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Anisotropy Effects During Dwell-fatigue Caused by δ-phase Orientation in Forged Inconel 718
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
Dept of Applied Physics, Chalmers University of Technology, Gothenburg and GKN Aerospace engine Systems, R&T Centre, Trollhättan.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.ORCID iD: 0000-0002-8304-0221
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. no 692, p. 174-181
Keyword [en]
Anisotropy, Nickel-based superalloys, Fatigue, Mechanical characterisation, Scanning electron microscopy
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-135825DOI: 10.1016/j.msea.2017.03.063ISI: 000400718400021OAI: oai:DiVA.org:liu-135825DiVA: diva2:1084256
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
In thesis
1. Cracks in superalloys
Open this publication in new window or tab >>Cracks in superalloys
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:nbn:se:liu:diva-144397 (URN)10.3384/diss.diva-144397 (DOI)9789176853856 (ISBN)
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

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Available from 2019-03-20 10:52

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