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- Canovic, Sead, 1979, et al.
(författare)
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Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O-2 at 600 and 800 degrees C
- 2010
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 157:6, s. C223-C230
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Tidskriftsartikel (refereegranskat)abstract
- The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800 degrees C for 72 h in dry O-2 and in O-2 with 10 vol % H2O. The mass gains were 3-5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800 degrees C, while no significant effect was observed at 600 degrees C. A thin two-layered oxide formed at 600 degrees C: an outer (Fe, Cr)(2)O-3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800 degrees C a two-layered oxide formed in both environments. The inner layer consisted of inward grown alpha-Al2O3. In dry O-2 the originally formed outward grown gamma-Al2O3 had transformed to alpha-Al2O3 after 72 h. Water vapor stabilized the outward grown gamma-Al2O3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800 degrees C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid O-2.
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| 2. |
- Liu, Fang, 1975, et al.
(författare)
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Cyclic oxidation of two FeCrAlRE foils at 1100 oC - The influence of the concentration of minor alloying elements on scale microstructure
- 2008
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 595-598, s. 707-716
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Konferensbidrag (refereegranskat)abstract
- Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils,with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction(GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray(EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloyshave the same weight gain after 500 hours exposure. During the early stages the scale consistsmainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on thealloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions ofMn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at themetal/oxide interface.
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| 4. |
- Canovic, Sead, 1979, et al.
(författare)
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CVD TiC/alumina and TiN/alumina multilayer coatings grown on sapphire single crystals
- 2010
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Ingår i: International Journal of Refractory Metals & Hard Materials. - : Elsevier BV. - 0263-4368. ; 28:2, s. 163-173
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Tidskriftsartikel (refereegranskat)abstract
- Multilayers of TiC/alpha-Al2O3 and TiN/kappa-Al2O3, consisting of three (1 mm thick) alumina layers separated by thin TiC or TiN (similar to 20 nm thick) layers, have been deposited onto c- and r-surfaces of single crystals of alpha-Al2O3 by chemical vapour deposition (CVD). The aim of this paper is to describe and compare the detailed microstructure of the different multilayer coatings by using transmission electron microscopy (TEM). The general microstructure of the alumina layers is very different when deposited onto different surfaces of alpha-Al2O3 single crystal substrates. On the c-surface the alumina layers grew evenly resulting in growth of single crystal layers of TiC or TiN and alumina throughout the coating. However, when deposited on the r-surface the alumina layers generally grow unevenly. The kappa -> alpha transformation occurs in TiN/kappa-Al2O3 multilayer coatings when deposited on both c- and r-surface substrates. In each layer the transformation starts at the interfacial pores in the upper part and proceeds inwards. The inner layer starts to transform first before the transformation proceeds to the outer layers. The microstructure of the transformed alpha-Al2O3 layers is different compared to as-deposited alpha-Al2O3, e.g. several voids and dislocations are present within the transformed alpha-Al2O3 layers. No pores were observed within the as-deposited alumina layers while a small number of pores was observed at the interfaces below the TiC and TiN layers. However, linkage of pores was observed within the transformed alpha-Al2O3 layers. There are also pores present in the substrate below the innermost TiC and TiN layers. It is believed that they are formed due to chemical etching during the deposition of the TiC and TiN layers. Single crystal TiC/alumina and TiN/alumina layers grow epitaxially on the c-surface substrates with close-packed planes growing on close-packed planes. On the r-surface, epitaxy is present only at some rare locations in alpha(r-sub)-TiC/Al2O3. However, in alpha(r-sub)-TiN/Al2O3 no epitaxy was observed. (C) 2009 Elsevier Ltd. All rights reserved.
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| 9. |
- Canovic, Sead, 1979
(författare)
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Microstructure of Alumina (Al2O3) Grown by Oxidation of FeCrAl Alloys and by Chemical Vapour Deposition
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of two parts: the detailed microstructure of (i) CVD alumina multilayer coatings, and (ii) alumina scales formed on three different FeCrAl alloys. The coatings and oxide scales were characterized by using analytical electron microscopy and different surface analysis techniques.The first part of this work investigates the interfacial structure in hot-wall CVD TiC/Al2O3 and TiN/Al2O3 multilayer coatings deposited both on cemented carbide substrates (which are commercially used) and on α-Al2O3 single crystals (model systems) with different surfaces. In TiN/κ Al2O3 multilayers epitaxial columns were frequent and the same orientation relationships were found when deposited on both types of substrates. One difference between the two cases (cemented carbide substrates and model substrates) was that γ-Al2O3 could grow (for a short distance) on the TiN layers in the former case, while no γ-Al2O3 was found in the latter case. In addition, when TiN/κ-Al2O3 multilayers were deposited on the model substrates the κ→α phase transformation occurred. The microstructure of the transformed α-Al2O3 layers was different compared to as-deposited α-Al2O3, e.g. several voids and dislocations formed within the transformed α-Al2O3 layers. Also TiC/α-Al2O3 multilayers have been deposited on different surfaces of single crystals of α-Al2O3. In this case, the TiC layers were oxidized in-situ prior to the alumina deposition. The general microstructure of the alumina and TiC layers was very different when deposited onto different surfaces of α-Al2O3 single crystal substrates.The second part of this work documents detailed microstructural investigations of three different alumina-forming FeCrAl alloys exposed to high temperature oxidation under various conditions. The oxidation behaviour of the investigated alloys follows a similar trend no matter if they are cast or manufactured by powder metallurgy. Before oxidation, on the surface of the FeCrAl alloys, a thin native oxide containing Fe, Cr and Al, with relatively high Cr content was formed. After oxidation at relatively low temperatures, 500‒600°C, an oxide containing a mixture of Fe, Cr and Al with a thickness of less than 100 nm formed on the Cr-rich native oxide. At higher temperatures (700°C or above) two-layered alumina scales formed, separated by a Cr-rich band that is believed to be the remnant of the pre-existing native oxide and hence represent the original alloy/gas interface. Accordingly the inner alumina layer is formed by inward oxygen diffusion, while the outer part forms by outward cation diffusion. The inner alumina layer consists of α-Al2O3 in both dry and wet O2. The outer alumina layer initially forms rapidly growing metastable phases, which transform to α-Al2O3 with time. The phase transformation starts at the Cr-rich band and proceeds outwards. The presence of water vapour inhibits the phase transformation, which is believed to be due to the stabilization of γ-Al2O3 by water vapour. Hence, a higher oxidation rate was observed in presence of water vapour.
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| 10. |
- Canovic, Sead, 1979, et al.
(författare)
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Oxidation of Co- and Ce-nanocoated FeCr steels: A microstructural investigation
- 2013
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 215, s. 62-74
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Tidskriftsartikel (refereegranskat)abstract
- The effect of novel Co and Ce nanocoatings on oxidation behaviour and chromium volatilization from a commercial Fe-22Cr steel (Sanergy HT) developed for solid oxide fuel cell interconnect applications is investigated. Three different coatings (10 nm Ce, 640 nm Co and 10 nm Ce + 640 nm Co) are studied. Uncoated and nanocoated samples are exposed isothermally at 850 C in the air with 3% H2O for 168 h. The detailed microstructure of the different coatings is investigated. The surface morphology and microstructure of the oxide scales are characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX). Cross-section TEM thin foils are prepared by using a combined FIB/SEM (focused ion beam/scanning electron microscope) instrument. A 640 nm cobalt coating strongly inhibits Cr volatilization but has only minor effects on oxidation rate. In contrast, a 10 nm Ce coating decreases the oxidation rate but has no significant effects on chromium volatilization. Combining the two coatings, i.e., applying a 640 nm Co coating on top of the 10 nm Ce, effectively reduces Cr evaporation and slows down the rate of alloy oxidation.
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