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Title: Fracture and fatigue of gamma based titanium aluminide intermetallic alloys.
Author: Jenkins, Nigel Barry.
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 1999
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This thesis presents the room temperature mechanical properties of two y-based titanium aluminides. The fracture toughness, fatigue crack growth resistance (FeGR), and tensile properties of duplex, fully lamellar, and near fully lamellar microstructures have been assessed. The inter relationships between fracture toughness, FeGR, and tensile results were also studied. Two alloys of nominal composition Ti-45AI-2Mn-2Nb (45-2-2) and Ti-48AI-2Mn-2Nb (48- 2-2) were used for mechanical property evaluation. The ingots were produced by plasma arc cold hearth (PACH) melting at the IRe in Materials for High Performance Applications. The material was obtained in both the as-cast and isothermally forged conditions (the cast microstructure was near-fully lamellar, whilst the forged microstructure was fine grained equiaxed-y). Subsequent heat treatments were used to produce fully lamellar type microstructures. The colony size and lamella thickness was varied by altering the duration at, and cooling rate, from the solution heat treatment temperature. The effects of microstructure (equiaxed-y versus fully lamellar), lamellar colony size, lamella spacing, specimen thickness, loading rate, and precracking on the mechanical properties have been determined. For fully lamellar microstructures the measured fracture toughness and FCGR is critically dependent on the micromechanisms of fracture. Increases in both properties can be achieved by a fully translamellar mechanism of fraCture at the crack tip. Substantial decreases are observed if the colonies fail by an interlamellar mechanism. For example, the fracture toughness can be halved, and the slope ("m-value" from the Paris relationship) of the FeGR curve can increase from 5 to 40 if the fracture surface exhibits a high proportion of interlamellar failure. It was found that the fracture toughness decreased by approximately 1 MPa-V'm per 10% increase in the amount of interlamellar failure ahead of the precrack. It was found that increasing lamellar colony size from 800-4000 Jlm gave little difference in fracture toughness but larger colony sizes could lead to steeper FCGR curves due to a high proportion of interlamellar fracture at the crack tip. Increasing the cooling rate during heat treatment of fully lamellar microstructures led to a slight increase in fracture toughness and tensile strength (due to finer lamella spacing). However, increased cooling rate could also lead to slightly steeper FeGR curves due apparently to smoother colony boundaries which cause an increased proportion of intergranular failure. Increasing the specimen thickness (for a fixed lamellar colony size of 800 Jlm) from 4 to 14 mm led to an increase in the fracture toughness, and the "initiation" stress intensity factor during FCGR testing. The KQ fracture toughness increased from 9 to 19 MPa.vm with increasing specimen size. Because of the large colony size relative to the specimen dimensions, the number of grains sampled in the process zone beneath the sharp crack is critical. Increasing the number of grains reduces the influence that the low energy interlamellar fracture mechanism can have on the measured fracture toughness (hence leading to an increased KQ value). Measurements of crack length during the tests confirmed that significant stable crack extension (due to interlamellar failure) was delayed in the thicker testpieces. The effect of loading rate was significant on the fracture toughness values of fully lamellar microstructures, but there was little effect for the as-forged duplex microstructure. For the fully lamellar microstructure the mean fracture toughness increased from 13 to 18 MPa.vm with loading rates of 0.0025 to 10 MPa.vm.s-1 (higher toughness values were achieved at both the very low and very high ends of the loading rates studied). Whilst for the duplex microstructure the mean fracture toughness only varied by 0.7 MPa.vm over the same range of loading rates. A test loading rate of between 0.05 to 1.0 MPa.vm.s-1 should be used to obtain conservative fracture toughness values. It was found that the fracture toughness values were slightly higher (50% increase) for fully lamellar microstructures if tested with just a sharp notch (i.e. no precrack), but much higher for duplex microstructures (an increase of 300%). This indicates that the fully lamellar microstructure is more resistant to a sharp defect. Additionally, the specimens must be precracked in-order to obtain valid fracture toughness values. The FCGR curves for the fully lamellar microstructures were non-linear, and this was related to varying proportions of interlamellar failure at the crack tip as it propagates (the more interlamellar failure the steeper the curve). Indeed, the m-value increased from 5 for a translamellar, to 40 for a predominantly interlamellar failure mechanism at the crack tip. Decreasing the colony size and/or increasing the testpiece thickness reduced the extent of this variation (a direct result of more grains being sampled in the process zone). The slope of the FeGR curve showed no clear variation with decreasing lamella spacing. Tensile testing revealed that the duplex microstructure had a higher proof stress and fracture strength compared to the fully lamellar microstructures. This is a consequence of the finer grain size of this microstructure (50 pm and >500 pm respectively). DecreaSing the lamella spacing leads to an increase in the strength of fully lamellar microstructures. The results indicate that a fine grained fully lamellar microstructure, with a fine lamella spacing, would give the best combination of fracture toughness and fatigue crack growth resistance. A higher fracture strength is found for fine grained duplex microstructures, however such microstructures have extremely poor fracture toughness and fatigue crack growth resistance.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Metallurgy & metallography Metallurgy Materials Biodeterioration