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Title: Investigation into the effect of surface conditions and environment on the high temperature fatigue response of titanium aluminides
Author: Duff, S. L.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2003
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The low cycle fatigue response of TiAl-45-2-2-XD has been assessed at a temperature of 650°C. Plain specimens were provided with various surface finishes: as machined, pre-oxidised, Electro-Chemi-Milled (ECM), ECM plus polish and non standard ECM. A Gleeble machine was also used to produce thermal shock resistance specimens for testing. The resulting fatigue data, however, demonstrates that the S/N response is not influenced by surface treatment, or, by thermal cycling. This lack of sensitivity to surface condition suggests that the surface, and hence fatigue crack initiation, may not be important under the present conditions. Round cylindrical notch (RCN) and corner crack (CC) specimens were tested in both air and vacuum at different R-values to highlight potential notch and environmental effects. No stress concentration effect was displayed by the RCN tests with their fatigue response similar to the plain specimens. Residual surface stresses from the grinding process may have counteracted the effect. There are transition points in the fracture behaviour with observed lives to failure falling into two distinct families. The most significant transition occurs at a peak stress in air of 500 MPa and a life of 3 x 103 cycles at R = 0.1. This appears to be a transition condition below which failures did not occur within 105 cycles. It is argued that the 'low life' family of results is determined by crack propagation behaviour. The significantly longer lives achieved in vacuum than in air can then be related to the detrimental effects of environment on crack propagation rates. A common defect or 'weak link' in the microstructure is postulated from which the crack initiates under the action of the environment. This weak link concept requires suitably orientated interfaces preferably in large grains where colonies are readily available for the crack to progress rapidly. Evidence of such α2/γ interfaces has been found. The crack initiation sites were often defined by a flat, faceted area reflective to light and noticeable to the naked eye. These shiny areas invariably occurred in near surface positions.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available