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Cracks in superalloys
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Gas turbines are widely used in industry for power generation and as a power source at hard to reach locations where other possibilities for electrical power supplies are insufficient. New ways of producing greener energy is needed to reduce emission levels. This can be achieved by increasing the combustion temperature of gas turbines. High combustion temperatures can be detrimental and degrade critical components. This raises the demands on the high temperature performance of the superalloys used in gas turbine components. These components are frequently subjected to different cyclic loads combined with for example dwell-times and overloads at elevated temperatures, which can influence the crack growth. Dwell-times have been shown to accelerate crack growth and change cracking behaviour in both Inconel 718, Haynes 282 and Hastelloy X. On the other hand, overloads at the beginning of a dwell-time cycle have been shown to retard the dwell-time effect on crack growth in Inconel 718. More experiments and microstructural investigations are needed to better understand these effects.

The work presented in this thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", where I have mainly looked at fatigue crack growth mechanisms in superalloys subjected to dwell-fatigue, which can have a devastating effect on crack propagation behaviour. Mechanical testing was performed under operation-like cycles in order to achieve representative microstructures and material data for the subsequent microstructural work. Microstructures were investigated using light optical microscopy and scanning electron microscopy (SEM) techniques such as electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). 

The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at maximum load (0 % overload) in the fatigue cycle. With the introduction of a dwell-time there is also a shift from transgranular to intergranular crack growth for both Inconel 718 and Haynes 282. The crack growth rate decreases with increasing overload levels in Inconel 718 when an overload is applied prior to the dwell-time. At high temperature, intergranular crack growth was observed in Inconel 718 as a result of oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along δ-phase boundaries with subsequent oxidation of the δ-phase. 

This thesis comprises two parts. Part I gives an introduction to the field of superalloys and the acting microstructural mechanisms related to fatigue and crack propagation. Part II consists of five appended papers, which report the work completed as part of the project.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 50
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1897
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-144397DOI: 10.3384/diss.diva-144397ISBN: 9789176853856 (print)OAI: oai:DiVA.org:liu-144397DiVA, id: diva2:1175579
Public defence
2018-03-23, ACAS, A-huset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-01-18 Created: 2018-01-18 Last updated: 2018-01-18Bibliographically approved
List of papers
1. Influence of Overloads on Dwell Time Fatigue Crack Growth in Inconel 718
Open this publication in new window or tab >>Influence of Overloads on Dwell Time Fatigue Crack Growth in Inconel 718
2014 (English)In: Materials Science and Engineering: A, Vol. 612, p. 398-405Article in journal (Refereed) Published
Abstract [en]

Inconel 718 is one of the most commonly used superalloys for high temperature applications in gasturbines and aeroengines and is for example used for components such as turbine discs. Turbine discs can be subjected to temperatures up to ~700 °C towards the outer radius of the disc. During service, the discs might start to develop cracks due to fatigue and long dwell times. Additionally, temperature variations during use can lead to large thermal transients during start-up and shutdown which can lead to overload peaks in the normal dwell time cycle. In this study, tests at 550 °C with an overload prior to the start of each dwell time, have been performed. The aim of the investigation was to get a better understanding of the effects of overloads on the microstructure and crack mechanisms. The microstructure was studied using electron channelling contrast imaging (ECCI). The image analysis toolbox in Matlab was used on cross sections of the cracks to quantify: crack length, branch length, and the number of branches in each crack. It was found that the amount of crack branching increases with an increasing overload and that the branch length decreases with an increasing overload. When the higher overloads were applied, the dwell time effect was almost cancelled out. There is a strong tendency for an increased roughness of the crack path with an increasing crack growth rate.

Place, publisher, year, edition, pages
Elsevier, 2014
Keywords
nickel based superalloys, fatigue, fracture, mechanical charcterization, electron microscopy
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-109348 (URN)10.1016/j.msea.2014.06.068 (DOI)000340331300049 ()
Available from: 2014-08-14 Created: 2014-08-14 Last updated: 2018-01-18Bibliographically approved
2. Time- and Cycle-Dependent Crack Propagation in Haynes 282
Open this publication in new window or tab >>Time- and Cycle-Dependent Crack Propagation in Haynes 282
2016 (English)In: Journal of Materials Science and Engineering: A, ISSN 2161-6213, Vol. 658, p. 463-471Article in journal (Refereed) Published
Abstract [en]

Haynes 282 is a promising superalloy candidate for several high-temperature applications in both aero and land-based gas turbine engines. To study the crack growth behaviour under time-dependent conditions relevant to such applications, a test program was carried out at room temperature up to 700 °C with conditions ranging from pure cyclic to sustained tensile loading. At 650 °C and high stress intensity factors the crack growth was fully time-dependent for dwell-times of 90 s and longer. At lower stress intensities, the behaviour was mainly controlled by the cyclic loading, even under dwell conditions. The behaviour under dwell-fatigue conditions was well described by a liner superposition model.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Nickel based superalloys, fatigue, fracture, mechanical characterisation, electron microscopy
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-126922 (URN)10.1016/j.msea.2016.01.111 (DOI)000372560800054 ()
Note

At the time for thesis presentation publication was in status: Manuscript

Name of manuscript was: Time-dependent crack propagation in Haynes 282

Funding agencies: Agora Materiae, graduate school, Faculty grant SFO-MAT-LiU [2009-00971]; Swedish Energy Agency; Siemens Industrial Turbomachinery AB; GKN Aerospace Engine Systems; Royal Institute of Technology through the Swedish research program TURBO POWER

Available from: 2016-04-07 Created: 2016-04-07 Last updated: 2018-01-18Bibliographically approved
3. Anisotropy Effects During Dwell-fatigue Caused by δ-phase Orientation in Forged Inconel 718
Open this publication in new window or tab >>Anisotropy Effects During Dwell-fatigue Caused by δ-phase Orientation in Forged Inconel 718
2017 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, no 692, p. 174-181Article in journal (Refereed) Published
Abstract [en]

Inconel 718 is a commonly used superalloy for turbine discs in the gas turbine industry. Turbine discs are often subjected to dwell-fatigue as a result of long constant load cycles. The effect of anisotropy on dwell-fatigue cracking in forged turbine discs have not yet been thoroughly investigated. Crack propagation behaviour was characterised using compact tension (CT) samples cut in different orientations from a real turbine disc forging. Samples were also cut in two different thicknesses in order to investigate the influence of plane strain and plane stress condition on the crack propagation rates. The samples were subjected to dwell-fatigue tests at 550 °C with 90 s or 2160 s dwell-times at maximum load. Microstructure characterisation was done using scanning electron microscopy (SEM) techniques such as electron channelling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and light optical microscopy (LOM). The forged alloy exhibits strong anisotropic behaviour caused by the non-random δ-phase orientation. When δ-phases were oriented perpendicular compared to parallel to the loading direction, the crack growth rates were approximately ten times faster. Crack growth occurred preferably in the interface between the γ-matrix and the δ-phase.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Anisotropy, Nickel-based superalloys, Fatigue, Mechanical characterisation, Scanning electron microscopy
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-135825 (URN)10.1016/j.msea.2017.03.063 (DOI)000400718400021 ()
Note

Funding agencies: Agora Materiae, graduate school, Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [SFO-Mat-LiU 2009-00971]; Swedish Energy Agency; Siemens Industrial Turbomachinery AB; GKN Aerospace Engine Sy

Available from: 2017-03-24 Created: 2017-03-24 Last updated: 2018-01-18
4. Grain Size Depending Dwell-Fatigue Crack Growth in Inconel 718
Open this publication in new window or tab >>Grain Size Depending Dwell-Fatigue Crack Growth in Inconel 718
2018 (English)In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648, article id 1700930Article in journal (Refereed) Published
Abstract [en]

Inconel 718 is a commonly used superalloy for turbine discs in the gas turbine industry. Turbine discs are normally subjected to dwell-fatigue as a result of long constant load cycles. Dwell-times have been shown to give rise to increased crack propagation rates in superalloys at elevated temperatures. Dwell-time crack propagation behavior in Inconel 718 has been tested at 550 °C using Kb test samples with 2160 s dwell-times at maximum load and “pure fatigue” tests. The dwell-time effect has been studied for differently processed Inconel 718, that is, fine grained bar, grain enlarged bar, and cast material. This has been done in order to investigate the effect of grain size on crack propagation. Microstructure characterization is conducted using scanning electron microscopy techniques such as electron channeling contrast imaging and electron backscatter diffraction. Time dependent crack propagation rates are strongly affected by grain size. Propagation rates increase with decreasing grain size, whereas crack tip blunting increased with increasing grain size.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2018
Keywords
electron microscopy, fatigue, fracture, mechanical characterzation, nickel base superalloys
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-144395 (URN)10.1002/adem.201700930 (DOI)000435928600009 ()
Note

Funded by Swedish Energy Agency Siemens Industrial Turbomachinery ABGKN Aerospace Engine Systems Royal Institute of Technology through the Swedish research program TURBO POWER

Available from: 2018-01-18 Created: 2018-01-18 Last updated: 2018-08-30

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