Experimental evidence and first-principles verification of deformation of basal twist grain boundaries in Ti

Recent experimental evidence indicates that Basal Twist Grain Boundaries (BTGBs) are responsible for the nucleation of fatigue cracks in Ti alloys . They are composed of two hexagonal close-packed α crystals rotated about the [0001] axis separated by a grain boundary parallel to the (0001) plane. To clarify the origin of this critical behavior, the mechanical response of bicrystal micropillars containing BTGBs was characterized by means of micropillar compression tests. Afterwards, the deformation mechanisms around the BTGB were analyzed by means of high-resolution scanning and transmission electron microscopy . It was found that shear deformation occurred by the glide of one basal grain over the other basal grain at very low values of the critical resolved shear stress (45 - 205 MPa), much lower than that of basal slip (400 - 600 MPa). To understand this mechanism, the Grain Boundary Energies (GBEs) of coincident site lattice BTGBs with different twist angles were determined by means of first-principles calculations of supercells . It was found that the GBE variations with respect to twist angle and in-plane translation were negligible for all coincident site lattice BTGBs investigated, leading to a very low shear resistance.