Fracture toughness data of stoichiometric FeAl and NiAl single crystals were measured in four-point-bending tests and the crystallographic orientations of the resultant fracture surfaces were determined.
For FeAl single crystals unexpectedly high fracture toughness values were measured even at low temperatures (17 MPa  at 77K). However, these crystals failed in a brittle manner for temperatures up to 300K. The crystallographic orientation of the fracture surfaces were strongly dependent on the environment due to hydrogen embrittlement.
The NiAl specimens were precracked and the fracture toughness was measured in the weakest crystal orientation. The fracture surfaces were similar to those obtained in previous investigations. Although the room temperature (RT) KIC-value was the highest for pure weak-oriented NiAl (7.5 MPa ) the BDT temperature was found in the same region where it occurs for the hard orientation (673K to 693K). The onset of the BDT will be discussed.