Dissociation of the superlattice dislocations in Ni3(Al,Ta) was studied by using transmission electron microscopy weak-beam methods. Single-crystal specimens were deformed at liquid-nitrogen temperature, at room temperature and at 270C, and the dislocations on both primary (111) and cube cross-slip planes (010) were investigated. When using g(4g) or g(5g) weak-beam conditions, twofold dissociations of the superlattice dislocations were observed (deviation parameter = 0.28 to 0.37/nm). After applying the standard correction to the measured spacings, the values of the antiphase boundary energy, γ, were calculated using anisotropic elasticity theory. The results deduced for γAPB(111) and γAPB(100) were 250 and 225mJ/m2, respectively. Compared with pure Ni3Al, the addition of 1at%Ta led to an increase in the γAPB values of more than 100% whereas their ratio remained nearly unchanged. A fourfold dissociation of the (111) glide dislocations was observed using weak Bragg reflection images with a very large deviation parameter (applying 2g(5g) conditions, of s = 0·56/nm. From the measured separations, an estimate of the complex stacking fault energy γCSF of 300mJ/m2 could be deduced. The mechanical properties of Ni3Al alloys, and those of ordered Ni3Fe, were compared and it was proposed that the actual structure of the dislocation core was the most important parameter for the cross-slip behaviour that caused the unusual mechanical properties of Ni3Al alloys.

TEM Observation of the Fourfold Dissociation of Superlattice Dislocations and the Determination of the Fault Energies in Ni3(A1,Ta). N.Baluc, H.P.Karnthaler, M.J.Mills: Philosophical Magazine A, 1991, 64[1], 137-50