Anisotropic plasticity and cavity growth during upset forging of Ti–6Al–4V

Orientation imaging microscopy was used to determine the effect of local crystallographic texture on the nucleation and growth of cavities in Ti–6Al–4V with a colony-α microstructure during upset forging to a 35% height reduction at 815 °C and a strain rate of 0.1 s−1. In contrast to uniaxial deformation, the stress–strain history in the bulged equatorial region was complex. Cavities developed preferentially along prior-β grain boundaries nominally perpendicular to principal stress directions, particularly where there were 90° colony misorientations, even in regions where global secondary-tension stresses were small or even negative. High cavity nucleation and growth rates were strongly correlated with the 90°-misoriented colonies, regardless of the orientations of the adjacent colonies. A rationale was developed to account for the particular sensitivity of 90°-misorientations based on colony orientation, slip systems, Taylor and Schmid factors, strain ratio and deformation history. Colonies with orientations in which prism slip was highly favored were stable and exhibited strong plastic anisotropy. When a neighboring colony had a 90°-misorientation, highly non-uniform strains developed in the boundary region, resulting in large triaxial stress concentrations that facilitated cavity nucleation and plastic strain conditions that favored cavity growth.