Deformation processes at crack tips in NiAl single- and bicrystals

Pre-cracked single- and bicrystals were examined in order to understand the quasi brittle behaviour of NiAl single crystals and the strength of grain boundaries. Single crystals were tested in-situ within a modified scanning force microscope (SFM). Sequences of elastic deformation, dislocation emission, followed by discontinuous crack growth on a submicron scale were observed within the SFM. In addition the fracture toughness of bicrystals with stoichiometric and off-stoichiometric composition pre-cracked along and perpendicular to the boundary was measured. The results proved that NiAl is inherently ductile. However, depending on purity, discontinuous crack growth occurred with local crack opening displacements (COD) of less than 100 nm and slip band lengths of less than 10 μm, showing clearly that local dislocation pinning can initiate brittle crack growth after blunting. In all cases the toughness of grain boundaries was more than a factor of two lower than the toughness of the adjacent component crystals. This proves that grain boundary brittleness is an inherent property of pure NiAl. However, boundaries with a surplus in Ni were tougher. For cracks precracked perpendicular to the boundaries dislocation-boundary interaction can either increase or decrease the toughness depending on the orientation of the adjacent grains.