The effect of notch size on fatigue crack threshold and propagation in cast gamma titanium aluminide

Cracks were initiated and grown from notched specimens with notch lengths of 0.03 (FIB), 0.06, 0.1 and 0.5 mm (diamond blade), in cast gamma titanium aluminide at 400 oC. Specimens with longer 0.1 and 0.5 mm notches initiated from the notch and show no effect from initial notch size on either crack growth threshold or propagation. Specimens with shorter 0.03 and 0.06 mm notches initiated naturally, cracking from microstructural features away from the notch. The fracture surfaces of these specimens were characterised to distinguish areas of initiation, fatigue and overload. Multiple initiation sites were observed on these fracture surfaces. It is inferred that, for this alloy and process route, microstructural initiation is dominant where notches are less than ~100 µm in size.