Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys

Abstract

Ultra-high purity Fe–Cr–Al–Y model alloys with controlled additions of impurities such as phosphorus and carbon, and potentially more beneficial elements such as titanium and zirconium have been prepared by induction melting in water-cooled, silver crucibles. 1 mm thick samples were then prepared by hot and cold rolling and annealing prior to cyclic oxidation in air at temperatures in the range 1100–1300°C. Other impurities were kept to a minimum of <10 ppm. scanning electron microscopy, Auger surface analysis and Scanning Transmission Electron Microscopy were used to characterise the samples both before and after oxidation.

Weight gain studies during oxidation showed that the high phosphorus containing alloy went into breakaway very quickly, after only 200 hours at 1300°C, while the Ti and Zr rich samples lasted for 1900 hours and 3300 hours respectively. In some cases, chromium or titanium rich precipitates were found along the alloy grain boundaries, often associated with carbon, while in other cases precipitates were found along the oxide metal interface. Although phosphorus was found at this interface in some of the samples, it was not always present, and may not be the only contributing factor to the premature breakaway failure of the oxides. A complete review of the microstructural evolution of these samples during oxidation will form the main topic of this paper.     

Life-time extension of alumina forming FeCrAl–RE alloys: influence of alloy thickness

Abstract

Oxidation lifetimes of the FeCrAl–RE alloys at ultra high temperatures are governed ultimately by chemical failure of the protective alumina scale, often accelerated by prior mechanical failure resulting in scale cracking and spallation. Critical parameters involved in these failure modes are expected to be influenced by alloy thickness. Therefore, the effect of alloy thickness upon the lifetime oxidation behaviour, in air, at 1,300°C has been studied for three commercial FeCrAl-RE alloys, Kanthal AF, Kanthal APM and PM2000, that have been fabricated by the three principal fabrication routes. As a result, these alloys have different creep properties, which also could have a significant influence on oxidation behaviour.     

High-temperature oxidation of FeCrAl(Y,Pt) alloys in oxygen–water vapour

Abstract

High-temperature oxidation behaviour of FeCrAl(Y, Pt) alloys was studied in oxygen and oxygen – water vapour (47 vol%) for 18 ks at 1473, 1573 and 1673 K, by mass gain measurements, the amount of spalled oxide, X-ray diffraction, scanning electron microscopy and field emission-transmission electron microscopy – energy dispersive spectroscopy. After oxidation at 1473K for 18 ks in oxygen, the mass gain of the FeCrAl alloy with appropriate addition of both yttrium and platinum was the smallest value, and alumina – alloy interface of the alloy showed good coherency by TEM. After oxidation at 1673K in oxygen – water vapour (47 vol%) for 18 ks, the amount of spalled oxide of FeCrAl0.5Pt alloy was zero. This result is thought to relate to the formation of concentrated platinum layer at the oxide – alloy interface. Spalling of oxide scale on FeCrAl and FeCrAl0.1Y alloys was observed, however, oxide scale on FeCrAl0.5Y alloy showed good adherence during five cycles (one cycle=18 ks) in oxygen – water vapour (47 vol%) at 1573 K.     

Oxidation behaviour and microstructural evolution of FeCrAl ODS alloys at high temperature

Abstract

The oxidation behaviour of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (MA 956) and of a dispersion-free FeCrAl alloy (APM) were studied during isothermal exposures in air between 1,100°C and 1,300°C. After short heat treatments, the oxide film developed on MA 956 was more protective than that developed on APM. Longer isothermal exposures were conducted on MA 956 in order to examine the long-term behaviour of the oxide layer and the microstructural evolutions of the alloy. Chemical analyses of the substrate for increasing treatment duration revealed a continuous decrease of the matrix aluminium content due to oxide growth. After 4 months at 1,300°C, aluminium content of the alloy decreased to a critical value below which a continuous and protective oxide film could no longer be formed. Longer exposure times were carried out in order to relate the different stages of the catastrophic breakaway oxidation and identify the parameters involved in this phenomenon. Other substrate evolutions like cavity growth and nature and morphology changes of the Y₂O₃ particles were observed and are discussed in relation with the oxidation behaviour.     

Significance of crystallographic grain orientation for oxide scale formation on FeCrAl ODS alloys studied by AFM and MCs+-SIMS

Abstract

Single crystalline specimens were prepared by spark erosion of large grains in an extremely coarse textured bar of an FeCrAl based ODS alloy. These flat single crystalline specimens were studied in respect to oxidation behaviour at 800°C and 1,000°C in air. Initially the oxide scales appeared to grow by outward diffusion of aluminium but subsequently scale growth proceeded by grain boundary oxygen diffusion. The composition, grain size and growth rate of the transient oxide appeared to be dependent on the crystallographic orientation of the alloy. After the inward diffusion of oxygen had become dominant, the oxides on specimens with different crystallographic orientations showed clear differences in their growth rates. Correlation of SIMS results with quantitative grain size analysis performed by AFM showed that the different oxide growth rates, which pertained during longer exposure times, were related to differences in the oxide grain size and therefore the density of oxygen diffusion paths with different crystallographic orientations.     

The effect of gas composition and contaminants on scale growth rates of FeCrAl alloys

Abstract

Fe–20Cr–5Al alloys were exposed to several environments crucial for industrial applications. Experiments in a simulated fuel-rich exhaust gas, in combustion gas, in air with additions of SO₂ and HCl, and a N₂–NO mixture were compared to the oxidation kinetics in air.

While the addition of HCl and SO₂ enhances oxide growth via increased spallation, the N₂–NO mixture and the simulated exhaust gas seem to act as a shield gas due to the reduced oxygen partial pressure. As the local geometry can also change the gas composition during oxidation, e.g. inside crevices, results from an experiment with a deep bore are presented, simulating such a situation.     

耐事故燃料包壳用FeCrAl不锈钢的研究进展

3.11日本福岛核事故后,国内外围绕提高核燃料元件的事故包容能力和固有安全性课题开展了大量的耐事故燃料(ATF)的研发工作,其中性能更先进的包壳材料是ATF研究的前沿和难点。Fe Cr Al合金优良的高温性能结合管材制备工艺的技术成熟度和经济性,促使该合金包壳成为近期ATF包壳研发的首选目标。简介了国外在ATF包壳候选材料的筛选和Fe Cr Al不锈钢上的研究进展,综述了ATF包壳用Fe Cr Al不锈钢高温蒸汽氧化性能、力学性能、中子辐照性能和应力腐蚀性能等方面的研究现状,指出了Fe Cr Al包壳研制和工程应用等方面需突破的关键技术和研究方向,其中成分优化控制及制备工艺、中子辐照性能和应力腐蚀性能等工程应用的评价是未来研究的重点和难点。     

Zr702表面激光熔覆FeCrAl涂层微观组织及性能研究

目的 优化激光熔覆工艺参数,制备综合性能良好的FeCrAl涂层,提高Zr702基体的表面性能。方法 通过同轴送粉方式,利用激光熔覆技术在Zr702表面制备了Fe CrAl涂层,采用配备能谱仪（EDS）的扫描电镜（SEM）、X射线衍射仪（XRD）、数字显微硬度计、高速往复摩擦试验机及马弗炉研究了不同激光功率（1 300、1 400、1 600、1 800 W）与扫描速度（7、8、9 mm/s）对FeCrAl涂层成型质量、显微硬度、耐磨性及抗高温空气氧化性能的影响。结果 从所制备涂层的宏观形貌及微观组织可以观察到,在激光功率为1 600W、扫描速度为9 mm/s时,涂层成型质量最好。显微硬度测试结果表明,随着激光功率和扫描速度的增大,显微硬度呈减小趋势,但涂层显微硬度均高于Zr702基体,平均显微硬度约为基体的2.70～3.78倍。耐磨性能测试结果表明,涂层的磨损量小于Zr702基体,在激光功率为1 600 W、扫描速度为7 mm/s时,磨损量最低。氧化结果表明,800℃氧化时涂层未表现出良好的防护作用,但在1 000℃氧化时,涂层样品皆表现出优于Zr702基体的抗高温空气氧化性能。结论 ...     

The effect of aluminium depletion on the oxidation behaviour of FeCrAl foils

Abstract

In thin FeCrAl foils, the formation of a chromia layer within or underneath the alumina layer has been observed after consumption of the aluminium from the alloy. For Aluchrom I SE, the growth law of the alumina-forming step has been evaluated and an activation energy for oxygen diffusion of 383 ± 36 kJ mol^?1 has been determined. For the growth mechanism of the chromia layer, three models are introduced and discussed. In agreement with a model proposed by H. E. Evans, the measurements of the aluminium content resulted in complete aluminium consumption before the beginning of chromia formation. Because of strong deformation of the thin samples during oxidation, a model is proposed to calculate the alloy thickness based on the amount of aluminium consumption. From a comparison of these calculated values with the measured thicknesses, the elongation of the sample due to creep processes could be determined.     

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.     

X-ray determination of stresses in alumina scales on high temperature alloys

Abstract

The resistance of alumina scales to cracking and spalling under the influence of growth and thermal stresses is a critical aspect of the environmental resistance of high temperature structural alloys, oxidation resistant coatings, and bond coats for thermal barrier coatings. However, the relative magnitudes of the stresses and their distribution are often not known. In this study several X-ray diffraction techniques are being used to measure the strains in alumina scales on a variety of high temperature alloys both during oxidation and after cooling to room temperature. The corresponding stresses are being calculated using appropriate elastic constants. The results include the observations that: (1) Growth stresses are higher in alumina formed on FeCrAl alloys as compared to that formed on nickel-base alloys, such as NiAl or single crystal superalloys (studies have not yet been performed on NiCrAl or CoCrAl alloys). (2) Yttrium additions to do not result in lower growth stresses in alumina scales on FeCrAl alloys even though the additions decrease the amount of lateral scale growth. (3) Growth stresses can be relaxed by plastic deformation of both the alloy and oxide. The implications of these results with regard to alumina adhesion are discussed.     

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.     

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     

Water vapor effects on the cyclic oxidation resistance of alumina forming alloys

Abstract

Water vapor is present in most environments in which alloys are used at elevated temperatures and there are papers in the literature that show water vapor usually has adverse effects on the oxidation resistance of alloys. However, the exact effect of water vapor is dependent on the particular alloy under consideration. This paper is concerned with the oxidation of alloys that rely upon the development of α-Al₂O₃ scales for oxidation resistance. This paper describes two major deleterious effects of water vapor on the oxidation of such alloys. One effect involves increased spalling of α-Al₂O₃ which will be shown to be less significant in the case of alloys with extremely adherent α-Al₂O₃ scales. It is proposed that water vapor causes the α-Al₂O₃-alloy interfacial toughness to be decreased, however this effect is not sufficient to cause spalling of extremely adherent α-Al₂O₃ scales. Another effect of water vapor is that it causes more transient oxides to be formed during the selective oxidation of aluminum in alloys. This condition becomes more severe at lower temperatures. Possible mechanisms by which water vapor affects the selective oxidation of aluminum in alloys will be described.     

Preliminary study on the fabrication of 14Cr-ODS FeCrAl alloy by powder forging

A simple powder forging process was presented herein to fabricate an Fe-14Cr-4.5Al-2W-0.4Ti-0.5Y2O3 ODS FeCrAI alloy.The forged alloy exhibits a high density that exceeds 97％of the theoretical density.The ODS alloy was investigated in terms of the residual porosity,morphology and phase structure of oxide nanoparticles,impact toughness and tensile properties.It was found that refined grains were obtained during powder forging.A residual porosity less than 1.1％has no impact on the precipitation of oxide nanoparticles.The average diameter of the oxide particles is 7.99 nm,with a number density of 2.75 x 1022 m-3.Almost all of the oxides are identified as orthorhombic YAlO3 particles.The refined grains and uniformly distributed oxide nanoparticles enable the alloy to show excellent mechanical strength and ductility below 700℃,and enable the ductile-to-brittle transition temperature to be close to room temperature.However,a slight decrease in strength at 1000℃and the Charpy upper shelf energy has been suggested to be due to the residual porosity.These results indicate that powder forging can be used as a promising technique for the fabrication of ODS alloys.     

表面预氧化镍钛丝微弧氧化涂层的研究

通过微弧氧化法(MAO)在表面预氧化镍钛(NiTi)丝(直径(0)0.5mm)表面制备含Ca、P的均匀微孔涂层,对不同处理时间形成的涂层进行扫描电镜(SEM)形貌观察和电子探针(EPMA)成分分析,并探查了涂层的血液相容性.实验结果表明,表面预氧化的NiTi丝在微弧氧化处理时,随微弧氧化时间的延长,氧化层中Ti/Ni提高,Ca、P含量也提高.微弧氧化20s时,涂层质量较好,微孔均匀分布,微孔孔径1μm左右,表面没有裂纹,而且此时涂层有较好的生物相容性.     

Microstructural characterization of oxide scales formed on steels P91 and P92

Abstract

In this paper, an in-depth study of steam oxidation of two 9Cr ferritic-martensitic steels (P91 and P92), for advanced power plant, has been carried out. The steels investigated were exposed to a 100% flowing steam environment at 650°C for 1,000 – 3,000 hours. Metallographic analysis showed a multilayered scale was formed on both substrates consisting of an outer Fe-rich phase and inner Fe – Cr spinel, of varying oxide stoichiometry, as well as Cr –Mn-rich bands. The results of scale thickness measurements showed that the oxide formed on the P92 steel was thicker compared to that on the P91 steel. Crystallographic analysis showed that the P91 steel exhibited a martensite to ferrite transformation adjacent to scale – substrate interface which was not observed on the P92 steel. The outer oxide layer on both substrates exhibited a region of equiaxed grains followed by a region of columnar crystal growth. However, on the P91 steel the longer exposure time (3,000 hours) resulted in the outer layer having a region with coarsened equiaxed magnetite grains. The roles of alloying elements (Cr, Mo, W and Mn) were investigated to provide a better understanding of the oxide growth behaviour.     

Effect of dynamic strain aging on the appearance of the rare earth texture component in magnesium alloys

Binary Mg alloys were prepared containing Zn, Ce and Gd. These were extruded and the resulting mechanical properties were determined. The intensities of the rare earth (RE) texture components were measured and linked to the extrusion conditions. Tension and compression testing was carried out on samples taken from extruded bars and a Mg-0.5Ce cast alloy. Over particular temperature and strain rate ranges, dynamic strain aging (DSA) was observed. The ranges over which DSA occurred during testing are compared with the conditions under which the RE texture components were produced during extrusion. It is concluded that formation of the RE texture components can be enhanced by extruding when the rate sensitivity is negative, i.e. under conditions where DSA is taking place.     

Characterisation of alumina scales on Fecralloy using impedance spectroscopy

Impedance measurements of alumina scales on Fecralloy substrates were made at temperature from 400 °C to 650 °C. The alumina scales were also examined using scanning electron microscopy, transmission electron microscopy, coupled with energy dispersive X-ray analysis. Impedance spectra of the alumina scale in modulus format indicate that the thickness of the alumina can be measured based on impedance measurements whereas the change of impurity level in the alumina can be monitored according to variation of activation energy for electrical conduction.     

Effect of impurities on the oxidation mechanism of nickel at 800°C

Abstract

The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.