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On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy

On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy
On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy
The combined effects of environment (oxidation) and mechanical load (fatigue) that control crack propagation in a single crystal Ni-based superalloy have been investigated with particular focus on the intermediate service temperature range. Fatigue tests have been carried out at different frequencies, hold times and environments, to study the parameters influencing crack propagation at 550 °C. The direct current potential drop method was used to monitor the crack growth while STEM-EDS were used to analyse the fracture mode and crack tip regions.

It was found that the micro-mechanism of fatigue crack propagation at intermediate temperatures is a complex process with several competing mechanisms acting on the crack tip simultaneously. Crystallographic slip processes by γ′ shearing are active at these temperatures while at the same time thermally activated processes that promote crack propagation through the γ channels also take place. In addition, the effects of oxidation were found to be two-fold. It was demonstrated that these temperatures are not high enough to cause macroscopic embrittlement of the crack tip but finger-like protrusions were found to penetrate the material ahead of the crack tip at the nano-scale. The kinetics of such a mechanism were accentuated by the plastic strains at the crack tip, which given enough time, can promote cleavage fracture at the γ/γ′ interface. At the same time, given that the crack driving force is lower than a transition value, oxide formation on the crack tip surfaces can bridge the opening of the crack tip and reduce the effective driving force.
crack tip, intermediate temperature, oxidation, Single crystal
0921-5093
648-661
Evangelou, Evangelos
e2a94a06-efae-4c69-b9d3-394c0c46660c
Soady, Katherine
be674732-80c9-4a19-b1af-8924a1695dc3
Lockyer, Scott
ad19e5ed-2384-4ce3-a247-7bc98e1b60b2
Gao, Nong
9c1370f7-f4a9-4109-8a3a-4089b3baec21
Reed, Philippa
8b79d87f-3288-4167-bcfc-c1de4b93ce17
Evangelou, Evangelos
e2a94a06-efae-4c69-b9d3-394c0c46660c
Soady, Katherine
be674732-80c9-4a19-b1af-8924a1695dc3
Lockyer, Scott
ad19e5ed-2384-4ce3-a247-7bc98e1b60b2
Gao, Nong
9c1370f7-f4a9-4109-8a3a-4089b3baec21
Reed, Philippa
8b79d87f-3288-4167-bcfc-c1de4b93ce17

Evangelou, Evangelos, Soady, Katherine, Lockyer, Scott, Gao, Nong and Reed, Philippa (2019) On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy. Materials Science and Engineering: A, 742, 648-661. (doi:10.1016/j.msea.2018.10.095).

Record type: Article

Abstract

The combined effects of environment (oxidation) and mechanical load (fatigue) that control crack propagation in a single crystal Ni-based superalloy have been investigated with particular focus on the intermediate service temperature range. Fatigue tests have been carried out at different frequencies, hold times and environments, to study the parameters influencing crack propagation at 550 °C. The direct current potential drop method was used to monitor the crack growth while STEM-EDS were used to analyse the fracture mode and crack tip regions.

It was found that the micro-mechanism of fatigue crack propagation at intermediate temperatures is a complex process with several competing mechanisms acting on the crack tip simultaneously. Crystallographic slip processes by γ′ shearing are active at these temperatures while at the same time thermally activated processes that promote crack propagation through the γ channels also take place. In addition, the effects of oxidation were found to be two-fold. It was demonstrated that these temperatures are not high enough to cause macroscopic embrittlement of the crack tip but finger-like protrusions were found to penetrate the material ahead of the crack tip at the nano-scale. The kinetics of such a mechanism were accentuated by the plastic strains at the crack tip, which given enough time, can promote cleavage fracture at the γ/γ′ interface. At the same time, given that the crack driving force is lower than a transition value, oxide formation on the crack tip surfaces can bridge the opening of the crack tip and reduce the effective driving force.

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On the mechanism of oxidation-fatigue damage at intermediate temperatures in a SX Ni-based superalloy - Accepted Manuscript
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Accepted/In Press date: 20 October 2018
e-pub ahead of print date: 23 October 2018
Published date: 10 January 2019
Keywords: crack tip, intermediate temperature, oxidation, Single crystal

Identifiers

Local EPrints ID: 426340
URI: http://eprints.soton.ac.uk/id/eprint/426340
ISSN: 0921-5093
PURE UUID: b01d787a-6f32-4e6b-a38f-c2d16714e002
ORCID for Philippa Reed: ORCID iD orcid.org/0000-0002-2258-0347

Catalogue record

Date deposited: 23 Nov 2018 17:30
Last modified: 26 Nov 2021 05:30

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Contributors

Author: Evangelos Evangelou
Author: Katherine Soady
Author: Scott Lockyer
Author: Nong Gao
Author: Philippa Reed ORCID iD

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