EXPERIMENTAL INVESTIGATION OF THE FRACTURE-TOUGHNESS OF NIAL BICRYSTALS COMBINED WITH COMPUTER-SIMULATIONS

The dependence on temperature, strain rate and orientation of the fracture toughness was measured in NiAl single crystals and bicrystals, and the results were compared with the predictions of computer simulations. The fracture toughness tests were performed in four-point-bending with NiAl bicrystals notched either parallel or perpendicular to the grain boundary. The boundaries of the bicrystals had less than half the toughness of NiAl single crystals after fracture in liquid nitrogen. The fracture toughness of specimens notched perpendicular to the grain boundary was measured as a function of the distance from crack tip to grain boundary. For small distances the brittle to ductile transition temperature in bicrystals was lower than in single crystals. With decreasing distance, depending on the bicrystal orientation, the toughness either increased or showed a pronounced minimum at intermediate distances. Computer simulations based on dislocation dynamics were performed with the same bicrystal orientations. For the interaction of a dislocation pile-up with either a ''soft'' or a ''hard'' grain, similar toughness-distance dependences were obtained.