Materials at High Temperature Vol 20, Issue 3, 2003
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Preface
The Fifth International Conference on the Microscopy of Oxidation was held at the University of Limerick, Ireland from 26th to the 29th August 2002. Some 70 delegates attended the Meeting. This issue of Materials at High Temperatures contains 25 of the papers, a further 21 papers will be published in issue 4.
The general format of the Conference followed that of the four previous Meetings, there being sessions on the fundamentals of oxidation studies as well as chromia and iron oxide formation. Some of the issues associated with the oxidation of nuclear materials and light alloys were addressed in two further sessions and one afternoon was devoted to posters. A number of the sessions were centred on alumina formation and this reflects the continuing demand for high temperature materials that can withstand stress. The role of coatings and TBCs was also discussed and both areas continue to promote new applications of microscopy based methods for studying oxidation.
A wine reception was kindly sponsored by JEOL (UK) Ltd and FEI (UK) Ltd and we are very grateful for their generous support of the Meeting. The Conference Dinner was held in Bunratty Castle and this proved both a successful and entertaining evening.
It is planned to hold a sixth meeting in this conference series.
Simon Newcomb
Gordon Tatlock
Significance of minor alloying additions and impurities on alumina scale growth and adherence in FeCrAl alloys
H. Al-Badairy1, D. Naumenko2, J. Le Coze3, G.J. Tatlock1 and W.J. Quadakkers2
1Materials Science and Engineering, University of Liverpool, UK
2Forschungszentrum Jülich, IWV 2 52425 Jülich, Germany
3Ecole des Mines, F-42023 Saint-Etienne, France
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.
Keywords: FeCrAlY, ultra-high purity alloys, breakaway oxidation, microstructure
The morphology of alumina scales formed on Fe-20Cr-4Al-S alloys with reactive element (Y, Hf) additions at 1273 K
T. Amano1, H. Isobe1, K. Yamada1 and T. Shishido2
1Shonan Institute of Technology, Fujisawa 251-8511, Japan
2IMR, Tohoku University, Sendai 980-8577, Japan
The morphology of alumina scales formed on Fe–20Cr–4Al–S (185 ppm) alloys with reactive elements (10, 300, 800, and 3700 ppmY, 500, 900, 1900, and 4300 ppmHf) was studied in oxidizing atmospheres at 1273 K for 18, 1800 and 3600 ks by mass gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). No spalling of the oxide scales was observed on any of the alloys after any of the oxidation times. Mass gain decreased for alloys with small additions of reactive elements, and then increased with increasing reactive element content. Oxide scales on the 185 ppmS, the 500 and 900 ppmHf alloys showed a wavy morphology. On the other hand, oxide scales on the other alloys with reactive elements changed from small wavy to planar morphologies with increasing reactive elements after any of the oxidation times studied. Oxide scales formed on all of the alloys were recognized as mainly _-Al2O3. After oxidation for 3600 ks, Y3Al5O12 particles were observed for the 800 and the 3700 ppmY alloys, and HfO2 particles were also detected for the 1900 and the 4300 ppmHf alloys.
Keywords: oxide adherence, alumina scale, sulfur, yttrium, hafnium
The behaviour of commercial FeCrAlRE alloys in nitrogen-oxygen – water vapour bioxidant environments
M. J. Bennett, R. Newton and J. R. Nicholls
School of Industrial and Manufacturing Science, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK
The corrosion behaviour of a range of commercial FeCrAlRE alloys (MA956, ODM751, PM2000, Kanthal AF, Kanthal APM and Aluchrom YHf) have been examined in nitrogen–oxygen-H2O or N2–H2–H2O bioxidant environments, at temperatures between 1100°C and 1350°C. The corrosion behaviour is governed by the competition between oxidation leading to protective alumina formation/ maintenance and nitrogen ingress leading to nitridation of the matrix alloy. Key issues addressed by four series of experiments, have included: the influence of a pre-formed protective alumina scale; the oxidant level required to form/reheal a protective oxide scale; the role of mechanical failure of the scale above the critical thickness for cracking/spallation in oxygen rich environments; chemical failure of the protective oxide scale leading to breakaway (non-protective) attack and in particular, the potential roles in such failure processes of nitridation concurrent with, and following defective oxide scale formation, and of oxidation following nitridation. Detailed characterisation of the chemical composition and physical microstructure of the attack of the respective alloys was undertaken using a range of surface analytical techniques, including X-ray diffraction, optical and scanning electron microscopy and energy dispersive X-ray analysis.
Keywords: FeCrAlRE alloys, protective and breakaway oxidation, nitridation
Use of oxygen isotope to study the transport mechanism during high temperature oxide scale growth
S. Chevalier1, G. Strehl2, J. Favergeon1, F. Desserrey1, S. Weber3, O. Heintz1, G. Borchardt2 and J.P. Larpin1
1Laboratoire de Recherches sur la Réactivité des Solides, UMR5613 CNRS, 9 av. A. Savary, BP 47870, 21078 Dijon cedex, France
2Institut für Metallurgie, Technische Universität Clausthal, Robert Koch Strasse 42, 38678 Clausthal-Zellerfeld, Germany
3Laboratoire de Physique des Matériaux, Ecole des Mines; Parc de Saurupt, 54042 Nancy cedex, France
The examination of high temperature (HT) oxide scale growth mechanisms was performed using secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), in conjunction with 16O2/18O2 HT oxidation experiments. Cr2O3, NiO, ZrO2 and Al2O3 were studied because they constitute excellent representative thermally grown oxide scales: they grow by cationic diffusion (Cr2O3, NiO), anionic diffusion (ZrO2) or mixed anionic-cationic diffusion (Al2O3). The oxidation tests were performed first in 16O2 and subsequently in 18O2 at several temperatures (600–1000°C for NiO, 600°C for ZrO2, 1000°C for Cr2O3 and 1100°C for Al2O3). The oxygen isotope distribution observed by SIMS and SNMS profiles are discussed and related with the HT oxidation mechanisms proposed in the literature.
Keywords: 16O2/18O2 oxidation experiments, SIMS, SNMS, Cr2O3, NiO, ZrO2 and Al2O3, oxide growth mechanism
Application of TEM and SNMS in the study of thermally grown alumina scales
S. Chevalier1, J.P. Larpin1, P. Dufour1, G. Strehl2, G. Borchardt2, K. Przybylski3, S. Weber4 and H. Scherrer4
1Laboratoire de Recherches sur la Réactivité des Solides, UMR5613 CNRS, 9 av. A. Savary, BP 47870, 21078 Dijon cedex, France
2Institut für Metallurgie, Technische Universität Clausthal, Robert-Koch-Strasse 42, 38678 Clausthal-Zellerfeld, Germany
3University of Mining and Metallurgy, Faculty of Materials Science and Ceramics, Department of Solid State Chemistry, al. Mickiewicza 30, 30-059 Krakow, Poland
4Laboratoire de Physique des Matériaux, Ecole des Mines, Parc de Saurupt, 54042 Nancy, France
The oxidation behaviour of several alumina-forming alloys containing reactive elements was investigated at 1100°C in air under atmospheric pressure. Analytical techniques were applied to characterise the corrosion products and to understand the role of reactive elements on the alumina scale growth. Secondary neutral mass spectrometry was used to determine the in-depth compositional profile of the element composing the oxide scale, as well as the oxygen isotope, after sequential oxidation experiments using 16O2 and 18O2 enriched environments. Transmission electron microscopy revealed the microstructure of the oxide scale and located the reactive elements within the Al2O3 scale or at the metal–oxide interface. The combination of both techniques led to a better understanding of the oxide scale growth mechanism.
Keywords: alumina scale, reactive element effect, two-stage oxidation experiments, SNMS, TEM, segregation.
Improvement of high temperature oxidation resistance of rare earth metal-added Fe–20%Cr–5%Al alloys by pre-annealing treatment
K. Fukuda, K. Takao, T. Hoshi, Y. Usui and O. Furukimi
Technical Research Laboratories, Kawasaki Steel Corp., Chiba, Japan
Fe–20mass%Cr–5mass%Al alloy foils have recently been used as a catalytic converter substrate for automobiles because of their good oxidation resistance. In this study, the effect of pre-annealing on the oxidation behaviour of Fe–20mass%Cr–5mass%Al alloy foils containing a small amount of La–Zr and La–Hf was examined by a cyclic oxidation test at 1373K in air. The Al2O3 scale formed by oxidation was analyzed with a scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with an energy-dispersive X-ray spectrometer. The oxidation rate of these alloys was reduced by pre-annealing in a reducing atmosphere. The Al2O3 scale which formed on the pre-annealing alloys was characterized by a thinner outer equiaxed grain layer and larger inner columnar grain size than the scale layer which formed on as-cold rolled alloys of the same material. It is inferred that, with the pre-annealed alloys, the low-density of grain boundaries in the Al2O3 limited oxygen diffusion along the Al2O3 grain boundaries, resulting in a decreased growth rate of Al2O3 scale.
Keywords: Iron-chromium-aluminum alloy, foil, oxidation rate, rare earth metals, zirconium, hafnium, annealing treatment, hydrogen atmosphere, grain boundary segregation
Thermal oxidation of InAlP
M.J. Graham1, S. Moisa1, G.I. Sproule1, X. Wu1, J.W. Fraser1, P.J. Barrios1, D. Landheer1, A.J. SpringThorpe2 and M. Extavour2
1Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Canada K1A 0R6
2Nortel Networks, 3500 Carling Avenue, Ottawa, Canada K2H 8E9
Producing insulating layers on III–V semiconductors is crucial for a number of important device applications. Al-containing thermal oxides on AlGaAs and InAlAs have been found to possess good insulating characteristics and oxides on InAlP have recently been shown to be even more promising. This paper presents data on the thermal oxidation at 500°C in moist nitrogen (95°C) of MBE-grown InAlP layers (In0.525Al0.475P and In0.494Al0.506P) lattice matched to GaAs. The oxides (20–300 nm thick) have been characterized by Auger electron spectroscopy, X-ray photoelectron spectroscopy, Rutherford backscattering spectroscopy, transmission and scanning electron microscopy. Oxides are amorphous and appear to be a mixture of indium phosphates and aluminum oxide. The oxidation kinetics are parabolic, and the InAlP layer with the higher Al content oxidizes slightly faster. Electrical measurements performed on metal-insulator-semiconductor (MIS) structures indicate that the oxide has good electrical properties, exhibiting low current densities (up to 14 V), making the oxide films potentially useful for some device applications.
Keywords: III–V semiconductors, InAlP, thermal oxidation, surface-analytical techniques
Impurity and surface condition effects on pore formation at Al2O3/FeAl interfaces
Peggy Y. Hou and Chris Van Lienden
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Interfacial pores are commonly observed at the scale/alloy interface in NiAl and FeAl alloys after high temperature oxidation. The pores are often an order of magnitude larger than the oxide grains, and many are formed during the initial stage of oxidation. In order to better understand the development of these pores, the effects of pre-oxidation surface condition and surface impurities on pore formation at the oxide/alloy interface in Fe–40at%Al are investigated. Surface conditions included polishing to 4000 grit with SiC and polishing to 1 _m with diamond suspension. Surface impurities included a number of nitrate salts dissolved in water. Pore number distribution in each case was determined and related to the treatment effects. Testing was also performed on an Hf-containing Fe–40Al to evaluate the effect of adding a reactive element. It was found that surface roughness and the presence of impurities, even residues from water drops, increased the number of interfacial pores. Hf addition made the alloy more resistant to pore formation, but large pores developed under heavy salt deposits.
Keywords: Al2O3/FeAl interfaces
Analytical study of the evolution of alumina scales formed on an ODS alloy
Laurent Maréchal1, Bernard Lesage1, Anne-Marie Huntz1 and Régine Molins2
1LEMHE, CNRS UMR 8647, Université Paris XI, 91405 Orsay, France
2ENSMP, Centre des Matériaux, CNRS UMR 7633, BP 87, 91003 Evry, France
An original deflection technique, in association with TEM observations, allowed study of the transformation of metastable alumina phases to _ 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 _m 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
Keywords: alumina scales, oxide dispersion strengthened alloys
Time dependence of Mg-incorporation in alumina scales on FeCrAl alloys studied by FIB-prepared TEM cross sections
J. Mayer1, H.J. Penkalla2, A. Dimyati1, M. Dani3, P. Untoro3, D. Naumenko2 and W.J.Quadakkers2
1Gemeinschaftslabor für Elektronenmikroskopie, RWTH Aachen, D-52074-Aachen, Germany
2Forschungszentrum Jülich, IWV-2, D-52425 Jülich, Germany
3National Nuclear Energy Agency Kawasan PUSPIPTEK, Serpong 15314, Indonesia
Minor alloying additions and alloy impurities can be of crucial importance for the protective properties of oxide scales on high temperature alloys. An example of an impurity is magnesium. In the present study, the effect of Mg impurity (concentration 80 ppm) on the oxidation behaviour of a commercial FeCrAl alloy at 1200°C was investigated. By a combination of various analysis techniques, such as optical metallography, SEM/EDX/WDX, X-ray diffraction and TEM, it was found that Mg appeared to become easily incorporated in the surface oxide already after relatively short exposure times. Comparison of the oxidation products showed a change in the scale growth mechanism to occur in course of the oxidation treatment. Magnesium was found to migrate through the alumina scale and it became enriched in the outer part of the oxide scale forming a layer of a (Mg,Mn)Al2O4 spinel. An important effect associated with the Mg-enrichment and spinel formation was the development of porosity within the scale which may be of great importance for the growth rate and adherence of the protective alumina based scale.
Keywords: Mg impurity, alumina scale, FeCrAl alloys
Microstructural evaluation of dross formation on Mg- and non-Mg-containing Al alloys from industrial furnaces
K.L. More1, P.F. Tortorelli1, L.R. Walker1, J. Hryn2 and G. Krumdick2
1Oak Ridge National Laboratory, Oak Ridge, TN, USA
2Argonne National Laboratory, Argonne, IL, USA
The microstructural aspects of dross formation in industrial Al re-melt furnaces for several different alloy compositions have been evaluated. The Al alloys used in this study included 1350 (no Mg), 3004 (low Mg), and 5182 (high Mg). Dross specimens were collected directly from the different Al alloy melts in industrial furnaces using a consistent sampling protocol in order to compare the microstructure and phase development of the dross as a function of melt temperature and composition. The results showed that the sequence of phases formed during the re-melt process was the same for all the alloys examined; amorphous-Al2O3 forms first followed by either á-Al2O3+AlN (for non-Mg-containing alloys) or cubic MgO, then MgAl2O4, and lastly á-Al2O3 (for low- or high-Mg content). The formation of MgAl2O4 is associated with accelerated oxidation rates (known as breakaway oxidation) and this reaction proceeds until the Mg is depleted at the molten surface. At this point, aluminum oxidation is predominant and occurs at a significantly lower oxidation rate. The results obtained in this study are consistent with models developed for dross grown on similar Al alloys in laboratory environments and show that Mg oxidation (and the accelerated formation of MgAl2O4) dominates the oxidation process during Al melting, whether the Al contains low or high Mg contents. The oxide morphology within the dross layer differed according to the particular alloy being melted and thus the amount of Al recovery from dross can vary with composition.
Keywords: Al alloys, dross formation, microstructural evaluation
Internal oxidation and local equilibrium
J.E. Morral and Yali Li
Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3136, USA
Local equilibrium principles can provide boundary conditions for modeling internal oxidation. For example at the moving boundary between the oxidized and oxide free regions, the principles require that average concentrations and matrix concentrations are equal and strongly suggest that “enrichment” of the oxidized region with solute will not occur. Instead, depletion of solute is always predicted for dilute alloys. Also, a flux of solute from the internally oxidized region toward the free surface is always predicted except in the limiting case of a zero solubility limit.
Keywords: internal oxidation, local equilibrium
Parameters affecting transient oxide formation on FeCrAl based foil and fibre materials
D. Naumenko1, W. J. Quadakkers1, A. Galerie2, Y. Wouters2 and S. Jourdain3
1Forschungszentrum Jülich, IWV-2, D-52425, Jülich, Germany
2Laboratoire de Thermodynamique et de Physico-Chimie Metallurgiques, BP 75 Institut National Polytechnique de Grenoble, Domaine Universitaire, 38402 Saint Martin d’Heres, France
3N.V. Bekaert S.A., Bekaertstraat 2, B-8550, Zwevegem, Belgium
The effects of oxidation temperature and atmosphere on the formation of alumina scales on two commercial FeCrAl foil materials have been investigated. The oxidized specimens were characterized using a range of surface analysis techniques including SEM, XRD, laser induced optical spectrometry (LIOS), AES and XPS. During oxidation at temperatures exceeding 1000°C, the protective oxide scales formed on FeCrAl-alloys consist mainly of alpha alumina. At lower temperatures, however, formation of transient alumina modifications, has been observed. Although after longer oxidation times transformation into the stable alpha alumina occurs, the high initial growth rate of the metastable oxide phases could lead to a critical depletion of the Al-reservoir in thin walled (e.g. 20 (m) components, resulting in early breakaway failure. The occurrence of metastable oxides cannot simply be correlated with alloy composition.
Keywords: FeCrAl foil, transient oxide formation
A life prediction model for the chemical failure of FeCrAlRE alloys: preliminary assessment of model extension to lower temperatures
J. R. Nicholls, M. J. Bennett and R. Newton
School of Industrial and Manufacturing Science, Cranfield University, Cranfield, Bedfordshire MK43 0AL, UK
FeCrAl alloys are being deployed increasingly for industrial applications at elevated temperatures, primarily because their environment protection derives from the formation of an alumina scale. However, such protection is limited ultimately by chemical failure of the scale resulting in catastrophic, non-protective corrosion rates. The accurate prediction of component failure, therefore, is essential for economic and safety reasons. A model for the chemical failure of alumina scales has been developed, which predicts the lifetimes of commercial FeCrAlRE alloys in oxidising environments. This model is based on the consumption of aluminium in the alloy through scale growth and also takes into account the effect of mechanical scale failure/spallation accelerating the rate of aluminium depletion – over the temperature range (1100–1400°C) this model has been shown to be applicable, when the scale formed was _-Al2O3. Its growth and failure processes are essentially similar over the range, although, of course, rates are temperature dependent. In other potential technological applications, for example automotive catalytic converters, service temperatures can be much lower, in some instances as low as 750°C, while component sections are considerably thinner than for the high temperature (≥1100°C) structural applications. At these lower temperatures scale growth mechanisms are more complex, involving the formation initially of transitional aluminas, which in some instances transform with time into the more stable _ alumina. Additionally, the role of the reactive element (RE) and also of the mechanical interaction between substrate and scale can vary over the lower temperature range. Component lifetime prediction at these temperatures is vital also. So, to initiate this action, this paper will present a preliminary assessment of the ability of the existing high temperature model to predict the lifetime of FeCrAlRE alloys in oxidising environments at temperatures in the range 750–1050°C.
Keywords: FeCrAIRE alloys, component lifetime prediction
Effect of partial oxygen pressure on the initial stages of high-temperature oxidation of b-NiCrAl alloys
T.J. Nijdam, L.P.H. Jeurgensa and W.G. Sloof*
Delft University of Technology, Laboratory of Materials Science, Rotterdamseweg 137, 2628 AL Delft, The Netherlands.
aPresent address is: Max Planck Institute for Metals Research, Heisenbergstrasse 3, D-70569, Stuttgart, Germany
The effect of the partial oxygen pressure (pO2) on the initial stages of oxide-layer growth of a ã-Ni- 27Cr-9Al (at.%) alloy at 1373 K was studied using X-ray photoelectron spectroscopy, X-ray diffractrometry, scanning electron microscopy and electron probe X-ray microanalysis. The alloy was oxidised at a pO2 of 0.1 and 0.2 _ 105 Pa in an UHV processing chamber and a furnace, respectively. For both pressures, a double-layered oxide structure evolved upon oxidation, which consisted of a NiCr2O4/ Cr2O3 layer on top of an _-Al2O3 layer. Two different oxidation stages were identified: an initial, very fast oxidation stage followed by a second stage of slow, parabolic growth. The initial oxidation stage was associated with the preferential oxidation of Al, followed by the internal oxidation of Al and simultaneous formation of Cr2O3 (and some NiO). The onset of the second stage ran parallel with the coalescence of internal _-Al2O3 crystallites into a continuous closed layer. Then, the internal oxidation of Al and growth of Cr2O3 ceased. A lower pO2 reduced the activity of oxygen at the oxide/alloy interface during the initial, fast oxidation stage, thereby enhancing the rate of formation of a continuous closed _-Al2O3 layer and suppressing the formation of Cr2O3 (and NiO).
Keywords: _-NiCrAl alloys, partial oxygen pressure, high temperature oxidation
Water vapor effects on the cyclic oxidation resistance of alumina forming alloys
K. Onal, M.C. Maris-Sida, G.H. Meier and F.S. Pettit
Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261, USA
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 _-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 _-Al2O3 which will be shown to be less significant in the case of alloys with extremely adherent _-Al2O3 scales. It is proposed that water vapor causes the _-Al2O3-alloy interfacial toughness to be decreased, however this effect is not sufficient to cause spalling of extremely adherent _-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.
Keywords: water vapour, cyclic oxidation, alumina
The use of two reactive elements to optimize oxidation performance of alumina-forming alloys
B. A. Pint, K. L. More and I. G. Wright
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831- 6156, USA
Standard reactive element (RE) studies have characterized the behavior of single RE additions such as Y, La or Hf. However, several commercial alumina-forming alloys are “co-doped” with two or more RE additions which allows the total amount of RE dopant in the alloy to be reduced. The oxidation performance of both commercial and laboratory-made co-doped alloys shows better scale adhesion and/or slower scale growth rates than comparable alloys with one RE addition. Characterization of the alumina scales showed no significant change in the grain structure with co-doping; however, as the total RE addition was reduced in co-doped alloys, a smaller volume of RE-rich oxides was observed within the scale. Quantification of the amount of RE ionic segregation on alumina scale grain boundaries formed on single doped and co-doped alloys showed similar amounts of segregation.
Keywords: reactive elements additions, alumina scales
Structural comparisons of passivated SI(100) by atomic and molecular oxygen
Maja Randjelovic and Judith C. Yang*
Materials Science and Engineering Department, University of Pittsburgh, Pittsburgh PA 15213 USA
We compared the structural characteristics of a silica layer formed on Si(100) by oxidation in hyperthermal atomic oxygen and molecular oxygen at 493K. The laser detonation method was used to create primarily neutral atomic oxygen with kinetic energy of 5.1eV. The silicon oxides were characterized by High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). We determined that atomic oxygen forms amorphous silica that is almost twice as thick and nearly double the surface roughness as compared to molecular oxygen – formed silica at the same temperature and time conditions
Keywords: silica layer, atomic and molecules oxygen
Degradation of Fe–Cr–Al–RE and Ni–Cr–Al–RE foils in air and combustion gas atmospheres
N.J. Simms1, R. Newton1, J.F. Norton1, A. Encinas-Oropesa1, J.E. Oakey1, J.R. Nicholls2 and J. Wilber2
1Power Generation Technology Centre, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK
2School of Industrial and Manufacturing Science, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK
In the continuing drive to increase gas turbine operating efficiencies (and reduce environmental emissions), it is necessary to consider ways of improving the temperature capabilities of hot gas path sealing materials. One potential route is to investigate the possibility of using alternative materials within the traditional honeycomb structure. This paper presents the results of investigations into the high temperature oxidation performance of a range of commercial Fe–20wt%Cr–5wt%Al–RE and Ni–16wt%Cr–5wt%Al–RE foil materials in air and simulated combusted natural gas environments. The effects of exposures for periods of up to 1500 hours have been studied in the temperature range 950–1300°C. During each series of tests the foils were subjected to regular thermal cycles (to room temperature) with dwell periods at the target exposure temperatures ranging from 20 hours at the higher temperatures to 100 hours at the lower temperatures. The degradation kinetics of each foil sample were monitored using mass change measurements at each thermal cycle. In addition, samples were periodically removed for destructive examinations to enable more meaningful metal loss measurements to be made and degradation mechanisms to be established. In this way the principal parameters governing the oxidation performance were established, as well as times to the onset of breakaway oxidation (when these fell within the exposure periods studied at each temperature). Earlier models for the performance of Fe–Cr–Al–RE materials have been adapted to describe the performances of the foils observed in this study.
Keywords: Fe–Cr–Al–RE, Ni–Cr–Al–REfoils, gas turbines
The influence of reactive element additions to _-NiAlCr alloys on the morphology of thermally grown oxides
I. J. Bennett1 and W. G. Sloof2
1Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
2Delft Universtiy of Technology, Department of Materials Science and Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands
The effect of the reactive elements (REs), Y and Zr, on oxidation of _-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 _-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 _-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 _-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 _-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.
Keywords: reactive element additions, _-NiAlCr alloys, thermally grown oxides
The effect of gas composition and contaminants on scale growth rates of FeCrAl alloys
G. Strehl1, H. Hattendorf2, A. Kolb-Telieps2, R.Newton3, R.J. Fordham4 and G. Borchardt1
1Institut füur Metallurgie, TU Clausthal, 38678 Clausthal-Zellerfeld, Germany
2Thyssen Krupp VDM, 58778 Werdohl, Germany
3Cranfield University, SIMS, Cranfield, MK43 0AL, UK
4Institute for Energy, Joint Research Center, 1755 ZG Petten, The Netherlands
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.
Keywords: gas composition, FeCrAl alloys, scale growth rates
Microstructural studies of NiAl-based model alloys and commercial coatings after isothermal oxidation
H. Svensson1, J. Angenete1, K. Stiller1 and V. Langer2
1Department of Experimental Physics and 2Department of Inorganic Chemistry,
Chalmers University of Technology and Göteborg University, SE-412 96 Göteborg Sweden
Four commercial aluminide diffusion coatings (one Pt-free) and four polycrystalline NiAl model materials, isothermally oxidised in laboratory air at 1050°C for 1 hour, have been investigated. The coatings were deposited on a single crystal Ni-based superalloy, CMSX-4. Two of the model materials are binary alloys containing different Al amounts. The two other have the same Ni/Al ratio as the binary alloys but were alloyed with 4 at% Pt. In order to control the surface roughness, all materials were polished prior to oxidation. Microstructural studies have been conducted using X-ray diffraction and scanning electron microscopy. To get a deeper insight on the platinum influence on the oxide structure, two commercial coatings (one Pt free and the other modified with Pt) were also investigated using transmission electron microscopy. The results obtained are discussed according to the influence of Al and Pt contents in the materials.
Keywords: aluminide diffusion coatings, isothermal oxidation
Microstructural evidence for counter-diffusion of aluminum and oxygen during the growth of alumina scales
V. K. Tolpygo and D. R. Clarke
Materials Department, College of Engineering, University of California, Santa Barbara, CA 93106-5050, USA
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 á-Al2O3 grain boundaries during oxidation of Fe–Cr–Al alloys, nickel aluminides and platinummodified 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.
Key words: oxidation, alumina, diffusion, oxide scale, microstructure
Growth stress – microstructure relationships for alumina scales
P. F. Tortorelli1, K.L. More1, E.D. Specht1, B.A. Pint1 and P. Zschack2
1Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA
2University of Illinois at Urbana-Champaign, Argonne, Illinois, USA
Focused monochromatic synchrotron undulator radiation was used to measure stresses in growing _-Al2O3 on FeCrAl-based and NiAl alloys during the first few hours of high-temperature air exposures at 1000–1200°C. Selected specimens were then sectioned and examined by transmission electron microscopy to characterize the microstructure of the oxides grown during the in situ stress measurement experiments. In several cases at 1100 and 1200°C, rapid decreases in an initial tensile stress were noted within the first hour of exposure. It was found that differences in high-temperature stress development in the alumina were at least partially reflected in the observed scale microstructures. Relaxation processes limited the magnitude of growth stresses at the higher temperatures such that after a few hours, stresses were typically quite low.
Keywords: growth stress, alumina scales
Self-assembly of metal-oxide nanostructures: oxidation of Cu films by In situ UHV-TEM
Guangwen Zhou and Judith C. Yang
Materials Science and Engineering Dept, University of Pittsburgh, Pittsburgh, PA 15261, USA
The effect of temperature on the epitaxial Cu2O oxide morphology has been investigated by oxidizing Cu(100) thin films at temperatures ranging from 150° to 1000°C. The evolution of the island size and shape was visualized within an in situ ultra high vacuum transmission electron microscope. Dramatically different morphologies of oxide nanostructures can be achieved by an appropriate choice of the oxidation temperature. During oxidation at low temperatures (T < 400°C), the oxide islands adopt triangular shape, but at temperatures higher than 400°C, the islands have a more symmetrical geometry. Of particular interest, quasi-one-dimensional Cu2O structures with aspect ratios as large as 40:1 were formed at the oxidation temperature of 600°C. The in situ observation data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interfacial energies and the elastic stress relaxation in the three dimensional islands.
Keywords: oxidation, copper thin film; Cu2O, morphology, nanorod, shape transition, in situ ultra high vacuum transmission electron microscopy (UHV-TEM)