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Crystallographic Orientation Influence on the Serrated Yielding Behavior of a Single-Crystal Superalloy
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2013 (English)In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 6, no 2, 437-444 p.Article in journal (Refereed) Published
Abstract [en]

Since Ni-based single-crystal superalloys are anisotropic materials, their behavior in different crystal orientations is of great interest. In this study, the yielding behavior in both tension and compression for 〈001〉, 〈011〉 and 〈111〉 oriented materials at 500 °C has been investigated. The 〈011〉 direction showed a serrated yielding behavior, a great tension/compression asymmetry in yield strength and visible deformation bands. However, the 〈001〉 and 〈111〉 directions showed a more homogeneous yielding, less tension/compression asymmetry in yield strength and no deformation bands. Microstructure investigations showed that the serrated yielding behavior of the 〈011〉 direction can be attributed to the appearance of dynamic strain aging (DSA) and that only one slip system is active in this direction during plastic deformation.

Place, publisher, year, edition, pages
Basel: MDPI AG , 2013. Vol. 6, no 2, 437-444 p.
Keyword [en]
single.crystals; superalloy; yield phenomena; tension/compression asymmetry; dynamic strain aging
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-88406DOI: 10.3390/ma6020437ISI: 000315398600004OAI: oai:DiVA.org:liu-88406DiVA: diva2:603110
Available from: 2013-03-26 Created: 2013-02-05 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Nickel-Based Single-Crystal Superalloys: the crystal orientation influence on high temperature properties
Open this publication in new window or tab >>Nickel-Based Single-Crystal Superalloys: the crystal orientation influence on high temperature properties
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Superalloys are a group of materials that are used in high temperature applications, for example gas turbines and aero engines. Gas turbines are most commonly used for power generation, and it is only the very critical components which are exposed to the most severe conditions within the turbine, which are made from superalloy material.

Today, energy consumption in many parts of the world is very high and is tending to increase. This implies that all power generating sources, including gas turbines, must aim for higher efficiency. For the gas turbine industry, it is a continuous challenge to develop more energy-efficient turbines. One way to do this is to increase the temperature within the hot stage of the turbine. However, increased temperature in the hot stage also challenges the materials that are used there. Today’s materials are already pushed to the limit, i.e. they cannot be exposed to the temperatures which are required to further increase the turbine efficiency. To solve this problem, research which later can lead to better superalloys that can withstand even higher temperatures, has to be conducted within the area of superalloys.

The aim of this licentiate thesis is to increase our knowledge about  deformation and damage mechanisms that occur in the microstructure in superalloys when they are subjected to high temperatures and loads. This knowledge can later be used when developing new superalloys. In addition, increased knowledge of what is happening within the material when it is exposed to those severe conditions, will facilitate the development of material models. Material models are used for FEM simulations, when trying to predict life times in gas turbine components during the design process.

This licentiate thesis is based on results from thermomechanical fatigue (TMF) testing of Ni-based single-crystal superalloys. Results show that the deformation within the microstructure during TMF is localized to several deformation bands. In addition, the deformation mechanisms are mainly twinning and shearing of the microstructure. Results also indicate that TMF cycling seems to influence the creep rate of single-crystal superalloys.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 56 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1568
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-89950 (URN)LIU–TEK–LIC–2013:2 (Local ID)978-91-7519-709-8 (ISBN)LIU–TEK–LIC–2013:2 (Archive number)LIU–TEK–LIC–2013:2 (OAI)
Presentation
2013-03-22, C3, C-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-03-12 Created: 2013-03-12 Last updated: 2013-03-12Bibliographically approved
2. On Thermomechanical Fatigue of Single-Crystal Superalloys
Open this publication in new window or tab >>On Thermomechanical Fatigue of Single-Crystal Superalloys
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Thanks to their excellent mechanical and chemical properties at temperatures up to 1000 °C, nickel-based superalloys are used in critical components in high-temperature applications such as gas turbines and aero engines. One of the most critical components in a gas turbine is the turbine blade, and to improve the creep and fatigue properties of this component, it is sometimes cast in single-crystal form rather than in the more conventional poly-crystalline form. Gas turbines are most commonly used for power generation and the turbine efficiency is highly dependent on the performance of the superalloys.

Today, many gas turbines are used as a complement for renewable energy sources, for example when the wind is not blowing or when the sun is not shining. This means that the turbine runs differently compared to earlier, when it ran for longer time periods with a lower number of start-ups and shut-downs. This new way of running the turbine, with an increased number of start-ups and shut-downs, results in new conditions for critical components, and one way to simulate these conditions is to perform thermomechanical fatigue (TMF) testing in the laboratory. During TMF, both mechanical strain and temperature are cycled at the same time, and one fatigue cycle corresponds to the conditions experienced by the turbine blade during one start-up and shutdown of the turbine engine.

In the work leading to this PhD thesis, TMF testing of single-crystal superalloys was first performed in the laboratory and this was then followed microstructure investigations to study the occurring deformation and damage mechanisms. Specimens with different crystallographic directions have been tested in order to investigate the anisotropic behaviour shown by these materials. Results show a significant orientation dependence during TMF, in which specimens with a low elastic stiffness perform better. However, it is also shown that specimens with a higher number of active slip planes perform better during TMF compared to specimens with less active slip systems. This is because a higher number of active slip systems results in a more widespread deformation and seems to be beneficial for the TMF life. Further, microscopy shows that the deformation during TMF is localised to several deformation bands and that different deformation and damage mechanisms prevail according to in which crystal orientation the material is loaded. Deformation twinning is shown to be a major deformation mechanism during TMF, and the interception of twins seems to trigger recrystallization. This work also studies the effects of alloying a single-crystal superalloy with Si or Re, and results show a significant Si-effect where the TMF life increases by a factor of 2 when Si is added to the alloy.

Finally, this research results in an increased knowledge of the mechanical response as well as a deeper understanding of the deformation and damage mechanisms that occur in single-crystal superalloys during TMF. It is believed that in the long-term, this can contribute to a more efficient and reliable power generation by gas turbines.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 83 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1626
National Category
Materials Engineering Materials Chemistry
Identifiers
urn:nbn:se:liu:diva-111643 (URN)10.3384/diss.diva-111643 (DOI)978-91-7519-211-6 (ISBN)
Public defence
2014-11-28, ACAS, Hus A, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-10-27 Created: 2014-10-27 Last updated: 2014-10-27Bibliographically approved

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Segersäll, MikaelMoverare, Johan

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