CRACK-TIP DISLOCATIONS AND FRACTURE-BEHAVIOR IN NI3AL AND NI3SI

The ideal Griffith energies and the critical stress intensity factors for three cleavage modes are determined from the calculated elastic co nstants and surface energies. The propensity for crack tip deformation is estimated on the basis of the calculated antiphase boundary (APD), complex stacking fault and superlattice intrinsic stacking fault ener gies and the anisotropic coupling effect on dislocation mobility, i.e. non-Schmid effects. It is shown that while the calculated Griffith en ergy of Ni3Si is larger than that of Ni3Al, dislocation emission from a crack tip is easier and dislocation mobility is higher in Ni3Al than in Ni3Si. When a crack is loaded in a mixed mode (K-I + K-III), emiss ion of a super-Shockley partial from the crack trailing extended SISF is predicted and confirmed by in situ straining transmission electron microscopy observations of Ni3Al.