DEFORMATION PROCESSES AT CRACK TIPS IN NIAL SINGLE-CRYSTAL AND BICRYSTAL

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 modi fied scanning force microscope (SFM). Sequences of elastic deformation , dislocation emission, followed by discontinuous crack growth on a su bmicron scale were observed within the SFM. In addition the fracture t oughness of bicrystals with stoichiometric and off-stoichiometric comp osition pre-cracked along and perpendicular to the boundary was measur ed. The results proved that NiAl is inherently ductile. However, depen ding on purity, discontinuous crack growth occurred with local crack o pening displacements (GOD) of less than 100 nm and slip band lengths o f less than 10 mu m, showing clearly that local dislocation pinning ca n initiate brittle crack growth after blunting. In all cases the tough ness of grain boundaries was more than a factor of two lower than the toughness of the adjacent component crystals. This proves that grain b oundary brittleness is an inherent property of pure NiAl. However, bou ndaries with a surplus in Ni were tougher. For cracks precracked perpe ndicular to the boundaries dislocation-boundary interaction can either increase or decrease the toughness depending on the orientation of th e adjacent grains. (C) 1997 Elsevier Science S.A.