Characterisation of deformation and fatigue crack growth in titanium aluminides

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Xi, Yingzi (2021). Characterisation of deformation and fatigue crack growth in titanium aluminides. University of Birmingham. Ph.D.

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Abstract

In this study, deformation mechanisms of a lamellar structure gamma TiAl alloy (forged Ti4522XD) have been analysed at five test temperatures, room temperature (RT), 400, 650, 700 and 750 °C. The deformation mechanism analysis has been carried out by tensile testing with a low strain rate of 1.67x10-5/s-1. All test pieces fractured in a brittle mode. The fracture surfaces were examined using scanning electron microscopy (SEM). Yield stress reduced with temperature increase, while elongation increased with temperature increase. Transmission electron microscopy was used to analyse dislocations and twins. Dislocation morphology, Burger’s vector, slip plane, Schmidt factor and dislocation density were analysed and compared. The effects of lamellar orientation and temperature on fatigue crack growth threshold and behaviour have been studied on a fully lamellar structure gamma TiAl alloy (Ti4522) with a very large colony size (~2.3 mm). The notches were introduced to various depths in a large single colony through thickness on one edge of tensile samples to allow crack initiation and growth within one single colony. The fatigue crack growth threshold tests were carried out on such single edge notched tension samples with various notch depths, at RT, 400 and 650 °C with a stress ratio R=0.1 using a stress intensity factor range increasing method with crack growth and many tests were interrupted after fatigue crack growth and before catastrophic failure. Threshold values are plotted against temperature/lamellar orientation, to show temperature influence and orientation influence. In addition, microstructural characterisation such as to determine colony sizes, fractography and crack path analysis was carried out by scanning electron microscopy (SEM). Further microscale analysis such as dislocation analysis, deformation around the crack path and oxidation layer analysis was examined by transmission electron microscopy (TEM) and using focused ion beam- scanning transmission electron microscopy (FIB-STEM). Fracture toughness tests were carried out after microstructure analysis at RT. Although it is not a standard procedure, the results (Kc) can still give a sense of the fracture toughness (KIC) of different orientation.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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