Effects of aluminum on the oxidation of 25Cr-35Ni cast steels

The oxidation kinetics and morphological development during reaction of two cast austenitic steels at 1000°C in pure dry oxygen at 20 kPa are reported. Both steels contained approximately 25 wt.% Cr and 35 wt.% Ni and, in addition, one steel contained 3.3 wt. %. Both steels oxidized to form external scales consisting mainly of Cr₂O₃ with a thin outer layer of manganese rich spinel. Scale growth kinetics were parabolic, and somewhat faster rates were observed for the aluminum bearing steel. In both steels, deep internal oxidation occurred at the site of primary (interdendritic) carbides. The kinetics of this process were parabolic, and rate control was attributed to oxygen diffusion along the interface between internal oxide and matrix metal. In the aluminum-free steel, interdendritic carbides were converted to chromium rich oxide, but when aluminum was present, a sheath of aluminum rich oxide formed around the carbides. In this latter case, the rate of interdendritic penetration was somewhat slower. The aluminum bearing steel also formed large numbers of rod-shaped Al₂O₃ precipitates within the austenitic dendrites. Deepening of the Al₂O₃ precipitate zone also proceeded according to parabolic kinetics at a rate consistent with rate control by diffusion of oxygen along the oxide-alloy interfaces.     

Sulfate-induced high-temperature corrosion of nickel

Corrosion of high-purity nickel with deposits of sodium sulfate has been studied at 900°C in O₂+4.2 % SO₂ at gas pressures ranging from 700 to 0.5 Torr. Corrosion rates in the gas mixture are faster than in oxygen or sulfur dioxide alone. At the higher pressures the initial reaction is parabolic, but subsides after extended reaction. The parabolic reaction is concluded to comprise a diffusion-controlled transport of oxygen atoms and sulfur dioxide molecules through the molten sulfate layer with dissolved nickel oxide. Ni-S liquid solution is formed at the metal surface during the reaction. At reduced gas pressures the initial reaction is nonparabolic (linear and subsequent gradually increasing rate) prior to the parabolic stage. After extended reaction at sufficiently low partial pressure of SO₂ or reaction in 1 atm O₂ significant amounts of Ni-S liquid solution cannot be detected at the metal surface below the sulfate melt. The reaction mechanism is illustrated and discussed in terms of the stability diagrams of the Na-O-S and Ni-O-S systems.     

SiC/Ti6Al4V复合材料界面反应扩散系数和微观扩散机理

采用三元体系半无限扩散偶的高斯方法,求解了SiC/Ti6AL4V复合材料界面反应层中相关元素的扩散系数,计算的浓度分布和实测值一致.碳原子通过反应层的扩散服从间隙扩散机制,硅原子的扩散为空位扩散机制.由于碳扩散的振动能最低并且跃迁距离最短,而供硅扩散的空位不足,碳和硅在反应产物TiCx中具有最小的内禀扩散系数,分别为8.9403×10-16和4.7747×10-16 m2·s-1.研究表明,在SiC/Ti6AL4V复合材料界面反应的过程中,反应元素通过反应层TiCx的扩散是一个主要的控制步骤.     

Initial Oxidation of Pure Iron and the Effect of the Allotropic Transformation

Abstract

A volumetric method has been used to study the kinetics of the initial oxidation of oure iron at two pressures of dry oxygen (viz. _p O₂ = 7·6 and 29 cm mercury) in the range 800–1000° in which the first allotropic transformation point of iron occurs. It was found that the oxidation of both body-centred cubic (b.c.c.) and face-centred cubic (f.c.c.)–iron at _p O₂ = 7·6 cm and of only b.c.c. at _p O₂ = 29 cm follows a modified parabolic law; a true parabolic law was found to operate in the oxidation of f.c.c.–iron only at _p O₂ = 29 cm. An anomaly, which is obtained in the Arrhenius plot at the allotropic change point of iron (～911°), is attributed to the effect of phase-boundary reactions on the oxidation kinetics. It would appear from the concept of vacancy distribution in iron, that for a modified parabolic law, it is the metal/oxide interfacial reaction in f.c.c.–iron and the oxide/oxygeninterfacial reaction in b.c.c.–iron that governs the phase-boundary controlled reaction rate.     

Energetics and atomic transport at liquid metal/Al2O3 interfaces

The objective is to study interfacial mass transport mechanisms and to estimate interfacial energies for metal/Al₂O₃ systems. Experiments have been performed with molten drops of Ni, Cu, or Au on pure, polycrystalline alumina at oxygen partial pressures for which no adsorption is expected and with Al to determine the effect of extremely low p(O₂). After removing the metal drops, grain boundary grooves at the interface and oxide surface have been analyzed using AFM and SEM. Several sources of error are assessed, and corrections are proposed for large systematic errors that occur for root angles. These experiments yield higher grain boundary energies and lower M/Al₂O₃ interfacial energies than previously reported. Transport rates near the metal/ceramic interface are two to four orders of magnitude faster than on the oxide surface and the results suggest that diffusion through the liquid metal is usually the main atomic transport mechanism. Experiments with Al indicate that, at the far lower oxygen activities, transport is faster at both the interface and alumina surface and that the interfaces are more anisotropic and have lower energy.     

铁/铝扩散偶界面反应层生长机理分析

在不同的加热温度和保温时间条件下,对铁/铝扩散偶的元素扩散特征和界面反应层形成机理进行了探讨.结果表明,保温时间较短,界面结构为纯铁/FeAlx+Al/FeAl/纯铝,保温时间超过某一临界值,不稳定的FeAlx、准稳定的FeAl将转化为稳定的Fe2Al5和FeAl3金属间化合物,最终界面反应层结构为纯铁/Fe2A15+...     

SiC/Ti6Al4V复合材料界面反应产物的形成序列及扩散路径

通过SiC/Ti6Al4V钛基复合材料的制备及在不同条件下的热处理试验，利用SEM，EDS及XRD分析技术研究复合材料界面反应产物相的形成及反应元素的扩散路径。结果表明：反应元素如C，Ti，Si在界面反应层中出现浓度波动，合金元素Al并没有显著扩散进入界面反应产物层，而是在界面反应前沿堆积，其界面反应产物被确认为Ti3SiC2，TiCx，Ti5Si3C，和Ti3Si；在界面反应初期，存在着TiC＋Ti5Si3Cx双相区，当形成各界面反应产物单相区时，SiC／Ti6Al4V复合材料界面反应扩散的完整路径应为：SiC ｜ Ti3SiC2 ｜ Ti5Si3Cx ｜ TiCx ｜ Ti3Si｜ Ti6Al4V＋TiCx；界面反应产物层的生长受扩散控制，遵循抛物线生长规律，其生长激活能Q^k及k0分别为290.935 kJ·mol^-1,2.49× 10^-2 m·s^-1/2.     

铝/钢固态焊接合界面金属间化合物生长机制

在保持固态条件下,分别变化加热温度、时间对铝/Q235钢爆炸焊接头进行加热处理.分析了接合界面区反应层形貌等微观特征,探讨了加热温度、加热时间对反应层厚度的影响,研究了接合界面金属间化合物的生长行为.界面反应物是由靠近铝合金侧的反应物为Fe₄Al₁₃和靠近钢侧反应物为Fe₂Al₅构成.金属间化合物层随着加热时间的延长而变厚.结果表明,金属间化合物的生长满足抛物线法则,其生长激活能为33.26 kJ/mol.     

Reaction diffusion in continuous SiC fiber reinforced Ti matrix composite

SiC continuous fiber-reinforced pure Ti(TA1)matrix composites were fabricated by a vacuum hot pressing(VHP)methodand then heat-treated in vacuum under different conditions.The interfacial reaction and the formation of interfacial phases werestudied by using SEM,EDS and XRD.The results show that there exists reaction diffusion at the interface of SiC fibers and Timatrix,and the concentration fluctuation of reaction elements such as C,Ti and Si appears in interfacial reaction layer.The interfacialreaction products are identified as Ti3SiC2,TiCx and Ti5Si3Cx.At the beginning of interfacial reaction,the interfacial reactionproducts are TiCx and Ti5Si3Cx.Along with the interfacial reaction diffusion,Ti3SiC2 and Ti5Si3Cx single-phase zones come forth inturn adjacent to SiC fibers,and the TiC Ti5Si3Cx double-phase zone appears adjacent to Ti matrix,which forms discontinuousconcentric rings by turns around the fibers.The formed interfacial phases are to be Ti3SiC2,Ti5Si3Cx and TiCx Ti5Si3Cx from SiCfiber to Ti matrix.The interfacial reaction layer growth is controlled by diffusion and follows a role of parabolic rate,and theactivation energy(Qk)and(k0)of SiC/TA1 are 252.163 kJ/mol and 7.34×10?3m/s1/2,respectively.     

Modeling Two- and Three-Stage Oxygen Tracer Experiments during High-Temperature Oxidation of Metals with a High Oxygen Solubility

The numerical tool EKINOX-Zr has been upgraded to simulate oxygen tracer experiments during the high-temperature oxidation of a metal with a high oxygen solubility limit. The penetration of ¹⁸O tracer is calculated during the dynamic evolution of the ZrO_2-x/α_{Zr(O )}/β_zr system. The numerical approach allows to explicitly take into account the variation of the tracer diffusion coefficient through the oxide scale as a function of the vacancy concentration. A classical two-stages ¹⁶O₂/¹⁸O₂ tracer experiment has been simulated. It is shown that a classical fitting procedure on the ¹⁸O concentration profile obtained for short-time experiments leads to the identification of the oxygen chemical diffusion coefficient. The second type of tracer experiment is proposed using a three-stages ¹⁶O₂/¹⁸O₂/¹⁶O₂ oxidation. It allows the direct estimation of the diffusion coefficient from the transport of ¹⁸O peak in the growing oxide scale.     

Numerical simulation of interfacial reaction between titanium and zirconia

Based on the conservation laws of energy and mass,and taking into account the effect of chemical reaction between liquid titanium and zirconia ceramic mold on the concentration field and the temperature field,a comprehensive mathematical model for numerical simulation of heat and mass transfer has been established to study the interfacial reaction between liquid Ti and ZrO2 ceramic mold.With the proposed model,numerical simulations were preformed to investigate the effects of pouring temperature and holding time on the oxygen concentrations and reactive layer thickness in metal.The results showed that both the oxygen concentration and the thickness of reactive layer in metal increase with the increase of the holding time and the pouring temperature.The development of reactive layer thickness with time consists of three stages:inoculation (0-1 s),linear increase (1-5 s)and parabolic increase (after 5 s).     

High-Temperature Oxidation Behavior of a Polycrystalline Ti2SnC Ceramic

The isothermal-oxidation behavior of polycrystalline Ti₂SnC at 500–800°C in air has been investigated. The growth of the oxide scale on Ti₂SnC from 500°C to 700°C obeyed a parabolic law, whereas at 800°C it was a two-step parabolic process. Microstructure and composition analysis on the surface and sectioned samples demonstrated that the oxidation of Ti₂SnC was controlled by outward diffusion of titanium and carbon, and inward diffusion of oxygen. As the oxidation continued the oxygen potential in the inner layer was low, and metallic Sn was stable as an interfacial layer between the oxide scale and the Ti₂SnC matrix. This work confirmed the presence of metallic Sn in the oxidized Ti₂SnC and explained why metallic Sn was stable in the oxidized Ti₂SnC sample.     

Reoxidation of chromium with densified Cr2O3 Scales

Preoxidized chromium specimens have been high vacuum annealed at 1200° and 1300°C to produce densified Cr ₂O₃ scales. These specimens have been reoxidized at the same temperatures at 10^–6 atm O₂. The initial reoxidation is linear with time and is concluded to reflect a volume diffusion controlled transport through the densified scale. The corresponding parabolic rate constant (w² = k_pt)is given by k_p=1.4 · 10^–2 exp(–235,000/RT)(gram of O) ²/cm⁴ sec. It is tentatively concluded that outward chromium diffusion predominates in an inner layer of the Cr₂O₃ scales and inward oxygen diffusion in an outer layer. Under the experimental conditions it has not been possible to maintain growth of the Cr₂O₃ scales controlled by volume diffusion. The new oxide layer consists of fine crystallites; the oxide grows at grain boundaries within the scales. This causes sideways growth of the scale, breakdown of the originally densified layer, and an increased rate of reaction.     

The influence of oxygen partial pressure on the kinetics of duplex scale formation on 316 stainless steel

A previous study of iron tracer diffusion in duplex scale grown on vacuum annealed 316 steel had suggested that Fe cation diffusion was slower in the outer Fe₃O₄ layer than in the inner spinel. In order to obtain additional information on the location of the rate controlling step during duplex oxidation of 18 Cr-8 Ni type stainless steels, vacuum annealed 316 steel was oxidized at 600°C in atmospheres of controlled oxygen partial pressure covering the whole of the Fe₃O₄ stability range at that temperature.An oxygen partial pressure dependence of the primary parabolic rate of duplex scale formation of k_p ∝ P_O2^0.135 was observed, providing further evidence that the rate controlling process is located in the outer Fe₃O₄ layer.     

Duplex n- and p-Type Chromia Grown on Pure Chromium: A Photoelectrochemical and Microscopic Study

Chromia grown on pure chromium at 900 °C for 30 min at an oxygen partial pressure p(O₂) of 10^?12 atm has been characterized using photoelectrochemical and electron microscopy techniques. This study reveals a duplex scale: n-chromia with equiaxis morphology in the internal part (~650 nm thick) and p-chromia with columnar morphology in the external part (~900 nm thick). Grain orientation maps also revealed the presence of a c-oriented chromia layer at the interface between the n- and the p-subscales. This 〈 0001 〉 textured layer was identified as the first-grown chromia layer. It means that internal n-equiaxis chromia grew by anionic transport governed by oxygen vacancy diffusion, whereas external p-columnar chromia layer grew outwards and was controlled by chromium vacancy diffusion.     

Structure and durability evaluation of YSZ + Al2O3 composite TBCs with APS and HVOF bond coats under thermal cycling conditions

Plasma sprayed thermal barrier coatings (TBCs) are applied to gas turbine components for providing thermal insulation and oxidation resistance. The TBC systems currently in use on superalloy substates typically consists of a metallic MCrAlY based bond coat and an insulating Y₂O₃ partially stabilized ZrO₂ as a ceramic top coat (ZrO₂ 7–8 wt.% Y₂O₃). The oxidation of bond coat underlying yttria stabilized zirconia (YSZ) is a significant factor in controlling the failure of TBCs. The oxidation of bond coat induces to the formation of a thermally grown oxide (TGO) layer at the bond coat/YSZ interface. The thickening of the TGO layer increases the stresses and leads to the spallation of TBCs. If the TGO were composed of a continuous scale of Al₂O₃, it would act as a diffusion barrier to suppress the formation of other detrimental mixed oxides during the extended thermal exposure in service, thus helping to protect the substrate from further oxidation and improving the durability. The TBC layers are usually coated onto the superalloy substrate using the APS (Atmospheric plasma spray) process because of economic and practical considerations. As well as, HVOF (High velocity oxygen fuel) bond coat provides a good microstructure and better adhesion compared with the APS process. Therefore, there is a need to understand the cycling oxidation characteristic and failure mode in TBC systems having bond coat prepared using different processes. In the present investigation, the growth of TGO layers was studied to evaluate the cyclic oxidation behavior of YSZ/Al₂O₃ composite TBC systems with APS-NiCrAlY and HVOF-NiCrAlY bond coats. Interface morphology is significantly effective factor in occurrence of the oxide layer. Oxide layer thickening rate is slower in APS bond coated TBCs than HVOF bond coated systems under thermal cycle conditions at 1200 °C. The YSZ/Al₂O₃ particle composite systems with APS bond coat have a higher thermal cycle life time than with the HVOF bond coating.     

Low Pressure Oxidation of Manganese in Pure Oxygen at 550 °C and 650 °C

Oxidation of manganese in pure oxygen has been studied at 550 °C and 650 °C under different pressures. In all cases we observed the formation of two oxide layers of different thickness and composition. The outer layer undergoes blistering and spalling after a time depending on the experimental conditions as shown also by a change in the kinetic curves. However, the first stage of oxidation follows always a parabolic law, with a rate constant depending on the oxygen pressure. This effect is attributed to a control of the oxide growth by the diffusion through the outer Mn₃O₄ layer. To explain the observed dependence of the parabolic constant on the oxygen pressure, the oxide is assumed to be an electronic conductor with prevailing cation diffusion. Wagner's treatment of the parabolic oxidation is modified to take into account the presence of two kinds of cations in Mn₃O₄. The final expression of K_t shows a reasonable agreement with the experimental results, if the diffusion is attributed to the neutral cation vacancies; this mechanism is also justified by the relatively low working temperatures.     

The influence of borate coatings on the high-temperature oxidation of iron

Coatings of di-sodium tetraborate and of calcium metaborate have been found to protect iron against oxidation in the temperature range 750°–1050°C in oxygen. Protection is due to the compound iron boro-ferrite (4 FeO.Fe₂O₃.B₂O₃) that forms as a non-coherent, granular, blocking layer along the scale/metal interface, which effectively decreases the interfacial area for iron ion diffusion. Void formation, associated with the layer of complex oxide, may possibly enhance the blocking effect.     

Oxidation of FeCrAl alloy: influence of temperature and atmosphere on scale growth rate and mechanism

The oxidation behaviour of a FeCrAl alloy with little rare earth content (Y=0.01 wt.%) was investigated. Specimens of this alloy were submitted to long-term oxidation treatments (up to 30 days) at 900 and 1200°C, under gaseous atmospheres containing 21, 10 and 2 vol.% of O₂. The weight gain for unit area was measured vs. oxidation time. The alumina scale growth was found to occur, at least during the first days of treatment, according to Wagner's parabolic law. Afterwards, the layer rate growth decreases down to that expected on the basis of this law. The values of the parabolic rate constant for scale growth (K_p) chiefly depended on the treatment temperature, while only small variations of K_p resulted from significant changes in treatment of atmosphere composition. The morphology and the composition of surface layers were studied by SEM-EDS and XRD analyses. Whatever the treatment temperature, the surface layer contained α-Al₂O₃ and non-negligible amounts of Cr and Fe. The metal/scale interface was always flat, while the morphology of the scale/gas interface changed greatly with temperature. At 900°C an irregular scale/gas interface formed; this was characterised by the presence of long α-alumina whiskers protruding towards the gaseous atmosphere. Contrary, at 1200°C a flat scale/gas interface was observed. These different morphologies can be attributed to different mechanisms of layer growth.     

ZrO2(ZrB2)活性扩散障界面演变行为的研究

本实验采用电子束物理气相沉积(Electron Beam Physical Vapor Deposition, EB-PVD)技术在镍基单晶合金Rene N5基体上制备出N5/(ZrB₂+ZrO₂)/ NiCrAl和N5/ZrO₂/ NiCrAl两组试样。然后,同时对其进行900℃/5h恒温氧化和1000℃/250h、1000℃/300h及1000℃/350h循环氧化,并采用SEM、EDS分析方法研究了界面演变行为及退化失效行为,结果表明,ZrB₂的加入对活性扩散障界面反应的生成速率有一定的延缓,但不影响具有阻扩散功能的Al₂O₃扩散障层的最终形成;并且,ZrB₂的加入提高了活性扩散障结构的服役寿命,改变了活性扩散障结构的失效方式。