CLEAVAGE FRACTURE IN B2 ALUMINIDES

Cleavage fracture of B2 aluminide single crystals, including FeAl, NiAl and CoAl, has been investigated at temperatures below their ductile brittle transition temperatures. Single-edge notched bend specimens oriented along specific crystalline directions were tested by 4-point bending. The fracture resistance was highly anisotropic because of the existence of a preferred cleavage plane in these B2 aluminide crystals. With a deep through notch NiAl and CoAl crystals that have high ordering energies generally cleave on {110} planes, while substoichiometric FeAl having a low ordering energy shows {100} cleavage as do most b.c.c. metals. In the case of NiAl, a transition fracture region, composed of fracture facets on {511} transient planes, appears at the initial cracking stage, followed by final cleavage on {110}. Different stoichiometric effects on the fracture toughness of B2 aluminides are observed when the Al concentration is reduced. A general discussion on different mechanistic models has been used to explain the preferred cleavage planes in B2 structures. The intrinsic fracture toughness of an aluminide crystal can be determined by an ideal fracture test in which the cleavage plane is parallel to the notch plane and is normal to the applied stress. Because of geometric constraints an increased fracture resistance is obtained when the natural cleavage plane is not parallel to the notch plane, and the anisotropy of fracture toughness can be explained by a simple approach of resolved normal stress intensity.