It is well known that grain boundaries have a beneficial effect on strengthening properties in crystalline materials. There is however, a much lesser degree of understanding on how grain boundaries determine the propensity of a material for damage nucleation. In polycrystalline aggregates the mechanical response of individual grains to loading conditions is highly heterogeneous and dependent upon the morphology and relative orientation of the neighboring grains. Surface characterization methods such as electron backscatter diffraction (EBSD) present only a partial understanding of this heterogeneous deformation behavior. X-ray diffraction using high brilliance beams from synchrotron sources provide a powerful non destructive means to characterize the subsurface mechanical response of grains. Such high energy x-ray diffraction microscopy (HEDM) methods can measure changes in grain orientation, morphology and local strain evolution with high spatial resolution. The low crystal symmetry of hexagonal metals make them ideal candidates to study individual slip systems and identify specific types of dislocations. Deformation in hexagonal materials is strongly dependent upon the relative orientation between the c-axis and the loading direction. Conditions that lead to the nucleation of deformation twinning in hexagonal metals are not well understood. This makes it challenging for physically based crystal plasticity models to predict twinning events. Another important consideration in polycrystalline deformation is the dependence of the local stress state on the geometrically necessary dislocation (GND) density. In this work, two different samples of polycrystalline pure titanium having textures were characterized using two different HEDM techniques. The first specimen has a predominantly "hard" texture with respect to the loading direction. In-situ far field high energy diffraction microscopy (FF HEDM) was used to characterize the formation of discrete twinning events during a tensile test. The propensity for twin nucleation in a grain by slip transfer from neighboring grains for each of the identified twinning events is evaluated using a geometrical parameter along with conditions of spatial proximity. The second specimen has a largely "soft" texture with respect to the loading direction. The evolution of local morphology and local stress state during a four point bending experiment were captured using differential aperture x-ray microscopy (DAXM). The GND density was estimated for each voxel from the orientation gradient. In contrast to surface measurements on the same material deformed in bending, where prism slip bands nucleated mechanical twins, a greater amount of pyramidal slip was correlated to twin formation in the interior of the specimen, though it is not clear whether the twin or the slip initiated the correlated shear. Comparison with similar studies in titanium shows that the type of slip system likely to nucleate a T1 twin is strongly dependent upon the loading direction and initial texture. In this work a quantitative matching of the FF HEDM data and more recently collected Near Field (NF) HEDM dataset is done using criteria of maximum crystallographic misorientation and Euclidean distance. Additionally, a comparison was made between the kinematic descriptor (lattice reorientation as a function of load) and the grain averaged stress measures in FF HEDM. This was done in order to determine the limits of FF HEDM for assessing complex mesoscopic loading events such as deformation twinning. A means to visualize the heterogeneity in the local stress state is enabled by DAXM characterization. Moreover, the in-situ DAXM experiment enabled the estimation of the GND density from the lattice rotation gradient. The analysis was able to identify the contributions to the total GND density from individual slip systems. The local agglomeration of GND (pileups) is strongly dependent upon the local stress state and the transmissivity of dislocations across grain boundaries. In contrast, the global stress state does not have a strong correlation with local GND accumulation. The present work is a step towards developing a better understanding of local mechanical response in polycrystalline materials. It is expected that results from this study can help better inform constitutive relations governing crystal plasticity based models that simulate material deformation.
Read
