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Crack growth in single crystal nickel base superalloys under isothermal and thermomechanical fatigue
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, Faculty of Science & Engineering.
2019 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This work concerns the fatigue crack growth behaviour of nickel base single crystal superalloys. The main industrial application of this class of materials is in gas turbine blades, where the ability to withstand severe mechanical loading in combination with high temperatures is required. In order to ensure the structural integrity of gas turbine blades, knowledge of the fatigue crack growth behaviour under service-like conditions is of utmost importance. The aim of the present work is both to improve the understanding of the crack growth behaviour of single crystal superalloys and also to improve the testing and evaluation methodology for crack propagation under thermomechanical fatigue loading conditions. Single crystal superalloys have anisotropic mechanical properties and are prone to localization of inelastic deformation along the close-packed planes of the crystal lattice. Under some conditions, crystallographic crack growth occurs along these planes and this is a complicating factor throughout the whole chain of crack propagation life simulation; from material data generation to component calculation. Fatigue crack growth testing has been performed, both using conventional isothermal testing methods and also using thermomechanical fatigue crack growth testing. Experimental observations regarding crystallographic crack growth have been made and its dependence on crystal orientation and testing temperature has been investigated. Quantitative crack growth data are however only presented for the case of Mode I crack growth under isothermal as well as thermomechanical fatigue conditions. Microstructural investigations have been undertaken to investigate the deformation mechanisms governing the crack growth behaviour. A compliance based method for the evaluation of crack opening force under thermomechanical fatigue conditions was developed, in order to enable a detailed analysis of the test data. The crack opening force evaluation proved to be of key importance in the understanding of the crack driving force under different testing conditions.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2019. , p. 44
Series
Linköping Studies in Science and Technology. Licentiate Thesis, ISSN 0280-7971 ; 1829
National Category
Materials Engineering Applied Mechanics Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:liu:diva-153619DOI: 10.3384/lic.diva-153619ISBN: 9789176851500 (print)OAI: oai:DiVA.org:liu-153619DiVA, id: diva2:1274987
Presentation
2019-01-25, ACAS, A-huset, Campus Valla, Linköpings universitet, Linköping, 09:19 (Swedish)
Opponent
Supervisors
Note

In the printed version of the thesis the series name Linköping Studies in Science and Technology Licentiate of engineering thesis is incorrect. The correct series name is Linköping Studies in Science and Technology Licentiate thesis.

Available from: 2019-01-04 Created: 2019-01-04 Last updated: 2019-01-28Bibliographically approved
List of papers
1. Fatigue crack growth behaviour of an alternative single crystal nickel base superalloy
Open this publication in new window or tab >>Fatigue crack growth behaviour of an alternative single crystal nickel base superalloy
2018 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 109, p. 166-181Article in journal (Refereed) Published
Abstract [en]

The fatigue crack growth behaviour of a single crystal nickel base superalloy was studied at three different temperatures (20 degrees C, 500 degrees C and 750 degrees C) and three different crystallographic orientations. At the highest testing temperature, the influence of hold time at maximum load was also evaluated. Under some of the testing conditions, crystallographic crack growth occurred along {1 1 1} planes, which were non-perpendicular to the loading direction. The propensity for crystallographic cracking was observed to be strongly temperature dependent with a maximum occurring at the intermediate testing temperature of 500 degrees C. During non-crystallographic, Mode I crack growth the crack tended to avoid the gamma particles and propagated preferentially through the gamma matrix.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2018
Keywords
Single crystal superalloy; Fatigue; Crack growth; Crystallographic crack growth; Temperature dependence; Orientation dependence; Hold time influence
National Category
Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-145431 (URN)10.1016/j.ijfatigue.2017.12.003 (DOI)000425073100017 ()
Note

Funding Agencies|Siemens Industrial Turbomachinery AB in Finspang, Sweden; Swedish Energy Agency, via the Research Consortium of Materials Technology for Thermal Energy Processes [KME-702]

Available from: 2018-04-03 Created: 2018-04-03 Last updated: 2019-09-23
2. Thermomechanical fatigue crack growth in a single crystal nickel base superalloy
Open this publication in new window or tab >>Thermomechanical fatigue crack growth in a single crystal nickel base superalloy
2019 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 122, p. 184-198Article in journal (Refereed) Published
Abstract [en]

Thermomechanical fatigue crack growth in a single crystal nickel base superalloy was studied. Tests were performed on single edge notched specimens, using in phase and out of phase thermomechanical fatigue cycling with temperature ranges of 100-750°C and 100-850°C and hold times at maximum temperature ranging from 10s to 6h. Isothermal testing at 100°C, 750°C and 850°C was also performed using the same test setup. A compliance-based method is proposed to experimentally evaluate the crack opening stress and thereby estimate the effective stress intensity factor range ΔKeff for both isothermal and nonisothermal conditions. For in phase thermomechanical fatigue, the crack growth rate is increased if a hold time is applied at the maximum temperature. By using the compliance-based crack opening evaluation, this increase in crack growth rate was explained by an increase in the effective stress intensity factor range which accelerated the cycle dependent crack growth. No significant difference in crack growth rate vs ΔKeff was observed between in phase thermomechanical fatigue tests and isothermal tests at the maximum temperature. For out of phase thermomechanical fatigue, the crack growth rate was insensitive to the maximum temperature and also to the length of hold time at maximum temperature. The crack growth rate vs ΔKeff during out of phase thermomechanical fatigue was significantly higher than during isothermal fatigue at the minimum temperature, even though the advancement of the crack presumably occurs at the same temperature. Dissolution of γ′ precipitates and recrystallization at the crack tip during out of phase thermomechanical fatigue is suggested as a likely explanation for this difference in crack growth rate.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
single crystal superalloy, thermomechanical fatigue, crack growth, crack closure
National Category
Materials Engineering Applied Mechanics Manufacturing, Surface and Joining Technology Metallurgy and Metallic Materials Infrastructure Engineering
Identifiers
urn:nbn:se:liu:diva-154074 (URN)10.1016/j.ijfatigue.2019.01.014 (DOI)000462110100017 ()
Note

Funding agencies: Siemens Industrial Turbomachinery AB in Finspang, Sweden; Swedish Energy Agency, via the Research Consortium of Materials Technology for Thermal Energy Processes [KME-702]

Available from: 2019-01-28 Created: 2019-01-28 Last updated: 2019-04-08

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Palmert, Frans

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