Atomistic simulations were made of the crack-tip configuration in monocrystals and bicrystals of the B2-phase. The simulations were carried out by using molecular statics and embedded-atom potentials. The cracks were stabilized near to the Griffith condition, and the process of dislocation emission from the crack tip was studied. The behaviour of a semi-infinite crack was studied, under mode-I loading conditions, for various crack-tip geometries. Whereas [001](110) and [1¯10](110) mode-I cracks cleaved near to the Griffith loading value, dislocation emission from [010](100) and [01¯1](100) crack tips was observed. The dislocations which were emitted from the [01¯1](100) crack were observed to move far away from the tip. However, the dislocations which were emitted from the [010](100) crack were considerably less mobile and remained in the immediate vicinity of the crack tip after emission. The results indicated that, for some orientations, fracture in NiAl had dislocation emission characteristics which were typical of ductile fracture. The atomistic configurations of the tip region were different in the presence of a high-angle grain boundary. Various symmetrical tilt grain boundaries were studied, which corresponded to differing orientations and local compositions. It was found that, in the ordered alloy, cracks along symmetrical tilt boundaries exhibited a more brittle behaviour for Al-rich boundaries than for Ni-rich boundaries. The fracture involved a combination of dislocation-emission and micro-cleavage portions that were controlled by the local atomistic structure of the grain boundary.

Bulk and Intergranular Fracture Behaviour of NiAl. D.Farkas: Philosophical Magazine A, 2000, 80[6], 1425-44