Environmental and dwell effects on the damage tolerance properties of ATI 718PLUS® alloy

DOIResolve DOI: http://doi.org/10.1002/9781118516430.ch82
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TypeBook Chapter
Proceedings titleSuperalloys 2012
Conference12th International Symposium on Superalloys, Superalloys 2012, September 9-13, 2012, Seven Springs, PA, USA
Pages741749; # of pages: 9
SubjectDwell fatigue; Environmental damage; FCGR; Intergranular fracture; Waspaloy; Carbides; Cerium alloys; Crack tips; Cracks; Fatigue crack propagation; Fatigue of materials; Grain boundaries; Microstructural evolution; Oxidation; Stress corrosion cracking; Superalloys; Textures; Vacuum; Fracture
AbstractFatigue crack growth rate (FCGR) and dwell-FCGR tests were conducted at 704C° in a vacuum environment and compared with data obtained under lab air conditions for various microstructural conditions of ATI 718Plus Alloy (718Plus) and Waspaloy. The results indicated that environmental damage has a significant impact on the FCGR characteristics of both alloys, particularly in the near-threshold regime. The dwell fatigue crack propagation behaviour under vacuum conditions displayed identical behaviour irrespective of differences in microstructural features. The observed crack growth rates under vacuum decreased dramatically compared to lab air test results, highlighting the crucial impact of environmental damage on the dwell-fatigue crack propagation resistance of both alloys. The fracture mode for 718Plus was observed to be predominately intergranular in both the FCGR and dwell-FCGR test conditions, with the effect of long-term thermal exposure leading to a larger fraction of transgranular failure. Waspaloy fracture surfaces revealed a predominantly transgranular deformation mode. The fracture surfaces of 718Plus and Waspaloy all exhibited transgranular fracture under vacuum test conditions and were nearly indistinguishable in crack topography. Dynamic embrittlement is believed to be the governing mechanism for intergranular failure of 718Plus leading to grain boundary decohesion. Improvement of dwell-fatigue crack propagation resistance of 718Plus may be strongly related to the coarsening of the γ hardening phase, with potential further improvement via implementation of a bimodal γ heat treatment. EDX analysis showed that the 8 phase just at the crack tip becomes oxidized and may be a contributing factor in the susceptibility of the grain boundaries during the dwell-FCGR tests. The presence of Nb-rich carbides along the dwell-fatigue crack path of 718Plus was also identified. Carbides located behind the crack front were heavily oxidized and those carbides observed just ahead of the crack tip showed no oxidation. It is believed that oxidation of these NbC particles near grain boundaries may be linked to the environmental susceptibility of 718Plus during dwell-fatigue crack propagation. © 2012 The Minerals, Metals, & Materials Society. All rights reserved.
Publication date
AffiliationNational Research Council Canada (NRC-CNRC); Aerospace (AERO-AERO)
Peer reviewedYes
NPARC number21269281
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Record identifierf309ef86-6acf-497a-8d9b-6c9b89d552fe
Record created2013-12-12
Record modified2016-05-09
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