| 1. |
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| 2. |
- Bacos, M P, et al.
(författare)
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ALLBATROS advanced long life blade turbine coating systems
- 2004
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 24:11-12, s. 1745-1753
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Tidskriftsartikel (refereegranskat)abstract
- The scientific and technological objectives of this program are to increase the efficiency, reliability and maintainability of industrial gas turbine blades and vanes by• developing coatings that can warrant a 50 000 h life, i.e. twice that of the usual life, of the hot components (800–1100 °C) even with the use of renewable fuels such as biomass gas or recovery incinerator gas i.e. low-grade fuels with high pollutant levels,• characterising advanced existing coatings to assess lifetime and performance of coatings and coated materials,• providing material coating data and design criteria to use coating as a design element,• increasing the fundamental understanding of the behaviour of coated materials, their degradation, fracture mechanisms and engineering because of the strong need for a mechanism-based modelling of durability.These programmes permitted the selection of two reference coatings and the development of two innovative coatings. Concurrently work has been done in order to develop corrosion, oxidation and thermo-mechanical property models. Correlations between coatings development, experimental results and calculations will be discussed.
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| 3. |
- Chantziara, Katerina, 1993-
(författare)
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Fatigue, defects and failure mechanisms in high strength tool steels at different fatigue life regimes
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue response of metallic materials is considered of significant importance, particularly for high-demanding applications. It is proved that most of the engineering materials do not exhibit a conventional fatigue limit in the high cycle fatigue regime, but rather display a continuously decreasing stress-life response at even longer lifetimes. Consequently, investigations of the various failure mechanisms taking place are essential, especially at the high and very high cycle fatigue regimes.The development of new ultrasonic testing equipment made the fatigue testing beyond 107 life cycles possible in a much shorter time, enabling testing with fatigue fractures at stress levels lower than the traditionally proposed “fatigue limit”. Nowadays, a classification of fatigue life regimes in Low Cycle Fatigue (LCF), High Cycle Fatigue (HCF) and Very High Cycle Fatigue (VHCF) is commonly used. The main reasons for this specific grading are: i) the need for safe design of components and ii) the fact that the failure mechanisms are particular in each of the LCF, HCF and VHCF regimes.The main goal of the present thesis is to address the fatigue response of high strength tool steels. Considering the novel alterations in composition and production methods in alloy development, materials of high-quality are continuously being introduced to the market; understanding the fatigue response of these materials is crucial for potential utilization across diverse industries and applications. The generation of fatigue experimental data, the analysis of the different types of fatigue initiation defects found in each material, as well as the investigation of the fatigue mechanisms occurring during cyclic loading are the main subjects analyzed throughout the present study.
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| 4. |
- Cruchley, S., et al.
(författare)
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Cautionary note on use of focused ion beam sectioning as technique for characterising oxidation damage in Ni based superalloys
- 2014
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Ingår i: Materials at High Temperature. - : Science Reviews 2000 Ltd.. - 0960-3409 .- 1878-6413. ; 31:1, s. 27-33
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Tidskriftsartikel (refereegranskat)abstract
- Previous observations on Ni based superalloys, obtained through the use of focused ion beam (FIB) sample preparation and imaging, have reported the presence of subsurface voids after oxidation. In this present study, oxidised specimens of the Ni based superalloy, RR1000, were subjected to conventional sample preparation as well as both dual and single beam FIB preparation, with the aim of re-examining the previous observations of subsurface void formation. It is clear from FIB preparations that features previously interpreted as networks of voids have been demonstrated to be internal oxides by varying the sample tilt angles and imaging signal using either secondary electrons (SEs) or secondary ions (SIs). Conventional preparation methods illustrate the presence of subsurface alumina intrusions and the absence of voids, supporting previous evidence. The positive identification of voids and oxides by FIB can be complex and prone to misinterpretation and thus, the use of several imaging conditions and tilt angles must be used, along with conventional preparation methods, to confirm or refute the presence of voids underneath oxides.
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| 5. |
- Cruchley, S, et al.
(författare)
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Chromia layer growth on a Ni-based superalloy: Sub-parabolic kinetics and the role of titanium
- 2013
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Ingår i: Corrosion Science. - : Elsevier. - 0010-938X .- 1879-0496. ; 75, s. 58-66
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation of the Ni-based superalloy RR1000 has been undertaken in air over the temperature range 600-900 degrees C for times up to 5000 h. The surface oxide consisted of a protective Ti-doped chromia layer but with rutile forming on its outer surface. Sub-surface oxidation of Al and Ti also occurred. The thickening kinetics of the chromia layer were sub-parabolic with initial rates around two orders of magnitude higher than expected for Ti-free chromia. This enhancement and the sub-parabolic kinetics are accounted for by Ti-doping of the chromia layer. Over time the enhancement reduced because of Ti-depletion in the alloy.
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| 6. |
- Engel, B., et al.
(författare)
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Modelling the influence of plasticity induced softening on the low cycle fatigue and crack propagation behaviour of a nickel-based superalloy
- 2024
-
Ingår i: Computational materials science. - : ELSEVIER. - 0927-0256 .- 1879-0801. ; 231
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Tidskriftsartikel (refereegranskat)abstract
- Cyclic hysteresis loops generated during high-temperature, isothermal, low-cycle fatigue testing of the nickel-based superalloy RR1000 revealed a gradual decrease in elastic modulus of up to 25 % compared to the initial value in dependence of the accumulated plastic strain. Based on the experimental observations, a material model was developed and implemented within a finite element solver to investigate the effect of degrading stiffness on crack growth laws and crack growth predictions. It was shown that the stress intensity factor diminished by 6 to 8 % and crack growth rates were 14 to 20 % higher when stiffness degradation was accounted for.
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| 7. |
- Engel, B., et al.
(författare)
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The prediction of crack propagation in coarse grain RR1000 using a unified modelling approach
- 2020
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Ingår i: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 137
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Tidskriftsartikel (refereegranskat)abstract
- The polycrystalline nickel-base superalloy RR1000 is used as turbine rotor material in Rolls-Royce aero engines and has to withstand a wide variety of load and temperature changes during operation. In order to maximize the potential of the material and to improve component design, it is of great interest to understand, and subsequently be able to accurately model the crack propagation caused by thermo-mechanical fatigue conditions. In this work, experimental data is analysed and used to inform unified modelling approaches in order to predict the crack propagation behaviour of RR1000 under a variety of stress-controlled thermo-mechanical fatigue conditions.
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| 8. |
- Jones, J., et al.
(författare)
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The effect of phase angle on crack growth mechanisms under thermo-mechanical fatigue loading
- 2020
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Ingår i: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 135
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Tidskriftsartikel (refereegranskat)abstract
- The current paper describes TMF crack growth behaviour in an advanced nickel-based superalloy. Changes in behaviour are examined which occur as a function of the phase angle between applied stress and temperature. The fractography of the failed specimens reveals changes from transgranular to intergranular growth between high and low phase angle tests as a result of the onset of high temperature damage mechanisms. More targeted testing has also been undertaken to isolate the contributions of these mechanisms, with specific transitions in behaviour becoming clear in 90 degrees diamond cycles, where dynamic crack growth and oxidation strongly interact.
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| 9. |
- Leidermark, Daniel, et al.
(författare)
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Thermomechanical fatigue crack initiation in disc alloys using a damage approach
- 2018
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Ingår i: 12TH INTERNATIONAL FATIGUE CONGRESS (FATIGUE 2018). - : E D P SCIENCES.
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Konferensbidrag (refereegranskat)abstract
- A fatigue crack initiation model based on damage accumulation via a fatigue memory surface in conjunction with a plastic strain energy parameter was evaluated for thermomechanical fatigue loading in a gas turbine disc alloy. The accumulated damage in each hysteresis loop was summed up, and it was assumed that the damage at the stable state is repeated until failure occurs. Crack initiation occurs when enough fatigue damage has been obtained, and the number of cycles can thus be directly determined. The fatigue damage is highly coupled to the constitutive behaviour of the material, where the constitutive behaviour was modelled using a non-linear hardening description. Based on this, a stable state was achieved and the obtained damage could be extracted. A user-defined material subroutine was implemented, incorporating both the constitutive description and the fatigue damage accumulation. The framework was adopted in a finite element context to evaluate the thermomechanical fatigue crack initiation life of the disc alloy RR1000. From the evaluation it could be seen that a good prediction of the thermomechanical fatigue life was achieved compared to performed experiments.
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| 10. |
- Leidermark, Daniel, et al.
(författare)
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Thermomechanical fatigue life due to scatter in constitutive parameters
- 2021
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Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 198
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Tidskriftsartikel (refereegranskat)abstract
- For critical component application, such as aerospace turbine rotors, it is imperative to be able to make accurate in-service material behaviour and component life predictions for both design and monitoring of component life. The development of such predictive capability is dependent on the quality of the experimental data from which the material parameters are derived. This paper shows the effect that scatter which may be present within experimental data, manifesting itself within the constitutive parameters derived from this data, has on the resulting fatigue crack initiation life of the nickel-based superalloy RR1000. Industrial relevance was added to this investigation by the use of flight representative thermomechanical fatigue loading cycles and state of the art material behaviour and fatigue crack initiation models used within the finite element simulations conducted. The effect of the scatter in to the modelling approach on the outcoming predictions is made via a Monte-Carlo analysis. This analysis consisted of running the same simulation several times, but with the experimentally determined and validated baseline constitutive parameters varied via correction factors built into the model, for each run via a singular value decomposition procedure. It was found that small scatter in has only a very localised scatter out effect on the crack initiation predictions under the flight representative loading.
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| 11. |
- Messe, O. M. D. M., et al.
(författare)
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Characterization of Plastic Deformation Induced by Shot-Peening in a Ni-Base Superalloy
- 2014
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Ingår i: JOM. - : Springer Verlag (Germany). - 1047-4838 .- 1543-1851. ; 66:12, s. 2502-2515
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Tidskriftsartikel (refereegranskat)abstract
- The shot-peening process is currently employed in most industries to improve the longevity of components by inhibiting crack initiation as well as crack growth at the surface. The protective effect of shot peening has been mainly attributed to compressive stresses within the deformed layer. Intensive research has been carried out to quantify the near-surface residual stresses on entry into service and evolution throughout life. In nickel-base superalloys, the focus of research on the effects of shot-peening has performed using x-rays from either laboratory or synchrotron-based sources. However, this approach cannot evaluate in detail the deformation mechanisms nor the role of the gamma precipitates in a nickel-base superalloy; the latter is responsible for its unique properties. Our study uses a complementary range of techniques to investigate in detail the microstructure and deformation mechanisms associated with shot-peening in a coarse-grained nickel-based superalloy strengthened with coherent gamma precipitates. These include scanning electron microscopy and transmission electron microscopy, nanoindentation and micropillar compression. Accurate mapping of the dislocation structure produced throughout the deformed layers have been performed. Using an unconventional specimen preparation technique, it provides the basis for a more complete interpretation of how shot-peening inhibits fatigue cracking.
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| 12. |
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| 13. |
- Norman, Viktor, 1988-, et al.
(författare)
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On the Mechanistic Difference Between In-phase and Out-of-phase Thermo-Mechanical Fatigue Crack Growth
- 2020
-
Ingår i: International Journal of Fatigue. - : Elsevier. - 0142-1123 .- 1879-3452. ; 135, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- The crack driving mechanisms in a coarse grained nickel-base superalloy RR1000 when subjected to in- and out of phase thermo mechanical fatigue are investigated. It is found that the difference in fatigue crack growth rate between these two load conditions is accounted for by the different mechanical conditions at the crack tip region, rather than oxidation effects. This is based on digital image correlation and finite element analyses of the mechanical strain field at the crack tip, which demonstrate that in phase leads to larger crack tip deformation and crack opening. Notably, it is demonstrated that in- and out of phase crack growth rates coincide when correlated to the crack tip opening displacement.
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| 14. |
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| 15. |
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| 16. |
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| 17. |
- Stekovic, Svjetlana
(författare)
-
DevTMF – Code of Practice for Thermo- Mechanical Fatigue Crack Growth
- 2020
-
Ingår i: Recent developments in Thermo-Mechanical Fatigue. - Berlin : Elsevier.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- DevTMF (Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Behaviour and Damage Mechanisms) is focused on contributing to development of new materials and improving efficiency of aero engine components that will reduce fuel consumption and environmental impact. This is ultimately achieved by either introduction of novel engine design or development of new materials able to sustain complex loadings from take-off, cruise, descent and shut down. Specifically, Thermo-Mechanical Fatigue (TMF), which occurs at the rim of turbine discs, aero-foils and rear structures, requires assessment in terms of both crack initiation and crack propagation as the harsh thermal transients during take-off and descent may cause the formation of cracks.A previous European project on TMF funded by the FP5, which ended in 2005, has been successful at evaluating and addressing issues related to strain-controlled TMF testing. This project led to the development of a code of practice for TMF studies, which has been implemented by testing houses and equipment manufacturers worldwide. Apart from work on strain-controlled TMF, DevTMF has been aiming to evaluate and develop TMF crack growth (CG) methods that are essential to demonstrate structural integrity and certification requirements of being tolerant of handling damage as well as to assess remnant lives in cracked turbine components.The current paper presents work on the identification and evaluation of a range of factors influencing accuracy, replicability, repeatability and comparability of data generated by three laboratories carrying out stress-controlled TMF CG tests. It addresses the crack length measurement methods, the temperature and heating methods, and the temperature measurement techniques. It also provides recommendations and guidance for the pre-cracking procedures and the use of various specimen geometries as well as the Digital Image Correlation (DIC) technique for monitoring cracks. The majority of the TMF CG tests has been carried out on a coarse grain polycrystalline nickel-base superalloy using two types of phase angles, namely Out-of-Phase (OP) and In-Phase (IP) TMF cycles with a triangular waveform.
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| 18. |
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| 19. |
- Stekovic, Svjetlana, et al.
(författare)
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DevTMF - Towards code of practice for thermo-mechanical fatigue crack growth
- 2020
-
Ingår i: International Journal of Fatigue. - : ELSEVIER SCI LTD. - 0142-1123 .- 1879-3452. ; 138
-
Tidskriftsartikel (refereegranskat)abstract
- The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400-750 degrees C.
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| 20. |
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| 21. |
- Stekovic, Svjetlana, 1967-, et al.
(författare)
-
Effect of Nitriding on Microstructure and Mechanical Properties on a Ti64Alloy for Aerospace Applications
- 2022
-
Ingår i: ICAS PROCEEDINGS. - Bonn.
-
Konferensbidrag (refereegranskat)abstract
- Titanium is 40% lighter than steel and is very strong in relation to its low weight, which makes it veryinteresting for lightweight applications. However, the use of titanium in certain aircraft components islimited because titanium is a relatively soft metal that quickly deteriorates when mechanically stressed.In this research, a nitriding heat treatment has been developed for Ti64 (Grade 5) alloy with the aimto improve wear properties without negative effect on fatigue and strength. The mechanical propertieswere studied through hardness and wear tests performed at room temperature in laboratory air onuntreated and treated Ti64. Different measurements techniques were used to evaluate hardness onsurface as well as polished cross-sections due to uncertainties in hardness measurements of thinfilms. The wear properties were investigated with pin-on-disc tests. The microstructures and nitridedsurfaces were also investigated by optical microscopy, scanning electron microscopy (SEM) andsurface profilometry. The analysis has shown that the nitriding process has led to the formation of anuneven compound layer and a diffusion zone beneath it. The energy dispersive X-ray spectroscopy(EDS) mapping showed a high concentration of nitrogen in the compound layer and aluminium in thediffusion zone. The microhardness measurements and nanoindentation have revealed the formationof an approximately 2.5 μm thick diffusion zone. The wear tests results showed a large difference infriction behaviour between the nitrided specimens, which has been associated with the failure of thenitrided layer and the wear rate.
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| 22. |
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| 23. |
- Stekovic, Svjetlana
(författare)
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Influence of long term aging on microstructure, LCF properties and deformation behaviour of coated nickel-base superalloys at elevated temperatures
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The objective of this study is to examine and establish the influence of long-term aging on microstructure, low-cycle fatigue life properties and the fracture behaviour of coated polycrystalline and single-crystal nickel-base superalloys. Long-term aging in metallic coating results in the changes of mechanical properties due to the significant interdiffusion of the main alloying elements between substrate and coatings. For this purpose, a polycrystalline nickel-base superalloy IN792 and two single crystal nickel-base superalloys CMSX-4 and SCB coated with three different coatings were used. The coatings were an overlay coating AMDRY997, a platinum-aluminide diffusion coating RT22 and an innovative coating with interdiffusion barrier of NiW called IC1. Cylindrical solid specimens were first aged at 1050oC under 2000 h to simulate long-term exposure of aircraft engine service environment and then cyclically deformed with fully reversed tension-compression loading total strain amplitude control at two elevated temperatures of 500oC and 900oC and a constant strain rate of 10-4s-1 (6%/ min) in air atmosphere without any dwell time. This tests indicate that long-term aging influences the fatigue behaviour and fracture of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged and uncoated samples. Fatigue life of aged samples exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks in most cases were initiated at the surface of the coating, growing intergranularly perpendicular to the load axis. Major degradation mechanism in AMDRY997 coating deposited on CMSX-4 tested at 900oC is surface oxidation and interdiffusion with the substrate. Cracks in this aged coated system propagated transgranularly through the coating changing the path behaviour when passing the interdiffusion zone.
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| 24. |
- Stekovic, Svjetlana, 1967-
(författare)
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Isothermal low cycle fatigue of uncoated and coated nickel-base superalloys
- 2004
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. However, the superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface of the superalloys.The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with elements like aluminium, chromium, which form a thermodynamically stable oxide layer that acts as a diffusion barrier to slow down the react ion between the substrate material and the aggressive environment. There are also other degradation mechanisms that affect nickel-base superalloys such as aging of microstructure, fatigue and creep. Long-term aging in metallic coating results in the changes of mechanical properties due to the significant interdiffusion of the main alloying elements between substrate and coatings. However, application of the coatings has mechanical side effects, the significance of which is not yet fully investigated.This work covers a study on the fatigue behaviour of a polycrystalline, IN792, and two single crystal nickel-base superalloys, CMSX-4 and SCB, coated with three different coatings, an overlay coating AMDRY997, a platinum aluminide modified diffusion coating RT22 and an innovative coating with an interdiffusion barrier of NiW called ICl , under low cycle fatigue loading conditions. Both low cycle fatigue properties, cyclic strain and stress response and fracture behaviour of the uncoated, coated and long-term aged coated specimens are presented.The main conclusions are that at 500oC the presence of the coatings have, in most cases, reduced the fatigue lives of the nickel-base substrates while at 900oC the coatings do improve the fatigue lives of the superalloys except RT22 coated on some superalloys and under certain test conditions. The reduction of the fatigue life at 500oC can be related to early cracking of the coatings below their ductile to brittle transition temperature (DBTT), where their surface roughness can act as notches affecting fatigue crack initiation. The beneficial effect of the coating at 900oC may be due to slower crack propagation caused by oxidation at the front of the crack tip. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged and uncoated samples. The aged samples exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing intergranularly perpendicular to the load axis.
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| 25. |
- Stekovic, Svjetlana, et al.
(författare)
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Low-Cycle Fatigue and Damage of an Uncoated and Coated Single Crystal Nickel-Bse Superalloy SCB
- 2007
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Ingår i: International Journal of Materials Research. - 1862-5282. ; 98:1, s. 26-32
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents low-cycle fatigue (LCF) behaviour and damage mechanisms of uncoated and coated specimens of a single crystal nickel-base superalloy SCB tested at 500°C and 900°C. Four coatings were deposited on the base material, an overlay coating AMDRY997, a platinum-modified aluminide diffusion coating RT22 and two innovative coatings called IC1 and IC3 with a NiWdiffusion barrier in the interface. AMDRY997 and RT22 were used as reference coatings. The LCF tests were performed at three strain amplitudes, 1.0, 1.2 and 1.4%, with R ¼ % 1, in laboratory air and without any dwell time. The LCF life of the specimens is determined by crack initiation and propagation. Crack data are presented for different classes of crack size in the form of crack density, that is, the number of cracks normalised to the investigated interface length. Micrographs of damage of the coatings are also shown. The effect of the coatings on the LCF life of the superalloy was dependent on the test temperature and deposited coating. At 500°C all coatings had a detrimental effect on the LCF life of the superalloy. At 900°C both AMDRY997 and IC1 prolonged the fatigue life of the superalloy by factors ranging between 1.5 and 4 while RT22 and IC3 shortened the life of the coating–substrate system. Specimens coated with RT22 exhibited generally more damage than other tested coatings at 9008 C. Most of the cracks observed initiated at the coating surface and a majority were arrested in the interdiffusion zone between the base material and the coating. No topologically close-packed phases were found. Delamination was only found in AMDRY997 at higher strains. Surface roughness or rumpling was found in the overlay coating AMDRY997 with some cracks initiating from the rumples. The failure morphology at 900°C reflected the role of oxidation in the fatigue life, the crack initiation and propagation of the coated specimens. The wake of the cracks grown into the substrate was severely oxidised leading to the loss of Al and Ti to the oxide and resulting in the formation of a ϒ’ depleted zone. The cracks grew more or less perpendicular to the load axis in a Stage II manner.
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| 26. |
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| 27. |
- Stekovic, Svjetlana, 1967-
(författare)
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Low Cycle Fatigue and Thermo-Mechanical Fatigue of Uncoated and Coated Nickel-Base Superalloys
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such environments superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface. The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with a thermodynamically stable oxide layer that acts as a diffusion barrier. During service life, mechanical properties of metallic coatings can be changed due to the significant interdiffusion between substrate and coating. There are also other degradation mechanisms that affect nickel-base superalloys such as low cycle fatigue, thermo-mechanical fatigue and creep.The focus of this work is on a study of low cycle fatigue and out-of-phase thermo-mechanical fatigue behaviour of three uncoated and coated nickel-base superalloys. Polycrystalline IN792 and two single crystals CMSX-4 and SCB were coated with four different coatings; an overlay coating AMDRY997 (NiCoCrAlYTa), a platinum aluminide modified diffusion coating RT22 and two innovative coatings with a NiW interdiffusion barrier in the interface called IC1 and IC3. A low cycle fatigue and thermo-mechanical fatigue device was designed and set-up to simulate service loading of turbine blades and vanes. The low cycle fatigue tests were run at 500oC and 900oC while the thermo-mechanical fatigue tests were run between 250oC and 900oC.To simulate long service life, some coated specimens were exposed at 1050oC for 2000 h before the tests.The main conclusions are that the presence of the coatings is, in most cases, detrimental to LCF lives of the superalloys at 500oC while the coatings do improve the LCF lives of the superalloys at 900oC. Under TMF loading conditions, the coatings have negative effect on the lifetime of IN792. On single crystals, they are found to improve TMF life of the superalloys, especially at lower strains. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged specimens. The aged specimens exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing transgranularly perpendicular to the load axis.
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| 28. |
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| 29. |
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| 30. |
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| 31. |
- Stekovic, Svjetlana
(författare)
-
Strain controlled fatigue and fracture behaviour of uncoated and coated polycrystalline and single crystal nickel-base superalloys at elevated temperature
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, the low-cycle fatigue life and mechanisms governing the fracture behaviour of coated nickel-base superalloys are presented and discussed. Cylindrical solid specimens were cyclically deformed with fully reversed tension-compression loading total strain amplitude control at two elevated temperatures and a constant strain rate of 10-4 s-1 (6%/ min) in air atmosphere without any dwell time. Three nickel-base superalloys, IN792, CMSX-4 and SCB, were coated with three different coatings: an overlay coating AMDRY997, a diffusion coating RT22 and an innovative coating ICl. The cyclic stress response, low-cycle fatigue (LCF) life and final fracture behaviour at the two temperatures are observed and compared.At 500oC the coatings reduced fatigue life relative to the uncoated specimens while at 900oC the coated specimens showed longer life at all strain ranges than the uncoated specimens except RT22 under certain test conditions. The decrease in the fatigue life was caused by brittle coating cracking under their ductile to brittle transition temperature (DBTT). Over DBTT, lower yield strength of the coated superalloys with subsequent increase in ductility could cause the improvement of the fatigue life. These cracks could be also slowed by oxidation on front of the crack tip.All uncoated and coated superalloys exhibit hardening and higher stress levels at higher applied strain amplitudes and at 500°C. At 900oC softening occurred together with lower stress response level. The coatings lowered the stress level response of the superalloys from about 12% to 31 %. Higher hardening was observed for polycrystalline IN792 caused by dislocation pileups at the the grain boundaries.Most of the observed cracks initiated at the coating surface and majority was arrested in the transition zone except for IN792 where internal pores served as initiation sites for most cracks. Some improvement in the fatigue life have also been seen in coated IN792. No cracks found initiated from TCP phases were found. Cracks initially grew more or less perpendicular to the load axis in Stage II manner. Crack propagation path in IN792 is governed by grain or dendrite boundaries while in single crystals crack growth path is determined by concentration of deformation and damage in γ and γ' phases. Surface roughness or rumpling was found in the overlay coating AMDRY997 with some cracks initiated from the rumples maybe due to cyclic straining and not thermal cycling.
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| 32. |
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| 33. |
- Stekovic, Svjetlana, 1967-, et al.
(författare)
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Towards the elaboration of the European Code of Practice for TMF crack growth
- 2016
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In recent years, there has been an increasing interest towards the environmental impact of air travel. As a result, it is of vital importance to the European aviation industry to reduce the engine emissions. For this purpose, ACARE set new challenging targets aiming to reduce emissions, fuel consumption and raising temperature capabilities. This will ultimately be achieved by the introduction of novel engine design and new materials able to sustain complex loadings from take-off, cruise, descent and shut down. It is a prerequisite to understand the impact of such environments in current alloys.Specifically, Thermo-Mechanical Fatigue (TMF), which occurs at the rim of turbine discs, aerofoils and rear structures need to be assessed. At these locations, the harsh thermal transients during take-off and descent may cause the formation of cracks. A previous European project on TMF, which ended in 2005, has been successful at evaluating and addressing issues related to thermo-mechanical fatigue testing. This project led to the elaboration of a code of practice for TMF studies, which has been implemented by testing houses and equipment manufacturers worldwide. A European project has recently been awarded, under the umbrella of both `Horizon 2020' and Clean Sky 2, aiming to evaluate and model TMF crack initiation and growth.TMF crack growth (CG) methods are essential to demonstrate that structural components meet the certification requirements of being tolerant at handling damages and the presence of melt anomalies. They are also needed to assess remnant lives in cracked turbine components such as vanes. However, crack growth in TMF loading cycles cannot be currently reliably measured experimentally nor predicted by existing CG models. Therefore, isothermal propagation data are used to conservatively handle TMF effects, which can lead to over-designed components and loss of engine efficiency. Our project aims to address those deficiencies.
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| 34. |
- Syväjärvi, Mikael, 1968-, et al.
(författare)
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Using Buurtzorg model as innovation clustering model for regional capacity building in an entrepreneurial context
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Smart Specialization (RIS3) is an innovative approach/strategy to bring together local authorities, academia, businesses and society to boost growth and jobs in Europe. It prioritizes domains, areas and economic activities where regions have a competitive advantage. We introduce an innovation model to facilitate translation of ideas and knowledge into regional implementation. Instead of focusing on technology, the innovator / entrepreneur himself is in focus. The proposed model is based on the Buurtzorg model which originally focusses on supporting patients in health care, and which has an onion frame: the patient is surrounded by informal network (family, etc), next is the Buurtzorg support team, and final level is the formal network (society). The individuals need are steering the health care support, rather than adapting it to the social and economic constraints of the health care system. The team consists of specialists who decide how they organize the work, share responsibilities and make decisions. The team is self-managing and with entrepreneurial spirit. We will argue in this paper that a support mechanism as innovation model may be applicable to regional capacity building, and the support acts as a clustering process. Clusters are often limited by geographical constraints, such as having a number of local actors in a certain field. Clustering may be based on other values than given by physical ones. Our model approach is based on that there are innovators who have visionary ideas which are outside their traditional business. They could potentially be of great important for regional growth since they will create value chains, jobs etc, but given the non traditional innovation character they will not be realized unless there is a support mechanism. Similar to the Buurtzorg model, there is a team of specialists that will support the innovator and the innovator informal network. The specialist team has competencies related to smart specialization, regional growth etc (formal network). When RIS3 is applied to broad areas like advanced materials or nanotechnology such a mediation becomes highly complex. Both these areas include an extremely broad range of areas, examples include anything from food (through modifying functional properties by physical and chemical changes), digital communication or connected systems (new materials’ approaches for faster processors or use of higher/faster band frequencies), to construction related materials (buildings, transport, etc). Very likely they do not have actors located geographically close to create a cluster. For synergistic effects, activities in the RIS3 within advanced materials can be linked to other fields, as well as to implementation synergies. If the smart specialization of advanced materials can interact with efforts in building competence, smart industry, sustainable production, automation, digitalization etc, the advanced materials could then find a value chain in specific avenues instead of building ones in its own area. This is a viable route to hold together the fragmented and broad character of advanced materials field, where transferring/translation activities (competence, technical, etc) are a binding agent. Therefore, the specialist team is crucial for such transfer, and a core element in an innovation model.
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