Dislocation mechanisms in Ni3Al at room temperature.In situstraining experiments in TEM

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