HIGH-RESOLUTION ELECTRON-MICROSCOPY INVESTIGATION OF THE (710) TWIN IN NB

High-resolution transmission electron microscopy has been used to evaluate theoretical predictions of the atomic structure of a symmetric 16.3-degrees tilt grain boundary with [001] tilt axes which forms a twin about (710) in Nb, a body-centered cubic metal. The boundary has been fabricated by diffusion-bonding single crystals with flat, polished (710) surfaces misoriented by a 180-degrees rotation in ultrahigh vacuum. High-resolution electron microscopy has been performed on the interface along the common [001] direction. Images were recorded at four defocus conditions which gave strong contrast of the crossed {110} fringes in the bulk crystal on either side of the boundary, Models of the grain boundary atomic structure were predicted using interatomic potentials derived using the embedded atom method (EAM) and the model generalized pseudopotential theory (MGPT). The EAM predicts a multiplicity of structures differing by relative translations of the adjacent crystals, while the MGPT predicts only one mirror-symmetric grain-boundary structure. The theoretically predicted structures have been compared with the high-resolution images through image simulation. The identification of focus conditions has been aided by Fourier analysis of the amorphous edge of the specimen and comparison with calculated contrast transfer functions. The boundary was experimentally observed to have mirror symmetry to within relative crystal translations of +/-0.02 nm as viewed along the tilt axis, hence most of the structures predicted by the EAM can be ruled out. The angular-dependent interactions modeled in the MGPT thus appear to be important in determining the grain-boundary structure of niobium. But unambiguous structure determination is presently limited by the means available to compare simulated with experimental images, the limited resolution of the microscope, imperfect bicrystal orientation, and the inability to distinguish atom positions in the direction parallel to the electron beam in very thin specimens.