Comparisons between 2D and 3D experimental mesoscale measurements to examine slip transfer through grain boundaries in titanium and Ti-5Al-2.5Sn

Identification of slip processes at grain boundaries is complex and challenging for both characterization and modeling. To enable modeling of heterogeneous deformation near grain boundaries, credible observations with good statistics are necessary, as the size of grain boundary misorientation space combined with slip system activation is huge. Recent work with 2D characterization of slip transfer in Ti-5Al-2.5Sn (direct or indirect transmission of dislocations through grain boundaries) provides statistics. However, the detail provided by Transmission Electron Microscope (TEM) requires effort that precludes gathering of such statistics. Furthermore, extraction of a TEM foil removes the state of stress that drove the slip transfer process and observations may not represent the dislocation configuration that took place during deformation. To overcome this limitation, a 3D investigation using far-field measurements in pure Ti to examine twin nucleation by slip transfer reveals different constraints than observed in surface measurements. Use of differential aperture x-ray microscopy on in-situ deformation enables micron scale quantification of geometrically necessary dislocations (GNDs) present near grain boundaries while under load, so that the concurrent local elastic stress state associated with local heterogeneous deformation near boundaries can also be extracted. Insights gained from analysis of GNDs and local stress states indicates that the local kinematics of slip even along a single grain boundary vary greatly from place to place.