THE ATOMISTIC STRUCTURE OF A SIGMA=3,(111) GRAIN-BOUNDARY IN NIAL, STUDIED BY QUANTITATIVE HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY

The atomistic structure of the Sigma = 3, (111) grain boundary in NiAl has been determined by quantitative high-resolution transmission elec tron microscopy. In NiAl, which crystallizes in the B2 (CsCl) structur e this grain boundary constitutes an 'incoherent twin boundary'. Using an atomic resolution microscope (JEM-ARM 1250) we have obtained image s of the grain-boundary structure at a point resolution of 0.105 nm. T o extract the atomistic structure quantitatively from these images we have applied a new method of digital image processing. This method yie lds the positions of Ni and Al columns in the grain-boundary core with a precision of 0.015 nm. The refined structure features Al atoms on t he boundary plane. Compared with a geometric model the real boundary h as an excess volume. Except for the region close to the boundary the a tom configuration exhibits mirror symmetry with respect to the boundar y plane. Near the boundary, however, lateral relaxations of atom posit ions break the mirror symmetry. The common building rule behind these features seems to be a strong repulsion between closely spaced Al atom s. Owing to the relaxation of atom positions, the boundary core has an open: structure, suggesting intrinsic weakness and brittle fracture b ehaviour.