On the Effect of 3D Grain Boundary Orientation on Slip Transfer and Fracture in Ti

Grain boundaries play a crucial role during plastic deformation of polycrystals, acting as physical barriers that hinder dislocation motion and potential sites for damage nucleation and fracture. Their influence is more severe in metals with hexagonal closed-packed lattice because of the limited number of available slip systems as well as the large differences in the critical resolved shear stresses to active dislocation slip, which difficult the accommodation of plastic deformation in the microstructure. Thus, it is important to develop robust geometrical criteria to assess whether slip transfer/blocking will occur at a given grain boundary and what is the influence of grain boundary orientation on the nucleation of cracks. Most of the experimental information available to this end has been obtained by means of slip trace analysis and high-resolution digital image correlation (HR-DIC) on the surface of deformed specimens in which the grain orientation was determined by electron backscatter diffraction (EBSD). However, these results lack information about the grain boundary orientation perpendicular to the surface, which is known to play a significant role to control slip transfer/blocking as well as damage nucleation at grain boundaries.<br/><br/>In this talk, the mechanisms of slip transfer and blocking are analyzed in commercially pure Ti thin foils subjected to uniaxial tensile loading. The microstructure of the polycrystal was characterized by means of laboratory diffraction contrast tomography (LabDCT) to obtain information about the crystallographic orientation of the grains as well as about the grain boundary geometry on the specimen surface and through the thickness. The samples presented a strong basal texture typical of cold rolled thin foils, with a bimodal distribution of grain boundary misorientation angles with either very little or highly disoriented grains. After tensile deformation, a thorough slip transfer/blocking analysis was performed in more than 300 grain boundaries via slip trace analysis coupled with HR-DIC in selected areas. The strain concentrations around grain boundary areas captured by HRDIC allowed to uniquely identify slip blocking/transfer events while the slip traces observed on the surface of the sample were correlated with the crystal orientation of the grains to determine the active slip system. In addition, intergranular cracks were identified at grain boundaries and triple junctions as well as transgranular cracks parallel to the active slip systems.<br/><br/>The information on the deformation and damage nucleation mechanisms was combined with the full 3D information of the microstructure provided by LabDCT to assess the influence of the 3D grain boundary orientation slip transfer and damage at grain boundaries. Different metrics based on geometrical factors (grain boundary misorientation, Luster-Morris parameter, residual Burgers vector, LRB factor) and/or driving forces (critical resolved shear stresses for slip and/or twinning) were employed to assess the likelihood of slip transfer/blocking and fracture across grain boundaries. These results provide novel information to understand the effect of grain boundaries in polycrystal deformation and to simulate their influence on the mechanical behavior.