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Nickel-Based Single-Crystal Superalloys: the crystal orientation influence on high temperature properties
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
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: urn:nbn:se:liu:diva-89950Local ID: LIU–TEK–LIC–2013:2ISBN: 978-91-7519-709-8 (print)OAI: oai:DiVA.org:liu-89950DiVA: diva2:610550
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
List of papers
1. Deformation and Damage Mechanisms During Thermomechanical Fatigue of a Single-crystal Superalloy in the <001> and <011> Directions
Open this publication in new window or tab >>Deformation and Damage Mechanisms During Thermomechanical Fatigue of a Single-crystal Superalloy in the <001> and <011> Directions
2012 (English)In: Superalloy 2012: 12th International Symposium on Superalloys / [ed] Eric S. Huron, Roger C. Reed, Mark C. Hardy, Michael J. Mills, Rick E. Montero, Pedro D. Portella and Jack Telesman, The Minerals, Metals, and Materials Society, 2012, 215-223 p.Conference paper, Oral presentation only (Refereed)
Abstract [en]

The purpose of this paper is to investigate the differences in mechanical response and microstructural behavior when the single-crystal Ni-based superalloy CMSX-4 is subjected to thermomechanical fatigue (TMF) in two different crystallographic directions, <001> and <011>. An out-of-phase (OP) straincontrolled TMF cycle with R=-∞ in the temperature range 100 to 850 °C was used. As expected, the material exhibited, when loaded in the <001> direction, a higher number of cycles to failure compared to the <011> direction, when equivalent strain ranges were compared. High strain ranges led to crystallographic fractures along one of the {111} planes while low strain ranges led to non-crystallographic fractures. This result was valid for both <001> and <011> directions. Specimens with random fractures also showed recrystallization close to the fracture surface. Twinning was found to be a major deformation mechanism for most specimens. A change in deformation mechanism from twinning to shearing was found in specimens subjected to loading in the <011> direction when going from low to high strain ranges. This investigation also indicated that crack propagation is a consequence of recrystallization and not the other way around.

Place, publisher, year, edition, pages
The Minerals, Metals, and Materials Society, 2012
Keyword
singel-crystal supearlloy, thermomechanical fatigue, deformation mechanisms, twinning, shearing
National Category
Engineering and Technology Materials Engineering
Identifiers
urn:nbn:se:liu:diva-81528 (URN)978-0-470-94320-5 (ISBN)
Conference
Superalloy 2012: 12th International Symposium on Superalloys, September 9-13 2012, Seven Springs, USA
Available from: 2012-09-18 Created: 2012-09-18 Last updated: 2014-10-27Bibliographically approved
2. Crystallographic Orientation Influence on the Serrated Yielding Behavior of a Single-Crystal Superalloy
Open this publication in new window or tab >>Crystallographic Orientation Influence on the Serrated Yielding Behavior of a Single-Crystal Superalloy
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
Keyword
single.crystals; superalloy; yield phenomena; tension/compression asymmetry; dynamic strain aging
National Category
Materials Engineering
Identifiers
urn:nbn:se:liu:diva-88406 (URN)10.3390/ma6020437 (DOI)000315398600004 ()
Available from: 2013-03-26 Created: 2013-02-05 Last updated: 2017-12-06Bibliographically approved
3. Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy
Open this publication in new window or tab >>Creep and Stress Relaxation Anisotropy of a Single-Crystal Superalloy
2014 (English)In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 45, no 5, 2532-2544 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the TMF stress relaxation and creep behavior at 1023 K and 1223 K (750 °C and 950 °C) have been investigated for a Ni-based single-crystal superalloy. Specimens with three different crystal orientations along their axes were tested; 〈001〉, 〈011〉, and 〈111〉, respectively. A highly anisotropic behavior during TMF stress relaxation was found where the 〈111〉 direction significantly shows the worst properties of all directions. The TMF stress relaxation tests were performed in both tension and compression and the results indicate a clear tension/compression asymmetry for all directions where the greatest asymmetry was observed for the 〈001〉 direction at 1023 K (750 °C); here the creep rate was ten times higher in compression than tension. This study also shows that TMF cycling seems to influence the creep rate during stress relaxation temporarily, but after some time it decreases again and adapts to the pre-unloading creep rate. Creep rates from the TMF stress relaxation tests are also compared to conventional constant load creep rates and a good agreement is found.

Place, publisher, year, edition, pages
Springer, 2014
Keyword
Single-crystal superalloy, thermomechanical fatigue, creep, stress relaxation, deformation mechanism
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-89949 (URN)10.1007/s11661-014-2198-0 (DOI)000334428000026 ()
Note

On the day of the defense data of the Licentiate Thesis the status of this article was Manuscript.

Available from: 2013-03-12 Created: 2013-03-12 Last updated: 2017-12-06Bibliographically approved

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