Bulk and intergranular fracture behaviour of NiAl

Abstract

We present atomistic simulations of the crack tip configuration in single crystals and bicrystals of B2 NiAl. The simulations were carried out using molecular statics and embedded-atom potentials. The cracks are stabilized near a 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 for different crack tip geometries. While [001](110) and [110](110) mode I cracks cleave near the Griffith value of loading, dislocation emission is observed from [010](100) and [011](100) crack tips. Dislocations emitted from the [011](100) crack are observed to move far away from the tip. However, the dislocations emitted from the [010](100) crack are observed to be considerably less mobile and they remain in the immediate vicinity of the crack tip after emission. The results indicate that for some orientations the fracture process in NiAl has dislocation emission characteristics typical of ductile fracture. The atomistic configurations of the tip region are different in the presence of a large-angle grain boundary from in the bulk. Different symmetrical tilt grain boundaries were studied corresponding to different orientations and local compositions. It was found that, in ordered NiAl, cracks along symmetrical tilt boundaries show a more brittle behaviour for Al-rich boundaries than 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.