Relationship between flow stress recovery and dislocation structure in polycrystalline alpha-titanium

Abstract

Cell structure was observed in polycrystalline alpha-titanium deformed up to 10% true strain at room temperature. These cells appeared to be slightly elongated rather than equiaxed. The cell boundaries were identified to be the {101̄0} type planes.

Flow stress recovery of alpha titanium at 500 and 550ºC proceeded in two stages: (1) an initial rapid recovery during the first 6 hours and (2) a leveling off after 6 hours. Dislocation structure was studied by using transmission electron microscopy.

During the first stage of recovery, significant rearrangement of dislocations took place. At this stage fairly regular dislocation loops appeared to surround the subgrains. The segments of the dislocation loop were found to lie along <101̄0> type directions on the (0001) plane, and [0001] type directions on the {112̄0} planes. In the second stage, continued rearrangement of dislocations formed low angle boundaries. The segments of dislocations lying along the <101̄0> type direction reacted to form networks on the basal plane, and those lying on the {112̄0} planes continued to remain aligned along [0001] direction. It was postulated that some of the subgrains formed as a result of rearrangement and interaction of dislocations developed twist networks on the (0001) planes and tilt boundaries on the {112̄0} type planes.

An inverse relationship between the strength contributed by subgrain structure and the subboundary spacing predicted an increase in the subgrain size at the higher recovery temperature. The measured and the predicted value were in reasonably good agreement.