The origins of heterogeneous deformation during primary hot working of Ti–6Al–4V

A Ti–6Al–4V cylindrical specimen with a large grain colony microstructure was upset forged to 35% reduction at 815 °C at a nominal strain rate of 0.1 s−1. An orientation imaging microscopy (OIM) analysis was conducted in two representative areas, near the center with an estimated von Mises strain of 1.6, and 0.8 about midway from the center to the outer edge. The process of physically breaking up the lamellar microstructure (globularization) was examined, focusing on how the globularization efficiency was affected by the initial colony orientation. Microstructural maps based upon the electron backscattered pattern quality, crystal orientation, and an estimated Taylor factor (using a continuum assumption) were used to identify and quantify heterogeneous deformation phenomena. These analyses show that in regions where both prism and basal slip systems were not operational, macro shear bands developed, leading to kinked lamellar microstructural features. The shear bands concentrated shear in localized regions that were able to flow easily around remaining hard regions, leaving remnants of the hard regions intact. Also, development of large misorientations of 50–90° from the parent grain arising from a transformation from β to α are quantified and related to the globularization efficiency.