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Effect of Dwell-times on Crack Propagation in Superalloys
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2015 (English)Licentiate 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 supply are insufficient. There is a strong need for greener energy, considering the effect that pollution has had on global warming, and we need to come up with ways of producing cleaner electricity. A way to achieve this is by increasing the combustion temperature in gas turbines. This increases the demand on the high temperature performance of the materials used e.g. superalloys in the turbine. These high combustion temperatures can lead to detrimental degradation of critical components. These components are commonly subjected to cyclic loading of different types e.g. combined with dwell-times and overloads at elevated temperatures, which influence the crack growth. Dwell-times have shown to accelerate crack growth and change the cracking behaviour in both Inconel 718 and Haynes 282. Overloads at the beginning of the dwell-time cycle have shown to retard the dwell time effect on crack growth in Inconel 718. To understand these effects more microstructural investigations are needed.

The work presented in this licentiate thesis was conducted under the umbrella of the research program Turbo Power; "High temperature fatigue crack propagation in nickel-based superalloys", concentrating on fatigue crack growth mechanisms in superalloys during dwell-times, which have shown to have a devastating effect on the crack propagation behaviour. Mechanical testing was performed under operation-like conditions in order to achieve representative microstructures and material data for the subsequent microstructural work. The microstructures were microscopically investigated in a scanning electron microscope (SEM) using electron channeling contrast imaging (ECCI) as well as using light optical microscopy.

The outcome of this work has shown that there is a significant increase in crack growth rate when dwell-times are introduced at the 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. When an overload is applied prior to the dwell-time, the crack growth rate decreases with increasing overload levels in Inconel 718. At high temperature crack growth in Inconel 718 took place as intergranular crack growth along grain boundaries due to oxidation and the creation of nanometric voids. Another observed growth mechanism was crack advance along phase boundaries with subsequent severe oxidation of the phase.

This thesis comprises two parts. The first giving an introduction to the field of superalloys and the acting microstructural mechanisms that influence fatigue during dwell times. The second part consists of two appended papers, which report the work completed so far in the project.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. , 49 p.
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1739
National Category
Materials Engineering Energy Engineering Metallurgy and Metallic Materials
URN: urn:nbn:se:liu:diva-123306DOI: 10.3384/lic.diva-123306ISBN: 978-91-7685-871-4 (print)OAI: diva2:881294
2016-01-22, ACAS, A-huset, Campus Valla, Linköping, 10:15 (Swedish)
Swedish Energy Agency
Available from: 2015-12-10 Created: 2015-12-10 Last updated: 2015-12-16Bibliographically 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, 398-405 p.Article 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
nickel based superalloys, fatigue, fracture, mechanical charcterization, electron microscopy
National Category
Materials Engineering
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: 2016-10-08Bibliographically approved
2. Time-dependent crack propagation in Haynes 282
Open this publication in new window or tab >>Time-dependent crack propagation in Haynes 282
2015 (English)Manuscript (preprint) (Other academic)
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 timedependent 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. The main crack growth occurred transgranularly at room temperature and there was a transition in cracking behaviour from cycle dependent transgranular growth to time-dependent intergranular propagation at ~ 45 MPa for the high temperature tests. No effect of cyclic frequency could be observed at room temperature, and at lower frequencies the crack growth rate increased with temperature.

Nickel based superalloys, Fatigue, Fracture, Mechanical characterization, Electron microscopy
National Category
Materials Engineering
urn:nbn:se:liu:diva-123305 (URN)
Available from: 2015-12-10 Created: 2015-12-10 Last updated: 2015-12-10Bibliographically approved

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