OBSERVATION OF HIGH-INDEX (511) TYPE FRACTURE PLANES AND THEIR INFLUENCE ON TOUGHNESS IN NIAL SINGLE-CRYSTALS

The cleavage fracture of NiAl has been investigated by fracture toughness tests on single crystals. Unlike FeAl single crystals, which cleave along {100} planes similar to many bcc metals, the preferred cleavage plane in NiAl is {110}. In addition to the unusual {110} cleavage plane, NiAl single crystals also form transient high-index fracture facets near the notch of fracture toughness specimens. Like most cleavage facets, the transient fracture facets are macropically flat and optically shiny. Laue X-ray diffraction, scanning electron microscopy, and surface trace analysis were used to determine that the transient fracture facets are high-index crystallographic planes near {511}. The area fraction of fracture surface consisting of transient fracture facets was found to be a function of specimen orientation. A direct correlation between the measured fracture toughness and the transient fracture area was also observed. Alloying additions to NiAl for improved strength and toughness influenced the formation of {511} facets. In highly alloyed NiAl alloys, the tendency to form high-index fracture facets was reduced. The mechanism of formation of high-index planes is not clearly understood; however, experimental observations indicate that their occurrence is related to deformation in the material prior to crack instability. These facets were not present when NiAl was deformed under high strain rates. The measured fracture toughness values and fracture planes were highly sensitive to specimen geometry, orientation, test technique, and the alloying additions as well as to stoichiometry.