on the circumferential stress at the interface. This biaxial interfacial stress is promoted by a lattice expansion stress that accompanies the tetragonal to monoclinic crystal phase transition. In contrast with current research in the literature, this allotropic transformation is suggested to be beneficial, not detrimental, because it contributes to retard the thresholds for the change of circumferential stress sign, and thus breakaway oxidation. The tetragonal phase was revealed to localize at the interface and adopt the shape of prismatic isosceles triangles detected at early stages of oxidation. These growth morphologies are consistent with a cationic oxidation mechanism. Upon phase transition, the monoclinic variant quickly dominates the oxide scale above the interfacial regions and forces the overall oxidation to proceed by an anionic diffusion mechanism. The results of Raman spectroscopy compared well with those of atomic force microscopy.     

In Situ Measurement of Stresses and Phase Compositions of the Zirconia Scale During Oxidation of Zirconium by Raman Spectroscopy

Raman spectroscopy was used to determine the stress and phase compositions of the zirconia scale in situ during the oxidation of zirconium at 600–900 °C in ambient air. The results show that the compressive stresses in the zirconia scale vary with the oxidation temperature and the oxidation time. The tetragonal (t) phase forms at the metal/oxide interface and the t to monoclinic phase transformation occurs far away from the interface during the oxide scale growth. The compressive growth stress at the oxidation temperature is favourable to the formation of t phase.     

Ce对Ni—Cr—Cu合金抗氧化性的影响

作者研究了添加0.1％和0.8％Ce的Ni—Cr—Cu合金在空气中1200℃100小时等温氧化和500小时循环氧化。Ni—Cr—Cu合金中添加微量Ce后,显著降低了氧化速率,增加了氧化膜的剥落抗力。氧化速率降低是添加Ce后各种效应综合作用的结果。它们是:(1)由于Cr的扩散加快,富Cr保护膜更迅速形成;(2)聚集在膜/合金界面附近的含Ce氧化物与空位复合,减少了膜/合金界面的空洞;(3)固溶于氧化膜中的含Ce氧化物阻碍了Cr~(3+)沿氧化物晶界的短程扩散。 提高耐剥落抗力主要原因是:(1)添加Ce使氧化膜晶粒变细,从而改善了塑性变形和适应热应力的能力;(2)0.8Ce合金中稀土氧化物“钉扎”(Keying)作用改善了膜与合金粘附性,并改变了热应力的分布状态。     

An elastic analysis of growth stresses during oxidation

A quantitative analysis of oxide growth stresses is carried out for the model advanced by Rhines and Wolf in which new oxide forms along preexisting oxide grain boundaries. The mean oxide stress developed within the oxide is calculated using standard techniques from continuum dislocation theory. This analysis shows that the mean growth stress is compressive and is directed parallel to the oxide/ matrix interface. The growth stress is found to be independent of the oxide scale thickness, provided that the scale is thicker than the oxide grain size. However, in thin scales, the growth stress is very sensitive to oxide scale thickness. The compressive growth stress increases in direct proportion to the width of the new grain boundary oxide layer formed. The oxide scale is expected to either fail by buckling, or the growth mode will change to one in which additional compressive stresses are not generated.     

Geometrical effects on oxide scale integrity

The strains developed during oxidation on curved surfaces depend on the rate at which the bulk of the already formed scale is being displaced from an arbitrary reference point well within the metal substrate. A scale displacement vector is defined in terms of certain variables of scale formation and the strains arising can then be related to scale growth. Strain energy storage within the scale can be calculated for various oxidation kinetics and allowance made for stress relaxation due to oxide creep. Integrity limits for oxide scales growing on curved surfaces are then defined in terms of a critical strain energy criterion for oxide scale failure. The anodizing of aluminium and the growth of oxide scales on iron are considered as examples. It is also shown that the scale displacement vector as defined in this paper may have wider significance in areas where mechanical factors influence oxidation and where oxidation processes exert mechanical effects on their surroundings.     

用双面氧化弯曲方法测定Al2O3膜生长应力

提出了一种测定氧化膜生长应力的双面氧化弯曲方法,经不象常规弯曲法那样为防止试样一个侧在发生氧化度制保护除层,因而可用于较高温度及较长时间的氧化情况并可测定Ａｌ２Ｏ３膜的生长应力,由于高温下合金及薄氧化膜发生蠕变,从而释放膜内应力。新方法应用于合金的蠕变数据,并采用数值计算来获得氧化膜应力值,用这种方法测定Ｆｅｃｒａｌｌｏｙ（Ｆｅ－２２Ｃｒ－５Ａｌ－０．３Ｙ）合金在１０００℃空气中氧化中形成的Ａｌ２     

The deformation behavior of NiO scales on Ni in argon and air at temperatures from 20 to 800°C with respect to the relief of growth stresses

Stresses formed in oxide scales due to oxide growth are usually of a compressive nature, and there is still some debate on how these stresses are accommodated. While the deformation behavior of oxide scales under tensile stresses can be regarded as fairly well understood, there are many open questions concerning scale deformation and cracking under compressive straining. Therefore, the NiO scale formed on two different grades of Ni was chosen as a model system for compression tests with strain rates ranging from 8×10^–4 to 8×10^–8 s^–1 in the temperature range of 20 to 800°C. Test environments were air and argon, and accompanying acoustic-emission measurements were taken in order to detect the beginning of oxide-scale cracking during straining. As a result the critical-strain values at the beginning of mechanical-scale damage could be determined quantitatively and explained consistently by model considerations. Furthermore, SEM and TEM investigations, backed up by sulfur decoration of microcracks at the end of the tests, revealed that at elevated temperatures a major deformation mechanism under these conditions takes the form of dynamic equilibrium of continuous microcracking with superimposed oxide-healing processes, making compressive strains of 10% and more possible without macroscopic scale failure. It is, therefore, assumed that a major mechanism of growth stress relief in oxide scales occurs via microcracking and scalecrack healing.     

Effect of oxide grain structure on the high-temperature oxidation of Cr

Cr was oxidized in 1 aim O₂ at 980, 1090, and 1200°C. ElectropolishedCr and some orientations of etched Cr oxidize rapidly and develop compressive stress in the growing Cr₂O₃; other orientations oxidize slowly, apparently free of stress. SEM examination of fracture sections shows that the thick oxide is polycrystalline whereas the thin oxide on etched Cr is monocrystalline. It is deduced that the monocrystalline oxide grows by lattice diffusion of cations outward, and the polycrystalline layer by the two-way transport of cation diffusion outward and anion diffusion inward along oxide grain boundaries. The consequent formation of oxide within the body of the polycrystalline layer generates compressive stress and leads to wrinkling by plastic deformation. The activation energy for oxidation of Cr by cation lattice transport is 58 kcal/mole. Polycrystalline Cr₂O₃ forms on Fe-26Cr alloy, whether electropolished or etched; oxidation is accordingly rapid and accompanied by compressive stress.     

Effect of Mechanical Loading on the Oxidation Kinetics and Oxide-Scale Failure of Pure Ni

The oxidation kinetics and mechanism of oxide-scale failure of pure Ni oxidized under external static compressive and tensile loads were studied. The results showed that both types of mechanical loads accelerated the oxidation rate, but the effect was different for the two types. Compressive loading (CL) affected it by improving the plasticity of oxide scales, and tensile loading (TL) affected it by amplifying the compaction of the oxide–metal interface. As for the oxide-scale failure, CL can delayed cracking, TL accelerated brittle failure. The study analyzed the effect of external load on the oxidation kinetics and the failure mechanism of oxide scales.     

应力对Ni3Al合金表面氧化膜成分的影响

研究了Ｎｉ－２３Ａｌ－０．５Ｚｒ－０．１Ｂ和Ｎｉ－１７Ａｌ－８Ｃｒ－０．５Ｚｒ－０．１Ｂ（ａｔ．－％）在应力作用下表面氧化成分的变化。结果表明，在拉应力作用下氧化膜中的Ｎｉ含量明显增加。分析和计算发现由于Ｎｉ和Ａｌ具有不同的氧化体积膨胀，在拉应力作用下Ｎｉ在氧化表层的富集可以减小氧化表面的弹性形变能，降低体系自由能。     

钇在Fe-Cr-Al合金1200℃氧化中的作用

将含有0.51％Y和不含Y的两种Fe-Cr-Al合金,在1200℃空气中进行了500小时的氧化实验。用X-射线衍射仪、扫描电镜、电子探针及离子探针对不同氧化时间的试样进行了分析,结果表明,除了初期(约20小时)稍提高Fe-Cr-Al合金氧化速度外,总的说来,钇的添加降低了合金的氧化速度。同时,钇的氧化物分布在以α-Al_2O_3为主的氧化膜中,使得α-Al_2O_3氧化物成为柱状晶,这种构造的氧化膜可经受较大压应力作用,因此在冷热交变过程中氧化膜不易剥落。钇与扩散到基体中的氧优先反应生成Y_2O_3,对阻止合金基体的内氧化及力学性能变坏也起到一定的抑制作用。