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Strain accumulation and fatigue crack initiation at pores and carbides in a SX superalloy at room temperature

Strain accumulation and fatigue crack initiation at pores and carbides in a SX superalloy at room temperature
Strain accumulation and fatigue crack initiation at pores and carbides in a SX superalloy at room temperature

Pores and carbides inherited from SX superalloy manufacturing processes usually act as stress concentrators and are preferential sites for fatigue crack initiation. In this study, pore & carbide size, morphology and distribution in a SX superalloy MD2 has been evaluated by X-ray CT. Strain accumulation and fatigue cracking behaviour in MD2, particularly around the pores and carbides, has been investigated by ex-situ SEM-DIC at room temperature along with image-based modelling of the observed MD2 defect populations obtained through X-ray CT imaging. The deformation structures have also been examined by electron channelling contrast imaging under controlled diffraction conditions. The results indicate that the pores & carbides with complicated three-dimensional features are the dominant fatigue crack initiation sites. Deformation is concentrated within intense slip bands and an enhanced strain accumulation around pores is captured by SEM-DIC. Dislocation motion is mainly confined to the γ matrix channels with some dislocation shear cutting of γ′ precipitates also observed ahead of the crack tip. As the crack propagates, strain band density and dislocation density at the crack tip increases correspondingly. Image-based modelling using the observed defect populations in MD2 (micro)structure can effectively predict the stress concentrations and the resultant hot spot for subsequent fatigue crack initiation, which is consistent with the experimental observations.

Digital image correlation, Fatigue crack initiation, Ni-based single crystal superalloy, Pores & carbides, Strain localisation
0142-1123
22-33
Jiang, R.
b78f0919-0168-43cd-9cda-dd922d8776bf
Bull, D.J.
3569ba02-89de-4398-a14d-02c3f9b4eab2
Evangelou, A.
840a9ab3-d2f9-4dee-9d5a-9f9c18c43be8
Harte, A.
dc2a0db1-81e3-48ac-80f2-11e65e47ec18
Pierron, F.
a1fb4a70-6f34-4625-bc23-fcb6996b79b4
Sinclair, I.
6005f6c1-f478-434e-a52d-d310c18ade0d
Preuss, M.
2e254b7e-e613-44f6-9ca2-8c7a2779ab04
Hu, X.T.
4f9b9f3a-b5d4-4c4f-9a9b-6feafb23507d
Reed, P.A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17
Jiang, R.
b78f0919-0168-43cd-9cda-dd922d8776bf
Bull, D.J.
3569ba02-89de-4398-a14d-02c3f9b4eab2
Evangelou, A.
840a9ab3-d2f9-4dee-9d5a-9f9c18c43be8
Harte, A.
dc2a0db1-81e3-48ac-80f2-11e65e47ec18
Pierron, F.
a1fb4a70-6f34-4625-bc23-fcb6996b79b4
Sinclair, I.
6005f6c1-f478-434e-a52d-d310c18ade0d
Preuss, M.
2e254b7e-e613-44f6-9ca2-8c7a2779ab04
Hu, X.T.
4f9b9f3a-b5d4-4c4f-9a9b-6feafb23507d
Reed, P.A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17

Jiang, R., Bull, D.J., Evangelou, A., Harte, A., Pierron, F., Sinclair, I., Preuss, M., Hu, X.T. and Reed, P.A.S. (2018) Strain accumulation and fatigue crack initiation at pores and carbides in a SX superalloy at room temperature. International Journal of Fatigue, 114, 22-33. (doi:10.1016/j.ijfatigue.2018.05.003).

Record type: Article

Abstract

Pores and carbides inherited from SX superalloy manufacturing processes usually act as stress concentrators and are preferential sites for fatigue crack initiation. In this study, pore & carbide size, morphology and distribution in a SX superalloy MD2 has been evaluated by X-ray CT. Strain accumulation and fatigue cracking behaviour in MD2, particularly around the pores and carbides, has been investigated by ex-situ SEM-DIC at room temperature along with image-based modelling of the observed MD2 defect populations obtained through X-ray CT imaging. The deformation structures have also been examined by electron channelling contrast imaging under controlled diffraction conditions. The results indicate that the pores & carbides with complicated three-dimensional features are the dominant fatigue crack initiation sites. Deformation is concentrated within intense slip bands and an enhanced strain accumulation around pores is captured by SEM-DIC. Dislocation motion is mainly confined to the γ matrix channels with some dislocation shear cutting of γ′ precipitates also observed ahead of the crack tip. As the crack propagates, strain band density and dislocation density at the crack tip increases correspondingly. Image-based modelling using the observed defect populations in MD2 (micro)structure can effectively predict the stress concentrations and the resultant hot spot for subsequent fatigue crack initiation, which is consistent with the experimental observations.

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More information

Accepted/In Press date: 2 May 2018
e-pub ahead of print date: 3 May 2018
Published date: 1 September 2018
Keywords: Digital image correlation, Fatigue crack initiation, Ni-based single crystal superalloy, Pores & carbides, Strain localisation

Identifiers

Local EPrints ID: 422770
URI: https://eprints.soton.ac.uk/id/eprint/422770
ISSN: 0142-1123
PURE UUID: 90bf269a-c988-475a-b86c-2dbaa48b2fa8
ORCID for D.J. Bull: ORCID iD orcid.org/0000-0001-6711-6153
ORCID for F. Pierron: ORCID iD orcid.org/0000-0003-2813-4994
ORCID for P.A.S. Reed: ORCID iD orcid.org/0000-0002-2258-0347

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Date deposited: 03 Aug 2018 16:31
Last modified: 04 Aug 2018 00:38

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Contributors

Author: R. Jiang
Author: D.J. Bull ORCID iD
Author: A. Evangelou
Author: A. Harte
Author: F. Pierron ORCID iD
Author: I. Sinclair
Author: M. Preuss
Author: X.T. Hu
Author: P.A.S. Reed ORCID iD

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