TEM OBSERVATION OF THE FOURFOLD DISSOCIATION OF SUPERLATTICE DISLOCATIONS AND THE DETERMINATION OF THE FAULT ENERGIES IN NI3(AL, TA)

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-degrees-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 = 0.28 to 0.37 nm-1). After applying the standard correction to the measured spacings the values of the antiphase boundary energy-gamma were calculated using anisotropic elasticity theory. The results deduced for gamma-APB{111} and gamma-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 gamma-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 gamma-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.