Dwell Fatigue Microstructure in a Near-α Titanium Alloy

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