Bulk and intergranular fracture behaviour of NiAl

We present atomistic simulations of the crack tip configuration in single c rystals and bicrystals of B2 NiAl. The simulations were carried out using m olecular statics and embedded-atom potentials. The cracks are stabilized ne ar a Griffith condition and the process of dislocation emission from the cr ack tip was studied. The behaviour of a semi-infinite crack was studied und er mode I loading for different crack tip geometries. While [001](110) and [1(1) over bar 0](110) mode I cracks cleave near the Griffith value of load ing, dislocation emission is observed from [010](100) and [01(1) over bar]( 100) crack tips. Dislocations emitted from the [01(1) over bar](100) crack are observed to move far away from the tip. However, the dislocations emitt ed from the [010](100) crack are observed to be considerably less mobile an d they remain in the immediate vicinity of the crack tip after emission. Th e results indicate that for some orientations the fracture process in NiAl has dislocation emission characteristics typical of ductile fracture. The a tomistic configurations of the tip region are different in the presence of a large-angle grain boundary from in the bulk. Different symmetrical lilt g rain boundaries were studied corresponding to different orientations and lo cal compositions. It was found that, in ordered NiAl, cracks along symmetri cal tilt boundaries show a more brittle behaviour for Al-rich boundaries th an for Ni-rich boundaries. The fracture process occurs as a combination of dislocation emission and microcleavage portions that are controlled by the local atomistic structure of the grain boundary.