The oxidation of Fe3Al–(0, 2, 4, 6%)Cr alloys at 1000 °C

Thermomechanically treated Fe₃Al–(0, 2, 4, 6at.%)Cr alloys were isothermally oxidized at 1000 °C in air, and their oxidation characteristics were studied using thermogravimetric analyzer, X-ray diffractometer, scanning electron microscope, electron probe microanalyzer, and TEM/EDS. It was found that Cr decreased the oxidation resistance of Fe₃Al alloys to a certain extent. The oxide scales that formed on the unalloyed Fe₃Al alloys consisted primarily of α-Al₂O₃ containing a small percentage of dissolved iron ions. Less than 1% of dissolved chromium ions was additionally present in the oxide scale formed on the Fe₃Al–Cr alloys. An Al-free, Fe-enriched zone was formed beneath the oxide scale, owing to Al consumption to form the oxide scale. The oxide scale on all alloys had poor adherence.     

Oxydation a haute temperature d'alliages Fe-20Cr-xAl

The oxidation of Fe-20Cr-xAl (x=2, 4, 6) and Fe-20Cr-6Al-0.025 Ce alloys was studied in pure oxygen (1 bar), the temperature ranging between 800 and 1100°C. The parabolic law is obeyed when alloys containing 4 or 6 % Al are oxidised. An α-Al₂O₃ scale is formed. The oxidation rate does not depend on the composition. The reaction is more complex with the Fe-20Cr-2Al alloy. An iron and chromium oxide layer is formed at the beginning. Alumina precipitates in the metal. After a few hours a continuous α-Al₂O₃ barrier is formed. The lower oxidation rate (when compared with other alloys) is then justified and the reason why the parabolic law is not obeyed can be explained.     

Early stages of oxidation of yttrium-implanted Ni-20% Cr

The effect of ion-implanted yttrium on oxidation at elevated temperatures (500 to 1000° C) has been studied in Ni-Cr alloys using high-voltage electron microscopy (HVEM) ofin situ oxidized specimens, electron microscopy of scales oxidizedex situ, and Rutherford back scattering. The presence of yttrium enhances nucleation and growth of Cr₂O₃ at the alloy surface. Although the radiation damage induced by the ion implantation accelerates oxide nucleation and the initial rate of oxidation at 500° C, the growth of a mature oxide scale is slowed down, and the main influences seem to come from the chemical characteristics of the implanted yttrium. A Cr₂O₃ layer forms first on the yttrium-implanted alloys. Outward diffusion of Ni²⁺ cations through this layer forms an outer NiO scale. The initial growth process on unimplanted alloys is opposite. Here, NiO is the predominant initial oxide. As it thickens, a porous spinel and Cr₂O₃-enriched layer is formed between alloy and NiO. The inner oxide layer on yttrium-implanted alloys is fully dense and contains more than 50% Cr₂O₃. On the unimplanted alloy, the inner spinel layer is porous and contains a lesser enrichment in Cr₂O₃. The porous spinel delays formation of a protective Cr₂O₃ layer and gives poor scale adherence. The oxide growth mechanisms are discussed in the light of TEM results.     

Oxidation behavior of FeAl alloys with and without titanium

The influence of Ti addition on the high temperature oxidation behavior of FeAl intermetallic alloys in air at 1000°C and 1100°C has been investigated. The oxidation kinetics of FeAl alloys was examined by the weight gain method and oxide products were examined by XRD, SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5Al alloys could be described as different parabolas that followed the formula: (W/S)² = K _p t + C. The parabolic rate constant, K _p values are approximately 2.4 and 3.3 mg² cm^–4 h^–1 for Fe-36.5Al alloy and about 1.3 and 2.0 mg² cm^–4 h^–1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only -Al₂O₃ oxide for Fe-36.5Al alloy while there are -Al₂O₃ and TiO oxides for Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were addressed based on the microstructural evidence.     

Laser raman spectroscopic studies of the surface oxides formed on iron chromium alloys at elevated temperatures

Binary iron-base alloys containing chromium additions of 3, 9, 12 and 18 % were oxidized in air at elevated temperatures. Laser Raman spectroscopy has been used to determine the chemical compounds of the oxides of these alloys. It is found that the oxides formed on Fe-3Cr alloy at various elevated temperatures consist mainly of iron. However, for Cr additions ≥12 %, the surface oxide formed at 400°C consists of αFe₂O₃, Fe₃O₄ and spinel phases. With increasing oxidation temperature up to 850°C, the oxide scale consists of Cr₂O₃ and spinel phases only.     

The influence of implanted chromium and yttrium on the oxidation behaviour of TiAl-based intermetallics

The influence of implanted chromium and yttrium on the oxidation behaviour of Ti-50Al (at %) in air at 800 °C was investigated. It was found that implanted chromium and/or yttrium leads to a decrease of the oxidation rate because an alumina scale was formed on the implanted material in the early stages of oxidation, whereas a titania-based scale grew on the non-implanted material. With increasing oxidation time the difference in oxidation behaviour between the implanted and non-implanted alloy gradually disappeared. These results show that chromium and yttrium, provided their concentration is properly chosen, can have a similar positive effect on the oxidation behaviour of TiAl-based intermetallics as they have on the oxidation behaviour of conventional Fe-, Ni-, or Co-based alumina-forming high temperature alloys.     

Convolution and spallation of oxides formed on Fe–Al alloys after short oxidation times

Abstract

A study of the oxidation of Fe–5Al alloys with additions of silicon and yttrium has shown that high stresses can develop within the oxide scale. These stresses occur after very short oxidation times (<5 min at 1000 °C)for samples that do not contain yttrium, but are not present in the scalesformed on alloys containing yttrium. A stress gradient develops across the scale with the oxide at the metal/oxide interface under the greatest compressive stress. Transmission electron microscopy and analysis were used to study cross-sections through the metal/oxide interfaces. It was found that a stress gradient was not present after oxidation at lower temperatures or when yttrium was present.

MST/860     

Oxide adherence of Fe-20Cr-4Al alloys with small amounts of sulfur and reactive elements (Y,Hf)

Abstract

Oxide adherence of Fe–20Cr–4Al alloys with small amounts of sulfur, yttrium and hafnium was studied in air for 360 ks at 1,373, 1,473 and 1,573K by mass change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. After oxidation at 1,373K, spalling of oxide scales on 7ppmS, 53ppmS and 1,300ppmS alloys was recognized. However, spalling of oxide scales on the other alloys was not observed. After oxidation at 1,573K, spalling of the oxide scales on the alloys with sulfur increased roughly with increasing contents of sulfur, and spalling of oxide scales on the alloys containing yttrium was scarcely recognized, however, oxide scales on all of the alloys containing hafnium spalled at the entire surface. Oxide adherence on the alloys may relate to morphologies of oxide scales and oxide–alloy interface, size and distribution of chromium sulfide, Y₃Al₅O₁₂ and HfO₂ particles at the oxide–alloy interface and temperature of oxidation.     

Characterization of the breakaway al content in alumina-forming alloys

Abstract

Residual Al contents were measured in Fe- and Ni-base alloys in order to improve lifetime prediction models based on the consumption of the Al reservoir. Measurements were made using electron probe microanalysis (EPMA) on both foil (50–125 μm thickness) and plate (0.5–2mm) material. For FeCrAl, NiCrAl and Fe₃Al plate specimens, significant Al concentration gradients were measured. An Al gradient is not considered in current reservoir type models. Residual Al contents at the onset of breakaway oxidation were somewhat lower than previously reported for several FeCrAl alloys and significantly higher (»10at.%) for Fe₃Al and Fe–Al alloys. The implications of these results for performance and lifetime predictions are discussed.     

Adherence of alumina scale formed on Fe–20Cr–4Al alloys with noble metals (Pd,Pt)

Abstract

Adherence of oxide scale formed on Fe–20Cr–4Al (standard: 4 ppmS), Fe–20Cr–4Al (FZ: <1 ppmS) purified by floating zone melting, and Fe–20Cr–4Al–(0.05, 0.1, 0.3, 0.5)–Pd(Pt) alloys was studied in oxygen for 18 ks at 1373–1673 K, by mass gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Spalling of oxide scale on the standard alloy occurred along the alloy grain boundaries after oxidation at 1373 K, and was found to spall over the entire surface after oxidation at 1473 and 1573 K. However, the standard alloy showed good oxide adherence after oxidation at 1673 K. The other alloys showed good oxide adherence after oxidation at any temperature. Mass gain of the FZ and noble metal-containing alloys increased with increasing temperature of oxidation, and showed almost the same values at each temperature. Oxide scale on the standard alloy showed wavy morphology after oxidation at 1373 and 1473 K, and changed to planar morphology after oxidation at 1573 and 1673 K. On the other hand, oxide scale on the other alloys showed almost planar morphology after oxidation at any temperature. Oxide scale formed on all the alloys was only found to be α-Al₂O₃ after oxidation at any temperature.     

AES and SIMS investigations of the oxide films on Fe-40Cr-Ru alloys

Auger and SIMS depth profiling have been used to investigate the effect of ruthenium addition on the oxidation behaviour of the Fe-40Cr-Ru alloys oxidized in air at 500 °C. Auger results revealed that the oxide films formed on the Fe-40Cr-Ru alloys consisted of a thin iron oxide external layer and a chromium-rich internal layer. Ruthenium was not found in these oxide layers. Secondary ion mass spectrometry measurements also indicated a thin Fe⁺-rich film as an outer layer with a predominantly chromium oxide in the internal layer of the film. Furthermore, ruthenium was incorporated in the entire oxide film and promoted the formation of a thin compact film.     

The high temperature oxidation of CoCrAl alloys containing yttrium or hafnium additions

The effect of small amounts of yttrium (up to 1 wt.%) or hafnium (up to 1.5 wt.%) on the oxidation behaviour of CoCrAl alloys in the temperature range 1000–1200°C for times of up to 1000 h in air was studied. Both isothermal and cyclic tests were carried out; the cycle used consisted of 20 h at a high temperature after which the specimen was withdrawn from the furnace and was allowed to cool. The major part of the study was concerned with Co10 wt.% Cr11 wt.% Al alloys.In isothermal oxidation, increasing the yttrium decreased the overall weight gain, principally by decreasing the length of the initial transient period before the establishement of steady state kinetics. Hafnium also decreased the rate, with the lowest concentrations (0.05 and 0.1 wt.% Hf) having the greatest effect; further increases produced an increase in oxidation rate. In thermal cycling tests, increasing the yttrium again progressively improved the oxidation behaviour. Hafnium also produced an improvement, but in contrast with the isothermal behaviour a minimum of 0.3 wt.% Hf was required to produce a significant effect; further increases resulted in a small increase in oxidation.Metallographic examination suggested that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al₂O₃ into the metal. However, if the intrusions are too large, they can act as initiators of scale failure.Overall, hafnium additions appear to be preferable to yttrium additions.     

Effects of Pt on the short-term oxidation behavior of γ-Ni+γ′-Ni3Al alloys

Abstract

The aim of this study was to elucidate the beneficial role played by platinum addition in promoting the formation of a protective Al₂O₃ scale on γ′-Ni₃Al+γ-Ni alloys during oxidation at high temperatures. To do this, the early-stage oxidation behavior of γ′-Ni₃Al-based alloys of composition (in at.%) Ni–22Al and Ni–22Al with 10, 20, and 30 Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150°C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the γ′-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al₂O₃ scale during heating above about 600°C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in γ′. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L1₂ crystal structure of γ′, which has the effect of amplifying the increase in the Al:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ′ surface kinetically favors the formation of Al₂O₃ relative to NiO. A further contributing factor is that the Pt-containing γ′-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment reduces Ni availability and can increase the Al supply to the evolving scale, thus kinetically favoring Al₂O₃ formation.     

Effect of admixtures of oxygen on the oxidation of iron and Fe−Cr alloys in lead melts

We study the process of oxidation of Armco iron and Fe−16Cr and Fe−16Cr−1Al model alloys held in lead melts with different concentrations of oxygen for 1000 h at 650°C. It was discovered that the intensity of oxidation, the structure, and phase composition of oxide layers are determined by the activity of oxygen in the liquid metal. By the methods of layer-by-layer X-ray diffraction analysis and microscopic X-ray diffraction analysis, it was shown that, for low concentrations of oxygen in lead (C ₀≤10⁻⁶ wt.%), a thin (1–5μm) oxide [magnetite (Fe₃O₄] film is formed on the surface of iron. If alloys are held under the same conditions, then we also observe an increase in the concentration of chromium in the subsurface layers. For higher concentrations of oxygen (up to 10⁻⁵ wt.%), a film of magnetite (with inclusions of pure lead) is formed on the surface of unalloyed iron. In alloys, under the layer of magnetite, we detect the formation of oxide layers with the same composition as a solid solution of Fe₃O₄ and FeCr₂O₄ and the structure of spinel. These layers efficiently suppress the process of penetration of lead but do not completely terminate the process of diffusion of oxygen into the bulk of the material, which eventually leads to the internal oxidation of alloy. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 3, pp. 97–101, May–June, 1997.     

合金元素对块体纳米晶Fe3Al材料磁学性能的影响

为了研究合金元素对块体纳米晶Fe3Al材料磁学性能的影响,通过铝热反应熔化法制备了纳米晶Fe3Al以及分别含Ni质量分数10%、Cr质量分数10%、Mn质量分数10%和含Ni质量分数10%-Cu质量分数2%的块体纳米晶Fe3Al.在振动样品磁强计(VSM)上测得合金的磁滞回线,分析其磁性能,采用X射线衍射仪进行结构分析和平均晶粒尺寸计算.结果表明:各样品的磁滞回线呈倾斜状且狭长,磁滞损耗很小;含Ni质量分数10%的样品饱和磁化强度Ms较大,剩余磁化强度Mr和矫顽力Hc较其他样品最小,具有较好的软磁性能;添加合金元素后几种材料的晶粒尺寸变小,磁性能有较大变化,合金元素对纳米晶Fe3Al块体材料的磁性能影响明显.     

Growth and surface morphology of hot-dip Al–Si on 9Cr-1Mo steel

The high-temperature oxidation behavior of a commercial cold-rolled 9Cr-1Mo steel sheet that was hot-dipped in Al–7 wt.% Si was studied. The oxidation kinetics showed a two-stage behavior. The oxidation rate was significantly faster during the first hour, followed by a decreased steady-state growth rate at longer times. The hot-dipped steel had a higher oxidation rate than has been reported for FeAl alloys. During oxidation, many large pores and needles or rod-shaped FeAl were formed beneath the oxide scale. The formation of pores was attributable to the consumption of the limited aluminum that is caused by the outward diffusion of aluminum to form γ-Al₂O₃ and the inward migration of aluminum by Fe/Al inter-diffusion. The formation of γ-Al₂O₃ and internal voids filled with oxides in the aluminide layer was responsible for the increase in the rate of oxidation.     

Mechanisms of simultaneous oxidation-chloridation of austenitic stainless steels by NaCl in air

Abstract

Simultaneous oxidation-chloridation behavior of austenitic stainless steels by NaCl in air was investigated at 650°C. Surface oxide scales of the test alloys had double-layered structures that consisted of inner spinel layer and outer Fe₂O₃ layer. Condensed and gaseous chlorides were formed below the scale surface by NaCl vapor diffusing inward via pores or cracks of the scales. The rate of internal and uniform attack mainly depended on the porosity of the inner spinel layer. In order to predict the complex mixed oxidation behavior of alloys, new types of thermodynamic stability diagrams were utilized, which was constructed by computer-assisted calculations of phase equilibria of the multi-component reacting systems using the Thermo-CaIc program. The overall mixed oxidation process including the initial stage of reaction, the rapid formation and growth of the scale and the transport of gaseous species through the oxide scale was elucidated.     

Long-term oxidation kinetics of aluminide coatings on alloy steels by low temperature pack cementation process

The long-term oxidation kinetics of the P92 steel and iron aluminide diffusion coating formed on its surface by the pack cementation process have been investigated at 650 °C over a period of more than 7000 h both in 100% steam and in air under normal one atmospheric pressure by intermittent weight measurement at room temperature. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were used to analyse the oxidised surfaces. For the P92 steel substrate, the scale formed by oxidation is largely magnetite (Fe₃O₄) in steam and haematite (Fe₂O₃) in air. Despite this difference in the type of oxide scales formed, it was found that the long-term oxidation kinetics of the P92 steel substrate in both steam and air can be described by a logarithmic time relationship: Δm _t = k _lln(t/t° + 1); the constants k _l and t° were subsequently determined using a closest fit process for oxidations in steam and air. For the coating, the oxide scale formed in both steam and air was Al₂O₃, which provided the long-term oxidation resistance. It was observed that the long-term oxidation kinetics of the coating in both steam and air can be best described by Δm _t = Δm ₀ + k _c t ^1/3; the rate constant k _c of oxidation in steam and air was then determined by the least squares method. For both the P92 steel substrate and coating, the rate of oxidation is faster in steam than in air at 650 °C particularly in the case of the P92 steel substrate.     

Al effect in transport properties of nickel oxide and its relevance to the oxidation of nickel

Transport properties of Al (0.11 at%)-doped Ni_1−xO, at equilibrium and in presence of a driving force of diffusion, have been studied to understand the oxidation behaviour of dilute Ni-Al (0.5 wt%) alloys. At T < 1000 °C, the alloy oxidizes slower than nickel, in agreement with a process mainly controlled by the outward diffusion of Ni²⁺, but faster at T ≥ 1000 °C, where the inner porous layer shows a low adhesion to the metal, with a greater amount of Al³⁺. The increasing oxidation rates at the higher temperatures are in discrepancy with experiments performed in the stability range of undoped and Al-doped Ni_1−xO single crystals. A formal treatment of transport processes under non-equilibrium conditions has allowed us to analyse the role played by doping. It has been found both that the Al³⁺ enrichment near the metal/oxide interface is largely due to kinetic demixing processes through the oxide scale and that the increasing alloy oxidation kinetics, at T ≥ 1000 °C, is largely due to oxygen molecules which penetrate the oxide scale along cracks or fissures. It is suggested that the fissure formation along grain boundaries is due to the influence of Al on the nickel ion mobility, leading to a localized oxide particle growth within the scale itself.     

Effect of addition of Al and Cu on the properties of Sn–20Bi solder alloy

This study investigates the effect of the composite addition of Al and Cu on the microstructure, physical properties, wettability, and corrosion properties of Sn–20Bi solder alloy. Scanning electron microscopy and X-ray diffraction were used to identify the microstructure morphology and composition. The spreading area and contact angle of the Sn–20Bi–x (x?=?0, 0.1 wt% Al, 0.5 wt% Cu, and 0.1 wt% Al–0.5 wt% Cu) alloys on Cu substrates were used to measure the wettability of solder alloys. The results indicate that the alloy with 0.1 wt% Al produces the largest dendrite and the composite addition of 0.1 wt% Al and 0.5 wt% Cu formed Cu₆Sn₅ and CuAl₂ intermetallic compounds in the alloy structure. And the electrical conductivity of Sn–20Bi–0.1Al is the best, which reaches 5.32 MS/m. The spread area of the solder alloy is reduced by the addition of 0.1 wt% Al and 0.5 wt% Cu, which is 80.7 mm². The corrosion products of Sn–20Bi–x solder alloys are mainly lamellar Sn₃O(OH)₂Cl₂ and the corrosion resistance of 0.1 wt% Al solder alloy alone is the best. The overall corrosion resistance of Sn–20Bi–0.1Al–0.5Cu is weakened and the corrosion of solder alloy is not uniform.