合金上氧化物的体积比的分析

氧化物与形成该氧化物消耗的金属的体积比（Ｐｉｌｌｉｎｇ－Ｂｅｄｗｏｒｔｈ Ｒａｔｉｏ简称ＰＢＲ）是判断氧化膜完整性的一个重要判据,也是氧化膜内产生生长应力的主要因素之一,已发发表的关于ＰＢＲ的数据都是针对纯金属的,本文基于合金的氧化行为建立了一个简化的模型,给出了合金上形成的氧化物的ＰＢＲ的人体估算了Ａｌ２Ｏ３、ＮｉＯ及Ｃｒ２Ｏ３等几种氧化物膜以及它们的混合膜ＰＢＲ值,合金上氧化物的ＰＢＲ值与合金　成分有关,并与纯金属上形成的同种氧化物的PBR有显著差别.     

Effect of substrate thickness on oxide scale spallation for solid oxide fuel cells

In this paper, the effect of the ferritic interconnect thickness on the delamination/spallation of the oxide scale was investigated experimentally and numerically. At the operating environment of solid oxide fuel cells (SOFCs), a combination of growth stress with thermal stresses may lead to scale delamination/buckling and eventual spallation during SOFC stack cooling, even leading to serious degradation of cell performance. The experimental and numerical results show that the interfacial shear stresses increase with the growth of the oxide scale and also with the thickness of the ferritic substrate, i.e., the thick ferritic substrate can easily lead to scale delamination and spallation.     

Analytical Modeling on Stress Assisted Oxidation and its Effect on Creep Response of Metals

The aim of this paper was to investigate the effect of external mechanical loads on the kinetics and process of high temperature oxidation of metals or alloys through a modeling approach. In this model, the complicated interplay among the external stress, growth strain and creep strain, the oxide stress and thickness was identified. The stress accumulation in the oxide and the variation of scale thickness along with time and the effect of oxidation behavior on the creep properties of underlying metal during oxidation process can be expected. Generally, the external tensile stress would promote the growth rate of oxidation layer. Introducing a small external stress may lead to obvious changes of magnitude and state of stress in oxidation layer. At a given external stress, the creep strain rate was not kept constant due to the oxidation layer growth and stress transfer between oxidation layer and underlying metal.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

Einfluß von Natriumchlorid auf die Oxidation von hochlegierten Chrom- und Chrom-Nickel-Stählen

Effects of sodium chloride on the oxidation of high alloy Cr- and Cr-Ni-steels The effects on the oxidation were investigated of solid NaCl deposits on the oxide scales of the technical steels X10 CrMoV 12 1, X15 CrNiSi 20 12 and X10 CrNiSi 25 20 at 700°C. Independent of the alloy composition the presence of NaCl(s) initiated a markedly accelerated Fe₂O₃ growth on the surface of preoxidized samples, under formation of voluminous, nonprotective layers. Below these scales on the metallic matrix in all cases chloride was detected. The oxides grow according to the mechanism of active oxidation in which chlorine plays a catalytic role. The presence of the chloride at the oxide/metal interface reduces the adhesion of the oxide scale and leads to spalling upon cooling to room temperature. The effects observed are independent of the alloy composition, however, the thickness of the oxide scale is decisive which means the diffusion distance for the gaseous iron chloride.     

Oxidation and embrittlement of Ti-6Al-2Sn-4Zr-2Mo alloy

Ti-6Al-2Sn-4Zr-2Mo titanium alloy is a candidate material for multiwall thermal protection system concepts for advanced space transportation system vehicles. The total oxidation kinetics for this alloy, exposed to laboratory air in the 593–760°C range, were monitored by thermogravimetric analysis. The oxide thickness was measured by microscopy and the substrate contamination was estimated from microhardness measurements. Tensile elongation was determined for selected foil specimens after exposure to simulated space shuttle reentry conditions. The variation of total weight gain with time was found to have two distinct parabolic stages separated by a transient region. This transient was due to a process which involved an increase in the parabolic growth rate constant for the oxide and a simultaneous increase in oxygen solubility at the oxide metal interface. The time dependent increase in oxygen solubility at the interface was from about 7 at. % in stage 1 to about 18 at. % in stage 2. The diffusion coefficient for oxygen in the alloy was determined as a function of temperature using the difference between the total weight gain in stage 1 and the corresponding weight gain due to oxide growth. A model for the total oxidation kinetics, accounting for the two individual components namely oxide growth and solid solution formation, is proposed. The activation energy for the diffusion of oxygen in the alpha-solid solution is shown to be roughly equal to the activation energy for the degradation of tensile elongation for the alloy in the foil gage condition.     

Oxidation Mechanism of Steels in Liquid–Lead Alloys

The oxidation mechanism of steels in liquid–lead alloys (lead or lead–bismuth) was studied. Parametric dependencies of oxidation, including oxygen-concentration effects, oxidation-rate constant and corrosion-rate effects, are analyzed. An oxidation model is developed based on the assumptions that the chemical reactions are at equilibrium locally, and scale removal is due to mass-transfer corrosion. The model shows that outward-iron diffusion in the solid phase (oxide layer) controls the oxide growth and mass-transfer rate in the flowing-boundary layer determines the corrosion-product transport in the liquid phase (liquid–lead alloy). The oxide thickness depends on both the parabolic oxide-growth-rate constant and the mass-transfer-corrosion rate. For long-term operation, the outer layer of a duplex-oxide layer can be completely removed by flowing lead alloys and it is expected that a pure-chromium-oxide layer forms underneath the Fe–Cr spinel if iron is heavily depleted. The oxide thickness and steel weight change are very different from those of the pure parabolic law and they are classified into distinct and universal categories. The model is validated partially by application to interpreting the measured oxide behavior of several steels in a lead-bismuth eutectic-test loop.     

Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)

This paper is the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth mechanism of a duplex oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer grows by iron diffusion until Pb-Bi/oxide interface whereas the Fe-Cr spinel layer grows, at the metal/oxide interface, inside the space kept “available” by the iron vacancies accumulation due to iron outwards diffusion for magnetite formation. This growth mechanism is close to the “available space model”. However, this model is completed by an auto-regulation process based on oxygen supply.     

Stratified oxide scale growth on two Cr-Ni steels oxidized in high-pressure steam at 800°C

A commercial 18% Cr-11% Ni steel and a laboratory-made 21% Cr-11% Ni alloy have been oxidized in steam at 50 atm pressure and with approximately 0.2 ppm O₂ at 800°C for 50–1000 hr. The oxide scales have been studied by optical microscopy, replica electron microscopy, x-ray, and selected-area-electron diffraction and microprobe analysis. The diffusion processes have been studied by gold markers and autoradiography by means of the reaction¹⁸O (p, n)¹⁸F. On both materials internal oxidation in combination with external scale formation is observed. The interface between the external scale and the inner internally oxidized scale coincides very closely with the original metal surface. The oxide phases present have been identified. The scale is of fairly even thickness on the 21% Cr-11% Ni alloy and grows by a cubic rate law. The scale on the commercial steel is of very irregular thickness; areas with protective Cr-oxide alternate with areas of the above-mentioned structure, so-called nodules. The zones of internal oxidation in the nodules are frequently intersected by bands of compact oxide. The growth mechanism of the nodules is discussed in terms of the theory for internal oxidation. The reason for the beneficial effect of increased Ni content with constant Cr content is discussed briefly.     

The role of Cr atom in the early steam oxidation of Fe-based alloys: An atomistic simulation

This study employed the density functional theory to capture the atomic-level dissociation processes of steam and investigate the ions migration on Fe(001) and FeCr(001) surfaces, revealing the role of Cr atom in the early oxidation. Various coadsorption structures with different steam-derived species have been systematically examined to find the most energetically favored surface site. The results showed that the steam dissociation on the alloy surface underwent two steps. First, H₂O molecule on the top site was dissociated into OH group and H atom, which further combined with metal atoms on the bridge site and hollow site. Second, the OH group was decomposed into O and H atoms, which moved to two adjacent hollow sites and generated an oxide. On further oxidation, the Fe atom migrated outward and formed an outer Fe oxide, whereas the Cr oxide could only grow inward as O atom passed through oxide. It was found that the presence of Cr atom on the surface was thermodynamically beneficial, which could promote steam oxidation. The Cr atom could effectively block ion diffusion across the oxide scale and protect the underlying substrate from further oxidation. These results were in good agreement with experimental observation.     

Effect of reactive element oxide inclusions on the growth kinetics of protective oxide scales

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.     

The influence of HCl on the oxidation of iron at elevated temperatures

A study has been made of the effects of 0.1 to 1.0% HCl on the oxidation of iron in argon – 20% O₂ at 600 °C and 700 °C. The overall weight‐change kinetics included contributions from weight increases due to growth of oxide and chloride scale by solid‐state diffusion and to deposition of oxide following vaporization of FeCl₂ and subsequent reaction with oxygen, and weight losses due to complete removal of FeCl₂ vapour to the environment. The weight gains in HCl‐containing gases were less than that in the HCl‐free gas at 600 °C, but greater than that in the HCl‐free gas at 700 °C. The kinetics could be described by a modified Tedmon equation; it was found that much of the scale growth resulted from the chloride‐to‐oxide reaction following vaporization of FeCl₂ from local sites on the metal surface while the contributions from solid‐state diffusion were relatively small.     

划痕诱发的镍基合金应力腐蚀裂尖扩展驱动力分析

为了解表面划伤导致的不同氧化物形貌对镍基合金应力腐蚀(SCC)行为的影响,模拟了膜致应力下镍基合金划伤裂纹尖端的局部应力应变场。结果表明,楔形力是引发SCC裂纹扩展的主要驱动力。划痕裂纹前端的氧化物越厚,楔形力越大,并会增大SCC裂纹扩展速率。裂尖氧化物的形成导致了压应力、压应变和负的应变速率,并会阻碍半椭圆裂纹尖端上部和下部的SCC裂纹扩展。     

High-temperature oxidation of pure titanium in CO2 and Ar-10%CO2 atmospheres

The oxidation behavior of pure titanium has been investigated in the temperature range of 1000 K to 1300 K in CO₂ or Ar-10%CO₂. Optical microscopy, electron probe microanalyses, and X-ray measurements on the oxide scales formed during oxidation indicate that their structures are nearly independent of temperature and the corrosion atmosphere. The scales consisted of two layers, an external one and an internal one, having a rutile (TiO₂) structure. The parabolic rate law was confirmed for growth of the external scale and the permeation depth of oxygen in titanium with apparent activation energies of 266 and 226 kJ/mol, respectively. The rate-determining diffusion species in the oxidation processes are discussed.     

Einfluß von Chloriden auf die Oxidation des 2¼ Cr-1 Mo-Stahls

Effects of chlorides on the oxidation of the 2¼ Cr-1 Mo steel Effects of NaCl deposited on oxide scales were investigated for the oxidation of the 2¼Cr-1 Mo steel in the temperature range 450–650 °C. The presence of NaCl causes accelerated oxidation under formation of porous and cracked Fe₂O₃ layers and of solid FeCl₂ at the metal/oxide interface. By reaction of the chloride with the oxide scale Cl₂ is formed, which penetrates into the scale and forms FeCl₂ at the inner phase boundary, this is evaporating continuously and is oxidized to iron oxide upon diffusing to the surface, whereby the chlorine is set free again. An active oxidation results which is catalysed by Cl₂. The rate of the accelerated oxidation is determined mainly by the evaporation and outward diffusion of gaseous FeCl₂ and therefore by temperature by temperature and thickness of the oxide scale. Other parameters such as the flow velocity, the area covered with NaCl or the chlorine pressure, which is present at the outer oxide scale have less effect on the overall rate.     

The complementary nature of x-ray photoelectron spectroscopy and angle-resolved x-ray diffraction part II: Analysis of oxides on dental alloys

X-ray photoelectron spectroscopy (XPS) and angle-resolved x-ray diffraction (ARXRD) were used to analyze the oxide layer on three palladium-gallium-based dental casting alloys. The oxide layers were approximately 10 Μm thick. The use of the techniques helped to determine which mechanism was responsible for oxide formation—either (a) oxide layer growth via diffusion of oxygen through the scale to the metal, causing the scale to grow at the metal-oxide interface, or (b) an oxide layer formed by metal ions diffusing through the scale to the surface and reacting with oxygen, causing the scale to grow at the oxide-air interface. The oxide growth mechanisms were correlated to previous layer adhesion results determined with biaxial flexure testing.     

温度和相变影响的弹塑性本构关系及应用

通过考虑温度和相变对等效物性参数的影响,建立了包含附加应力和应变的弹塑性增量本构关系,并采用该本构关系和ABAQUS软件构建了淬火过程的数值模拟平台.该本构模型引入的附加应力和应变分别由温度变化和相演化引起的.具有明确的物理意义.通过对26Cr2Ni4MoV钢的圆筒零件水淬过程的模拟表明,该模型能够正确地分析残余应力沿半径方向的分布规律.并且在截面中间部位残余应力值和靠近外表面的应力峰值与实测值更加接近,说明全面考虑温度和相变影响的弹塑性增量本构关系可改善模拟计算的精度.     

Influence on the oxidation kinetics of metals by control of the structure of oxide scales

An explanation of the deviation from the parabolic law is the treatment which considers both shortcircuit and lattice diffusion in the oxide scale. In this study we examine how the oxidation kinetics are influenced by changing the structure of the scale of copper oxide in order to confirm the role of short-circuit diffusion in determining the oxidation rate. In addition we explain the oxidation kinetics of copper and nickel by using a model of the scale structure which includes recrystallization and grain growth. Results are as follows: (1) The nucleation and growth behavior of oxide have a direct effect on the structure and in turn the oxidation kinetics due to short-circuit diffusion. (2) A modified treatment is valid in the region where volume diffusion and short-circuit diffusion play an important role in which it is necessary to consider the scale structure such as the grain size distribution and the boundary width. (3) When recrystallization takes place it is necessary to consider the model of a two-layered scale structure which is different in properties and morphology. (4) In this region the rate curves are S-shaped when oxide recrystallization takes place and exhibit a transition from a parabolic to an nth-power relationship (n>2) when grain growth takes place.This research was performed at the University of Tokyo in partial fulfillment of the requirements for the degree of Doctor of Engineering.     

Oxidation mechanism of a Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy (Part III)

This paper is the third part of a global study on the oxidation process of a Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth kinetics simulation of a duplex oxide scale. The oxide layer is composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer growth is controlled by iron diffusion inside the Fe-Cr spinel and the magnetite lattice. An analytical simulation, matching with the experimental kinetics, is performed. The Fe-Cr spinel layer growth kinetics is also simulated. The protective scale is determined by simulation and by an original experiment using GD-OES.     

The Effect of Platinum on β-NiAl/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Interfacial Tensile Stress: A Combined Ab Initio DFT and Mechanics-Based Model Study

The beneficial effect of adding Pt in diffusion β-NiAl coating on reducing the tensile stress normal to the coating/α-Al₂O₃ scale interface has been investigated using a combination of ab initio density functional theory (DFT), phonon dispersion theory and mechanics-based interfacial stress modeling. The coefficient of thermal expansion (CTE) for Pt, β-NiAl and β-NiAl-6.25 at% Pt was calculated using the total energy DFT combined with the phonon dispersion theory. The calculated CTE of β-NiAl and β-NiAl-6.25 at% Pt and experimentally measured CTE of α-Al₂O₃ were used to evaluate the tensile stress of the undulated β-NiAl/α-Al₂O₃ and β-NiAl-6.25 at% Pt/α-Al₂O₃ interfaces resulting from the CTE mismatch between the coating and the oxide scale during cooling from elevated temperatures. It was found that the addition of Pt to β-NiAl is capable of lowering the interfacial tensile stress as a result of the reduced CTE of the Pt-modified β-NiAl coating, thus beneficial for improving thermal cyclic durability of the coating. The calculated results showed that the interfacial tensile stress is a function of oxide scale thickness and the interfacial wave amplitude and wavelength of an undulated interface. A thicker oxide scale and a rougher interface with a larger ratio of wave amplitude versus wavelength yield higher interfacial tensile stresses during thermal cycling. The addition of 6.25 at% Pt to β-NiAl coating reduces the coating/oxide scale interfacial tensile stress by about 27% over a wide range of scale thickness and interfacial wave amplitude and wavelength.