Initiation and propagation of short fatigue cracks in forged Ti6Al4V

Abstract

Forged Ti6Al4V alloy in two different microstructures was used for investigations on fatigue behaviour with special focus on crack initiation and short crack propagation characteristics. Both microstructures are in the bi-modal condition containing different amounts and sizes of primary alpha grains. Interrupted fatigue experiments were carried out using a servohydraulic test facility. Different stress levels were imposed at a constant R ratio of −1 and a frequency of 20Hz with a sinusoidal command signal. SEM together with the EBSD technique was applied for the crack observation as well as for the determination of local crystallographic orientation data with the objective of linking initiation sites and crack paths to microstructural features. It was found that most of the cracks initiate on boundaries between two lamellae in favourably oriented colonies. These cracks propagate on prismatic glide planes with high Schmid factors until they reach another boundary. In some cases crack splitting was observed leading to crack propagation in different directions on different slip planes resulting in a reduction of crack growth rate. The crack paths can be attributed either to prismatic slip planes or the basal slip plane, whereas basal slip was found inside the lamellae or in primary alpha grains. Crack deflection at boundaries is also a common feature, and is probably related to high tilt and twist angles between the grains involved.