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# Fatigue crack threshold and growth behaviour in a near fully-lamellar gamma based titanium aluminides

Wang, Shiyuan (2015)
Ph.D. thesis, University of Birmingham.

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## Abstract

Fatigue crack threshold ($$D$$K$$_{th}$$) and fatigue crack growth of a near fully-lamellar $$y$$-TiAl alloy (Ti-4522XD alloy) have been investigated in air at room temperature, 400, 650, 700 and 750 oC and at three R ratios (0.1, 0.5 and 0.8). Studies were carried out on both corner-cracked specimens and smooth specimens. A combination of a $$D$$K- increasing loading method and growing a crack from notch were applied throughout the tests. As a consequence of consistent material microstructure, use of standardized testing procedure and a sufficient number of tests, some trends in fatigue threshold and crack growth have been established, including: trends include: lack of dependence of ($$D$$K$$_{th}$$) values on test temperature; average ($$D$$K$$_{th}$$) values decrease with increasing R ratio; a strong dependence of crack growth rate on K$$_{max}$$ values at RT; a reduced dependence of crack growth rate on K$$_{max}$$ values and increased plasticity at elevated temperatures; crack blunting causes a reduction of fatigue crack growth rate at R=0.8 and at elevated temperatures; and little effect of test temperature on ‘fracture toughness’ values. Above all, the origins of naturally initiated cracks under cyclic loading are often found to be centered on up to four colonies which have failed by interlamellar fracture.

Type of Work: Ph.D. thesis. Bowen, Paul and Li, Hang Yue Colleges (2008 onwards) > College of Engineering & Physical Sciences School of Metallurgy and Materials TN Mining engineering. Metallurgy University of Birmingham 6347
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