TEM observation of the fourfold dissociation of superlattice dislocations and the determination of the fault energies in Ni₃(A1, Ta)

Abstract

The dissociation of the superlattice dislocations in Ni₃(Al, Ta) has been studied by transmission electron microscopy (TEM) weak-beam methods. Single-crystal specimens were deformed at liquid-nitrogen temperature, at room temperature and at 270°C, 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 s = 0·8 to 0·37 nm ⁻¹). 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 γ^APB(111) and γ^APB(100) are 250 ± 30 and 225 ± 25 mJ m⁻² respectively. Compared with pure Ni₃Al the addition of 1 at.% Ta leads to an increase in the γ^APB values of more than 100% whereas their ratio remains nearly unchanged. A fourfold dissociation of the (111) glide dislocations has been observed using weak Bragg reflection images with a very large deviation parameter (applying 2g (5g) conditions, s = 0·56 nm⁻¹). From the measured separations an estimate of the complex stacking fault energy γ^CSF = 300 ± 40 mJ m⁻² can be achieved. In the discussion the mechanical properties of Ni₃Al alloys and those of ordered Ni₃Fe are compared and it is proposed that the actual structure of the dislocation core is the most important parameter for the cross-slip behaviour that causes the unusual mechanical properties of Ni₃Al alloys.