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

High-resolution transmission electron microscopy has been used to eval uate 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 vacuu m. High-resolution electron microscopy has been performed on the inter face along the common [001] direction. Images were recorded at four de focus conditions which gave strong contrast of the crossed {110} fring es in the bulk crystal on either side of the boundary, Models of the g rain boundary atomic structure were predicted using interatomic potent ials derived using the embedded atom method (EAM) and the model genera lized pseudopotential theory (MGPT). The EAM predicts a multiplicity o f structures differing by relative translations of the adjacent crysta ls, while the MGPT predicts only one mirror-symmetric grain-boundary s tructure. The theoretically predicted structures have been compared wi th the high-resolution images through image simulation. The identifica tion of focus conditions has been aided by Fourier analysis of the amo rphous edge of the specimen and comparison with calculated contrast tr ansfer functions. The boundary was experimentally observed to have mir ror 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 i n the MGPT thus appear to be important in determining the grain-bounda ry structure of niobium. But unambiguous structure determination is pr esently limited by the means available to compare simulated with exper imental images, the limited resolution of the microscope, imperfect bi crystal orientation, and the inability to distinguish atom positions i n the direction parallel to the electron beam in very thin specimens.