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Title: Effect of cooling rate on the microstructural and mechanical properties of the γ-titanium aluminide TNB
Author: Butson, A. M.
Awarding Body: University of Wales Swansea
Current Institution: Swansea University
Date of Award: 2006
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The current research was commissioned to assess the effect of cooling rate from a previously optimised solution heat treatment on a gamma titanium aluminide alloy considered for commercial application as an aerofoil in a gas turbine engine at temperatures up to 700°. The effect of cooling rate on microstructural evolution and a range of mechanical properties was assessed. Ultimately a methodology of lifing gamma titanium aluminides is proposed. The gamma titanium aluminide, ‘TNB’, of nominal composition Ti-45 A1-Nb-0.2C (wt.%) was evaluated. The optimised solution heat treatment involved a soak at 1310°C, in the two-phase α + γ field, for 30 minutes. Three cooling rates were considered for evaluation ranging from air cool to furnace cool. The mechanical properties measured included monotonic tensile and LCF response and at 20°C and 700°C. Threshold crack growth values were experimentally determined using the constant Kmax method. The unexpected inclusion of pre-machined defects in the specimens allowed a comparison between measured and modelled values of DKth. Additionally two types of experimental determination of DKth are evaluated. Fractographic analysis was performed to determine the mechanisms of failure operating in this alloy. It was shown that cooling rate from the optimised heat treatment has a significant role in determining the final microstructure. Microstructures ranged from coarse grained α2 + γ lamellar microstructures found in the air cooled specimens to fine grained duplex microstructures observed in the furnace cooled specimens. The sensitivity to cooling rate was highlighted with different sized blanks producing different microstructures for equivalent cooling rates. All mechanical properties evaluated demonstrated sensitivity to microstructure and therefore applied cooling rate. A lifting methodology based on the Kitagawa/Takahashi approach enabled the construction of a plot showing safe-stress regimes, relating to crack size. Implications of the current research with respect to commercial use are discussed accordingly.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available