The atomistic structure of a Σ = 3, (111) grain boundary in NiAl, studied by quantitative high-resolution transmission electron microscopy

The atomistic structure of the Σ = 3, (111) gain boundary in NiAl has been determined by quantitative high-resolution transmission electron microscopy. In NiAl, which crystallizes in the B2 (CsCl) structure this grain boundary constitutes an 'incoherent twin boundary'. Using an atomic resolution microscope (JEM-ARM 130) we have obtained images of the grain-boundary structure at a point resolution of 0·105nm. To extract the atomistic structure quantitatively from these images we have applied a new method of digital image processing. This method yields 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 the boundary plane. Compared with a geometric model the real boundary has an excess volume. Except for the region close to the boundary the atom configuration exhibits mirror symmetry with respect to the boundary plane. Near the boundary, however, lateral relaxations of atom positions break the mirror symmetry. The common building rule behind these features seems to be a strong repulsion between closely spaced Al atoms. Owing to the relaxation of atom positions, the boundary core has an open structure, suggesting intrinsic weakness and brittle fracture behaviour.