Dislocation interactions in near-alpha Titanium alloy Ti6242Si under LCF

Abstract

Dislocation interactions were investigated in near-alpha titanium alloy Ti6242Si after low cycle fatigue. Samples from the gauge section and the crack initiation site on the fracture surface were studied. Grain pairs with different crystallographic orientations were analysed from the gauge section to understand the dislocation interactions near the boundary. Deformation was primarily caused by planar slip, localized into slip bands in primary alpha (αp) grains. Direct slip transfer was observed within grains having similar orientations. In contrast, slip transfer resulted in a different kind of dislocation nucleation in the neighboring hard grain where there was misorientation between the grains, with the observation of cross-slip. Strain transfer was observed between highly misoriented grain pairs. Crack nucleation occurred on an αp grain by basal plane splitting, due to the large tensile stress developed by a double ended pile-up. This kind of pile-up is suggested to result from the incomplete reversibility of dislocation motion during load reversals. The observation of superjogs on the basal dislocations in the crack nucleated αp grain provides a rationale for why cracks nucleate near, rather than on, the basal plane.