Electron channeling contrast imaging of dislocation structures in deformed stoichiometric NiAl

Observations of dislocations in the crack tip regions of loaded and unloaded bulk single crystals of NiAl have been carried out using the electron channeling contrast imaging (ECCI) technique. A significant amount of dislocation generation has been observed ahead of both loaded and unloaded cracks in both commercial and high purity materials, suggesting a degree of crack tip blunting can occur in these materials. Dislocations have also been observed along crack edges at points where crack deflection has occurred indicating that crack deflection may contribute to the toughening of the material. However, few dislocations are observed along portions of the crack edges where the cracks have propagated in a fairly straight manner, suggesting that dislocations cannot consistently be generated at the head of the cracks. Thus the cracks may propagate freely until they encounter regions where the dislocation generation is easier. As a result, the overall lack of toughness in NiAl may be in part due to an inability to consistently generate dislocations in this material.