Fatigue in gamma titanium aluminide
File(s)
Author(s)
Trant, Claire Frances
Type
Thesis
Abstract
Despite the limited ductility of Ti-45Al-2Mn-2Nb-0.8 vol.% TiB2-XD, it is relatively tough, but with poor fatigue strength and high crack growth rates. It is therefore imperative that cracks should not be allowed to initiate and propagate, making the fatigue crack growth threshold critically important. The dependence of this threshold on the initial defect size has been investigated in this alloy using a machined notch. When the machined notch was larger than 70 μm, the scale of the underlying microstructure, the threshold was found to be independent of the notch size. When the machined notch was smaller than the microstructural scale, crack initiation occurred away from the machined notch and from the underlying microstructure.
At 400°C in air the fatigue crack growth threshold for the machined starter notches was 4.03±0.11MPa√m for twelve specimens in trans-lamellar fracture mode through the randomly oriented colonies local to the notch. The initiation site on specimens that initiated away from the machined notch was determined, and this threshold could be calculated as approximately 4 MPa√m. Multiple notches were machined in a single specimen, and a notch chosen to measure the fatigue crack growth threshold that would initiate in inter-lamellar fracture mode. This measured threshold in inter-lamellar fracture mode at 400°C in air was 2.7 MPa√m.
The fatigue crack growth threshold is strongly microstructure and temperature dependent. This increased from 4.0 MPa√m at 400°C to 7.1 MPa√m at 750°C in air in trans-lamellar fracture mode. A step increase in the temperature at low values of the stress intensity factor range, ∆K, determined the inter-lamellar fracture mode fatigue crack growth threshold of 5.8 MPa√m at 750°C in air. Step increasing the temperature to 750°C at larger values of ∆K caused the crack growth rate to initially increase, then quickly retard. This is due to the initial inter-lamellar fracture mode, until reaching a colony boundary and crack propagation being retarded by this boundary. Step decreasing the temperature back to 400°C caused crack growth rate retardation due to the larger plastic zone previously formed at the higher temperature.
The resulting fracture surface can be categorised into regions of fatigue and overload. There is also a visible roughness change attributed to the plastic zone size reaching the scale of the microstructure and therefore a change in cracking mechanism from trans-lamellar to mixed trans-, inter- and intra- lamellar. An oxide scale also formed giving a clear visible indication of temperature application.
At 400°C in air the fatigue crack growth threshold for the machined starter notches was 4.03±0.11MPa√m for twelve specimens in trans-lamellar fracture mode through the randomly oriented colonies local to the notch. The initiation site on specimens that initiated away from the machined notch was determined, and this threshold could be calculated as approximately 4 MPa√m. Multiple notches were machined in a single specimen, and a notch chosen to measure the fatigue crack growth threshold that would initiate in inter-lamellar fracture mode. This measured threshold in inter-lamellar fracture mode at 400°C in air was 2.7 MPa√m.
The fatigue crack growth threshold is strongly microstructure and temperature dependent. This increased from 4.0 MPa√m at 400°C to 7.1 MPa√m at 750°C in air in trans-lamellar fracture mode. A step increase in the temperature at low values of the stress intensity factor range, ∆K, determined the inter-lamellar fracture mode fatigue crack growth threshold of 5.8 MPa√m at 750°C in air. Step increasing the temperature to 750°C at larger values of ∆K caused the crack growth rate to initially increase, then quickly retard. This is due to the initial inter-lamellar fracture mode, until reaching a colony boundary and crack propagation being retarded by this boundary. Step decreasing the temperature back to 400°C caused crack growth rate retardation due to the larger plastic zone previously formed at the higher temperature.
The resulting fracture surface can be categorised into regions of fatigue and overload. There is also a visible roughness change attributed to the plastic zone size reaching the scale of the microstructure and therefore a change in cracking mechanism from trans-lamellar to mixed trans-, inter- and intra- lamellar. An oxide scale also formed giving a clear visible indication of temperature application.
Version
Open Access
Date Issued
2019-09
Date Awarded
2020-02
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Dye, David
Lindley, Trevor
Sponsor
Rolls-Royce Group plc
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)