Microcrack formation under normal and dwell fatigue of IMI 834

Dwell sensitivity of titanium alloys at ambient temperature (-25 degrees C) is a well-reported phenomenon, although the question about the phenomena of crack initiation remains open. In this work, the normal and dwell fatigue response of a near alpha titanium alloy is studied. A dwell fatigue debit (number of continuous cycles to failure to number of cycles to failure with a dwell period) of up to 2 and 10 were observed at two different load levels. Analysis using scanning electron microscopy, electron back scattered diffraction and transmission electron microscope-based orientation imaging microscopy (TEM-OIM) revealed that the formation of microcracks at very early stages of dwell fatigue loading was the major reason for this decreased life. The initiation of these cracks was observed to be a strong function of the local crystallography of the constituent grains. A modified pile up model in which the shear stress on the pile up plane is calculated using stress redistribution at the grain level is used to explain the micromechanisms of crack nucleation in dwell fatigue. The elastic and plastic anisotropy of the hexagonal crystal, the microtextural characteristics and strong stress redistributions at the grain level results in the nucleation and propagation of these microcracks.