Dwell fatigue microstructure in a near-α titanium alloy

The dwell fatigue microstructure of a near-alpha titanium alloy was studied by optical microscopy and transmission electron microscopy. Optical microscopy revealed primary alpha grains as being crack initiation sites. The majority of the cracks nucleated on and propagated along specific crystal planes in the primary a grains perpendicular to the a platelets in their neighborina transformed beta grains. The propagation of the cracks is effectively blocked by alpha platelets with axes parallel to the directions of crack in the transformed beta grains. Transmission electron microscopy revealed the formation of different types of slip bands across alpha platelet boundaries in transformed fl grains and dislocation networks and subgrain boundaries in primary alpha grains. The most active slip system was identified as the < alpha > type dislocation, which slips on the basal planes of alpha phase. Slip bands in transformed beta grains were found to be on the basal planes of the alpha colonies, perpendicular to the beta platelets. The retained beta phase at the beta platelet boundaries have limited effect on blocking the propagation of the slip bands. The dwell effects are explained in terms of the dislocation movement in relation to the crystallography of the primary alpha grains and alpha platelets in the transformed beta grains.