Which tool to distinguish transient alumina from alpha alumina in thermally grown alumina scales?

Abstract

Alumina scales constitute excellent protective barriers when they form on alumina-forming steels. If they keep tightly adherent to the underlying substrate, they isolate it from the surrounding aggressive atmosphere at high temperature. The protectiveness of the alumina scale is highly dependant upon its growth mechanism. The nucleation and transformation of transient alumina (mainly γ-Al₂O₃ and θ-Al₂O₃) is known to play an important role on alumina scale formation. It is therefore fundamental to characterise these transient alumina especially during the early stages of the oxidation process. The morphology of the transient alumina was observed by scanning electron microscopy (SEM), their crystallographic phases determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectrometry (XPS) analyses were performed on reference samples and then compared to the alloys oxidised 5 and 30 minutes at 850, 900 and 950°C.     

Microstructural evidence for counter-diffusion of aluminum and oxygen during the growth of alumina scales

Abstract

A novel, two-stage oxidation experiment is described that enables the outward diffusion of cations in alumina scales during high-temperature oxidation to be analyzed on the basis of microstructural changes in the surface morphology of the scale. Using this technique, observations of aluminum out-diffusion along α-Al₂O₃ grain boundaries during oxidation of Fe–Cr–Al alloys, nickel aluminides and platinum-modified NiAl bond-coats are made. Although microstructural evidence for the inward grain boundary diffusion of oxygen is more difficult to obtain, it still can be demonstrated by the growth of the oxide above interface cavities on nickel aluminides and inside internal cracks in the alumina scales during cyclic oxidation of zirconia top-coated material. SEM examination of the crack surfaces after scale spallation provides a vivid illustration of two simultaneous processes, aluminum outward and oxygen inward diffusion along grain boundaries in the scale.     

Analysis and TEM examinations of corrosion scales grown on alloy 690 exposed to PWR environment

Abstract

The radioactivity in the primary circuit of pressurized water reactors (PWR) mainly comes from the activation of corrosion products in the core of PWR; corrosion products dissolve from the oxide scales developed on steam generator tubes of alloy 690. The controlling of this process, oxidation and dissolution, requires a detailed knowledge of the microstructure and chemical composition of oxide scales. Alloy 690 was studied as tubes and plates with three various surface states (as-received, cold-worked, electropolished). Corrosion tests were performed at 325°C and 120 or 155 bar in primary water conditions (B/Li(1000/2 ppm, [H₂] = 30 cm³ kg^?1 TPN, [O₂]<5 ppb); test durations ranged between 24 and 2160 hours. Corrosion tests were carried out in a static autoclave made of stainless steel, and in one recirculating autoclave made of titanium alloy (TITANE recirculating autoclave). Characterisation of the oxide scales grown in representative conditions of the primary circuit was performed by several techniques (SEM, (X)TEM, SIMS, XPS, GIXRD, RBS). These analyses revealed the essential role of the finegrained cold-worked superficial zone of perturbed microstructure presents on as-received and coldworked specimens. They also gave some insights on the growth mechanisms of corrosion products scales.     

Analytical study of the evolution of alumina scales formed on an ODS alloy

Abstract

An original deflection technique, in association with TEM observations, allowed study of the transformation of metastable alumina phases to α phase. This transformation is mainly characterised by a 14% volume decrease. Deflection experiments were performed on PM 2000 after pre-oxidation, in order to create a 3 µm alumina scale, and subsequent mechanical oxide removal of one large sample face. This technique allowed evaluation of the kinetics of transformation. The microstructural and chemical evolutions of the oxide, the interface (morphology, segregation) and the alloy (aluminium depletion, oxide dispersion) were characterised using TEM analyses and thermogravimetric measurements were done in the same conditions as the deflection tests to determine the oxidation rate constants     

Effect of yttrium on elastic strains and fracture of Cr2O3 scales on Ni-30Cr alloys

It has long been known that the addition of so-called reactive elements, such as Y, to alloys forming chromia scales improves the protectiveness of those scales by reducing the rate of scale growth and by reducing the tendency of the scales to spall during thermal cycling. It has been suggested that reduced spallation might arise from better scale adherence or from improved resistance to fracture.

In this study Ni-30Cr and Ni-30Cr-0.5Y alloy samples were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. Scale fracture events, which are detected by acoustic emission, were collected throughout the experiment. An in situ X-ray diffraction technique was used to measure the elastic strains in the oxide scales at the isothermal scale growth temperature and at several temperatures during cooling.

These measurements found higher elastic strains in the Cr₂O₃ scales, during both isothermal oxidation and cooling, but for fewer cracking events, on the yttrium-containing alloy than on Ni-30Cr. We infer that the addition of yttrium increased the adherence of the scale to the substrate, since the scale was able to withstand higher elastic stresses.     

Microstructural study of the theta-alpha transformation in alumina scales formed on nickel-aluminides

Abstract

Luminescence spectroscopy and scanning electron microscopy have been used to study the transformation from theta-Al₂O₃ to alpha-Al₂O₃ on the surface of platinum-modified nickel aluminide, (Ni,Pt)Al, bond coats on superalloys during initial stages of oxidation at 1,000-1,200°C. The transformation can proceed in a number of ways, depending on the surface roughness, leading to different microstructures of the stable alpha-alumina scale. The use of the luminescence method makes it possible to correlate microstructural features of the oxide with the local stress through piezospectroscopic shifts of both alpha- and theta-alumina optical spectra.     

High spatial resolution imaging of the segregation of reactive elements to oxide grain boundaries in alumina scales

Abstract

Although it is well established that reactive elements such as yttrium and hafnium can segregate to oxide/metal interfaces and oxide grain boundaries in thermally grown oxides, their distribution and role at these sites are less certain. For example, their effect on oxide growth, scale plasticity and spallation is still debated. It has also been reported that hafnium and yttrium rich oxide particles can be present within growing alumina scales and that the growth or shrinkage of these particles can affect the Y and Hf distributions in the aluminium oxide grain boundaries in their vicinity. Hence, we now report the use of very high spatial resolution imaging in the SuperSTEM electron microscope to investigate the distribution of Y and Hf in aluminium oxide grain boundaries at the atomic level.

The oxide scales studied were detached from Fe – 20Cr – 5Al alloy substrates doped with Y and Hf, which had been oxidised for up to 100 h at 1250°C in laboratory air. The scales were ion beam thinned prior to examination in the STEM, and a series of tilting experiments and through focal series were used to map out the distributions of the reactive elements. The influence of electron beam/sample interactions was also studied and some evidence for the movement of Hf and Y atoms along the grain boundaries to the surfaces of the thin oxide foils is also reported.     

Direct observations and analysis of the spallation of alumina scales grown on PM2000 alloy

The spallation during cooling or thermal cycling of alumina scales grown at high temperature on PM2000 alloy is directly observed and recorded using CCD cameras. This simple and direct method permits to determine easily and accurately the scale spallation kinetics. In addition, it provides many insights on the spallation mechanisms and processes and their evolution as well as on the effect of substrate microstructure on spallation.     

Photoelectrochemical and Raman imaging studies of chemical and mechanical properties of thermally grown titanium oxide scales

Abstract

The present work focuses on thin TiO₂ scales (up to 3 μm). The use of both micro-photoelectrochemistry (MPEC) and Raman imaging aims to get chemical and mechanical information at a micron scale. In particular, the MPEC technique allows images of the history of strain and adhesion levels of the oxide/metal interface to be realised. A set of photocurrent images recorded at different applied potentials allows different images to be constructed, Structural Quality Image (SQI) and Interface Cohesion Image (ICI). In addition, Raman spectroscopy images the chemical nature of the scale, topography of the surface and local stress level in the scale. Combining both techniques is a powerful tool in elucidating the mechanisms of thermal oxidation.     

On the application of SIMS to study the oxidation mechanisms of alumina formers

Abstract

Secondary ion mass spectrometry (SIMS) is widely used to investigate the oxidation mechanism of alumina formers, being various MeCrAl (Me=Fe, Ni, Co) alloys and β-NiAl intermetallic compound. Frequently, it is combined with dedicated oxidation exposures, mostly in atmospheres containing different amounts of oxygen tracer (¹⁸O₂) in order to investigate the scale growth mechanism. However, there are several practical aspects which have to be taken into account during planning the oxidation experiments as well as during interpretation of their results in order to avoid misleading conclusions. In particular, the effects related to the formation of non-uniform scales as well as to the relationship between the exposure conditions and the scale microstructure and morphology at various stages of its growth should be dealt with.

This paper discusses the implications of sputtering process with respect to elemental in-depth distribution profiling and the results of the oxidation mechanism investigation of various alumina formers by means of SIMS. It is shown that the obtained shapes of the SIMS profiles and images strongly depend on the choice of the exposure conditions and on the analysis parameters, including the spatial resolution. The exposure conditions are related to the scale microstructure and morphology, the evolution of which occurs during the oxidation process. Formation of duplex alumina scale is reported which comprises an inner more compact sub-layer and an outer ridged and less-compact sub-layer.     

The influence of reactive element additions to β-NiAlCr alloys on the morphology of thermally grown oxides

Abstract

The effect of the reactive elements (REs), Y and Zr, on oxidation of β-NiCrAl alloy at 1373 K in a gas mixture of argon with 20 vol.% oxygen at atmospheric pressure was evaluated using X-ray diffractometry, scanning electron microscopy, electron probe X-ray microanalysis and secondary ion mass spectroscopy. The oxide surfaces and interface morphologies, compositions and growth kinetics were studied for alloys with 0.32 at.% Zr and 0.24 at.% Y additions and for an undoped alloy. The oxide layer produced on the three different alloys contains mainly α-alumina and some intermediate alumina modification, Cr₂O₃ and RE-oxides. A needle-like morphology was seen on top of the oxide layer for the undoped and Zr alloy. Needle formation on the Y alloy was suppressed by the formation of a thin Y₂O₃ layer during the initial stage of oxidation. Needles were maintained to long oxidation times for the undoped alloy, but disappeared on the doped alloys indicating that some cation diffusion is possible when REs are present. Fewer intermediate alumina modifications are seen for the oxide layers on the RE alloys showing that the REs promote the formation of the α-alumina phase. Oxide layer growth occurs in two stages for all alloys. Initially, oxide growth is rapid with outward diffusion of aluminium. The second stage of oxidation is slow and is initiated by the formation of a closed α-alumina layer limiting further oxidation to inward oxygen diffusion. This stage is characterised by parabolic growth kinetics associated with a constant aluminium interface concentration. The oxide layer is thinnest for the Y alloy due the fine Y₂O₃ layer acting as a diffusion barrier. The oxide/alloy interface for the undoped alloy is flat and shows many voids, whereas voids are not seen for the RE alloys. This is due to the promotion of a closed α-alumina layer giving predominantly inward growth early in the oxidation process. Oxide pegs of the RE are also seen growing into the alloys. The lack of voids and the oxide pegs are advantageous for oxide layer adhesion to the doped alloys.     

硫酸中阳极氧化制备Al2O3多孔模板的BPANN模拟

采用24组硫酸阳极氧化多孔膜的样本数据建立BP神经网络模型,得到了多孔膜孔有序度与硫酸阳极氧化参数之间的最优关系,经检验该模型的预测结果与实测结果相一致.表明BPANN 是硫酸阳极氧化制备多孔膜孔有序度的较好的预测模型.     

TEM analysis of the growth of oxide scales at different temperatures in FeAl grade 3 intermetallic alloy

Abstract

Upon the isothermal oxidation of an ODS FeAl Grade 3 intermetallic alloy, a structured oxide scale is developed between 800 and 950°C. TEM studies have revealed a three-layered structure with a top nanoequiaxed alumina, a central alumina and a bottom Fe-Al spinel. At these temperatures, aluminium outward diffusion does not seem to be suppressed neither by the spinel sub-layer nor by the yttria present in the superalloy. However, the formation of metastable θ-Al₂O₃ seems to be significantly hindered. On the contrary, at 1000°C, the spinel phase no longer forms but a columnar alumina layer topped up with a nanoequiaxed structure. Yttria seems then to segregate at the scale/alloy interface and to randomly coarsen to produce an Y-Al oxide phase.     

Local and microstructure-related effects affecting the high temperature oxidation of alumina formers: a brief survey

Abstract

Local effects such as precipitation of alloyed additions, segregation of impurities and scale mechanical failure through its cracking and/or spallation often play an important role in oxidation-induced degradation of high temperature materials. This paper reviews a number of effects and processes which either are entirely local or exhibit different extents or rates in various regions in the scale, at the scale-substrate interface and/or at the substrate surface. Special attention is paid to the implications of the starting material inhomogeneity, various types of mechanical failure in the oxide layer, oxidant penetration towards and into the substrate and finishing of component edges. The experimental evidence was obtained using commercial and synthetic alumina formers oxidised under isothermal and thermal cycling conditions. The observed effects are discussed in terms of their influence on the oxidation and degradation rate and mechanisms at temperature range relevant to the application of alumina formers, i.e. exceeding 1070 K.     

TEM investigation of the oxide scales formed on a FeCrAlRE alloy (Kanthal AF) at 900°C in dry O2 and O2 with 40% H2O

Abstract

The base oxide scales on a commercial FeCrAl alloy oxidized isothermally at 900°C in dry O₂ or O₂ with 40% H₂O were studied in detail using analytical electron microscopy. Electron transparent cross-section foils prepared with a FIB/SEM in-situ lift-out technique were investigated using STEM/EDX and CBED. The oxide scales on the samples exposed to dry O₂ are slightly thinner than the scales formed in O₂+H₂O. The oxide scales exhibit a multilayered structure, with a Cr-rich layer in the middle, indicating the original metal/gas interface. An almost pure inner α-Al₂O₃ layer, containing columnar grains, was formed by inward oxygen diffusion, after exposures in both the dry and wet atmospheres. The outer oxide layer consisted of γ-Al₂O₃ in the wet case and of α-Al₂O₃/MgAl₂O₄ in the dry case. It is suggested that the α-Al₂O₃/MgAl₂O₄ phases resulted from a phase transformation of initially grown γ-Al₂O₃. The observations indicate that water vapour may stabilize the γ-Al₂O₃ phase.     

氧化物弥散强化高温合金抗氧化性能的研究进展

氧化物弥散强化商业高温合金因氧化物颗粒在基体中的弥散强化作用而具有较好的高温力学性能,如今被广泛应用于航空航天、能源、汽车等领域的高温部件。研究发现,氧化物颗粒的掺杂不仅可以使合金基体具有优异的高温强度,还可以显著提高基体的抗氧化性能。概述了氧化物颗粒种类、尺寸和含量对高温合金抗氧化性能的影响,从合金初期氧化行为、氧化膜生长速度、生长机制、粘附性能等角度重点关注不同性质氧化物(如活性元素氧化物和非活性元素氧化物)弥散质点在氧化过程中作用机理的异同,最后对未来的研究方向做出了展望。     

A TEM and SEM-cathodoluminescence study of oval defects in graded InGaAs/GaAs buffer layers

The nature of oval defects in compositionally graded InGaAs/GaAs buffer layers has been investigated by means of Transmission electron microscopy (TEM) and Cathodoluminescence (CL). CL spectra show that emissions from within the defects have energies lower than that from the defect-free regions, thus demonstrating that the defects are richer in In than the surrounding matrix. No In particles have been detected within the defect core. TEM investigations reveal that the defects form in the topmost strained regions close to the surface and originate from stacking faults.     

数值模拟在氧化铝陶瓷SLS/CIP过程中的应用

采用选择性激光烧结(SLS)和冷等静压(CIP)复合工艺制造高密度形状复杂的氧化铝零件,经高温烧结,氧化铝零件密度高达94.5%.应用修正的Cam-Clay模型模拟了零件的CIP过程,同时通过模拟和实验研究其在CIP过程中的致密化、变形及收缩规律.研究表明,在激光扫描平面内模拟与实验的尺寸相对误差不超过2.26%,证明该模拟可为氧化铝SLS零件的尺寸设计和CIP过程提供有利指导.     

氧化铝碳热还原氯化法制低价氯化铝反应的热力学研究

对氧化铝碳热还原氯化法制低价氯化铝的反应进行了热力学研究。经过热力学计算得到：在常压下，该反应发生的起点温度为2121．21K。而当系统压力下降时。该反应的起点温度也会下降；经过计算得到了该反应起点温度与系统压力间的关系，也得到了不同系统压力下反应吉布斯自由能与系统温度间的关系。