Transmission electron microscopy in situ straining experiments were performed under weak beam conditions, in Ni3Al(0.25at%Hg) at room temperature. They revealed rectilinear screw super-dislocations dissociated either in the two octahedral or in the cube cross-slip plane, and fully reversible cross-slip between these planes. Screw super-dislocations have the lowest mobility. They glide in octahedral planes through series of jumps and waiting times in sessile positions for which the antiphase boundary was contained in the glide plane. The jump amplitude could either be large, or equal to the super-partial separation. In order to explain these experimental results, a new sessile configuration of screw super-dislocations was introduced, with the antiphase boundary ribbon in {111}, and both super-partials cross-slipped in {100} on a short distance (about 20A). This configuration has some common points with that already proposed by Paidar, Pope and Vitek (1984), but was different in several aspects. It was consistent with observations of super-dislocations under different directions. It forms under the torque introduced by Yoo (1987), and a small difference of antiphase boundary energy between octahedral and cube planes. Under such conditions, all movements in {111} and all cross-slip events could be readily explained. It was shown that locking of super-dislocations occurs by cross-slip of their trailing super-partials, while the configuration of the leading one determined the type of subsequent movement.

Dislocation Mechanisms in Ni3Al at Room Temperature. In situ Straining Experiments in TEM. Molenat, G., Caillard, D.: Philosophical Magazine A, 1991, 64[6], 1291-317