| 1. |
- 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. |
- Hörnqvist Colliander, Magnus, 1979, et al.
(författare)
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Dynamic strain aging in Haynes 282 superalloy
- 2014
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Ingår i: MATEC Web of Conferences. - : EDP Sciences. - 2261-236X. ; 14, s. Art. no. 16002-
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Konferensbidrag (refereegranskat)abstract
- Haynes 282 is a newly introduced Ni-based superalloy, developed to provide a combination of high-temperature mechanical properties, thermal stability and processability. The present contribution investigates the effect of dynamic strain aging (DSA) on the deformation behaviour of Haynes 282 during monotonic and cyclic loading. It is shown that DSA (presumably related to carbon diffusion based on rough estimates of the activation energy) completely dominates the development of the stress during cycling at intermediate temperatures, leading to extensive cyclic hardening and serrated yielding. However, no clear effects on the fatigue life or the resulting dislocation structure could be observed. The tensile properties were not severely affected, in spite of the presence of extensive serrated yielding, although a reduction in ductility was observed in the DSA temperature regime. During monotonic loading at lower strain rates indications of an additional DSA mechanism due to substitutional elements were observed.
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| 3. |
- Johansson, Joakim, 1983, et al.
(författare)
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Microstructural examination of shear localisation during high-strain-rate deformation of Alloy 718
- 2016
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 662, s. 363-372
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Tidskriftsartikel (refereegranskat)abstract
- Chip formation during metal cutting involves high strain rates and large deformations. Under many conditions, the deformation is concentrated in narrow bands due to shear localisation from adiabatic heating. In order to understand the localisation process, it is necessary to increase the knowledge regarding the microstructural evolution during deformation. However, the deformation that occurs during chip formation is hard to measure. Therefore, this study utilises top-hat specimens deformed at high strain rates in order to generate localised shear bands in the Ni-based superalloy Alloy 718, with defined measurable deformation. The resulting shear bands in the top-hat specimens are compared with those generated in metal cutting chips and studied in order to characterise the deformation occurring at a microstructural level. The resulting microstructures in the top-hat specimens and machining chips are found to be similar, with heavily localised deformation into narrow shear bands and homogeneously sheared microstructure adjacent to the bands. The centres of the shear bands are heavily deformed with ultra-fine grains, indicating dynamic recrystallisation during the deformation. The results indicate that the shear deformation produced by high strain rate testing of top-hat specimens can provide an excellent means of replicating the conditions for shear localisation during metal cutting. However, care should be taken to design the tests so that the local conditions are representative in terms of strains and strain rates.
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| 4. |
- 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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| 5. |
- Jahns, Katrin, et al.
(författare)
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On oxide formation on a single crystalline Ni-based superalloy at 900 °C in SO2 containing atmosphere: The effect of surface treatment
- 2021
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 180
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation behavior of the single crystalline SCA425+ (17Cr-10Al-3Ta-Ni bal.) after two different surface finish procedures (grinding and polishing) was investigated after cyclic exposures at 900 degrees C in SO2 containing atmosphere up to 260 h. The microstructure and chemistry of the produced oxide scales were studied using highresolution methods, SEM/EDX, TEM, and STEM. The investigation revealed that grinding provides more effective diffusion paths, so that the material is able to form a protective continuous alumina layer, while an extensive internal oxidation takes place on the polished sample. In addition, a numerical model, based on the cellular automata approach, was developed and used to consider the effect of dislocation pipe diffusion in combination with diffusion processes and oxide formation. Good agreement between the model and experimental results was obtained.
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| 6. |
- Knutsson, Pavleta, 1980, et al.
(författare)
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A method for investigation of hot corrosion by gaseous Na2SO4
- 2013
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 73, s. 230-236
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Tidskriftsartikel (refereegranskat)abstract
- A novel in situ exposure method for investigating hot corrosion of aluminde coatings in the absence of an alkali sulphate melt is presented. The samples are exposed to Na2SO4(g) at a temperature above the dew point of the gas. Results from 100 and 500 h exposures at 900 degrees C of Ni-based In792 covered with SIF232 aluminide coating are presented. Already after 100 h of testing, attack characteristic for Type I hot corrosion is observed. The reactions taking place during the testing are discussed and the results are compared with those from the well-known ex situ salt hot corrosion test.
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| 7. |
- Lai, Haiping, 1979, et al.
(författare)
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High Temperature Corrosion of Ni-Based Alloys SCA425+and IN792
- 2013
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 80:5-6, s. 505-516
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports the high temperature corrosion of two Ni-base superalloys: a newly developed alloy, SCA425+, and the extensively used IN792. The composition of the two materials is quite similar, but SCA425+ contains more Cr and Al (17.1 and 10 at.% compared to 13.9 and 7.4 at.% in IN792). The results from exposures at 900 A degrees C in SO2 (3,000 ppm) + O-2 (69 vol%) + H2O (31 vol%) mixed gas for 260 h using 65 h/cycle are compared with those obtained from tests in laboratory air. The microstructure of the formed oxide scales was studied using several techniques, such as XRD, SEM, FIB, EDX, STEM and XPS. It is shown that in IN792 severe internal oxidation takes place after both types of exposures. On the other hand, the newly developed SCA425+ has the tendency to form an alumina layer proving that it has more potential to be used in the aggressive environments. Surprisingly the mass gains for SCA425+ alloy exposed in SO2-rich environment are lower than in laboratory air. The reason for this behavior is discussed.
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| 8. |
- Lai, Haiping, 1979
(författare)
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High Temperature Oxidation and Corrosion of Ni-Based Superalloys for Industrial Gas Turbines
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High temperature oxidation and corrosion of Ni-based superalloys used for blades in industrial gas turbines was studied using advanced high-resolution electron microscopy methods. Both coated and non-coated alloys were tested.In the case of alloys covered by protective coatings, the influence of Pt on the material corrosion properties at 900°C was investigated. Due to an extensive research it is known that Pt improves oxidation resistance of coatings above 1000°C. However, the effect of Pt on the oxidation and corrosion resistance at lower temperatures and in the presence of sulphates has not been studied to the same extent. To simulate hot corrosion occurring in gas turbines, a novel method, based on the evaporation of Na2SO4 in the furnace was developed. The investigation showed that the presence of Pt improves the protective properties of the coating against oxidation and hot corrosion. The propagation stage of the corrosion on the platinum-free coating was reached already after 100 h of exposure while the scale formed on the platinum-rich coating was thinner and denser, and still appeared to be protective even after 500 h. It was proposed that Pt suppresses the formation of interfacial cavities, degrading the scale adherence by the faster diffusion of Al ions, which can fill the voids.The performed work on uncoated superalloys includes studies of effects of a) the surface treatment; b) the aggressive environments and c) sulfur in the material on the corrosion properties of the newly developed alloy, SCA425+ and the extensively used alloy IN792. Cyclic exposures up to 260 hours were performed in different atmospheres such as: air, SO2+O2+Ar, SO2+H2O+Ar and H2O+O2. The study showed that due to the relatively high Al content, 10 at.%, SCA425+ might form a protective alumina layer, provided that it receives a surface treatment that creates a zone of defects allowing for a fast enough supply of Al to the metal surface. On the other hand, IN792 does not form a protective oxide scale in any of the investigated conditions and the structure and morphology of the oxide scale are not very sensitive to the surface treatment. Furthermore, it is shown that the most pronounced effect on the corrosion behavior of the investigated material has the presence of moisture in the atmosphere while the effect of SO2 up to 260 h exposure seems to be less important. In the presence of water vapor Cr is depleted from SCA425+ due to the evaporation of chromia. The loss of Cr2O3 was estimated using a comparison of the Cr depletion in the metal with the amount of Cr2O3 in the oxide scale. The results also indicate that the volatilization of chromia slows down with increased exposure. The experiment concerning the influence of sulfur in the material, desulfurization of SCA425+ in hydrogen indicates that S degrades the adherence of the oxide scale.
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| 9. |
- Lai, Haiping, 1979, et al.
(författare)
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The influence of platinum on the oxidation and sodium sulfate induced hot corrosion of NiAl diffusion coatings
- 2011
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Ingår i: Materials at High Temperatures. - 0960-3409. ; 28:4, s. 302-308
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation and corrosion properties at 900 degrees C of two coatings, one containing platinum and the other platinum-free, on IN792 were investigated. During the corrosion exposures, sodium sulfate salt was situated in the furnace together with the coated specimens. The temperature of the salt was kept above its melting point but lower than the temperature of the coating i.e. 900 degrees C. The exposure times ranged from 100 to 1000 h. The formed oxide scales were studied by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and scanning transmission electron microscopy techniques. It is shown that the presence of Pt improves the protective properties of the coating against corrosion. The propagation stage of corrosion on the platinum-free coating is reached already after 100 h of exposure. On the other hand the scale formed on the platinum-rich samples still appeared to be protective even after 500 h. The details concerning structure and chemistry of the scales formed are presented and discussed.
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| 10. |
- Tuzi, Silvia, 1987, et al.
(författare)
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Corrosion of pre-oxidized nickel alloy X-750 in simulated BWR environment
- 2017
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115. ; 486:1 April, s. 350-360
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Tidskriftsartikel (refereegranskat)abstract
- Samples of pre-oxidized Alloy X-750 were exposed to a simulated boiling water reactor environment in an autoclave at a temperature of 286 °C and a pressure of 80 bar for four weeks. The effect of alloy iron content on corrosion was investigated by comparing samples with 5 and 8 wt% Fe, respectively. In addition, the effect of two different surface pre-treatments was investigated. The microstructure of the formed oxide scales was studied using mainly electron microscopy. The results showed positive effects of an increased Fe content and of removing the deformed surface layer by pickling. After four weeks of exposure the oxide scale consists of oxides formed in three different ways. The oxide formed during pre-oxidization at 700 °C, mainly consisting of chromia, is partly still present. There is also an outer oxide consisting of NiFe2O4 crystals, reaching a maximum size of 3 ?m, which has formed by precipitation of dissolved metal ions. Finally, there is an inner nanocrystalline and porous oxide, with a metallic content reflecting the alloy composition, which has formed by corrosion. © 2017 Elsevier B.V.
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