The stability of screw dipoles in Ni3Al studied by TEM

Transmission electron microscopy (TEM) methods were used to study the dislocation structures in Ll(2) ordered binary Ni3Al deformed at a temperature below and at a temperature above the onset of the anomalous increase of the yield stress (compression axis oriented for single slip). The corresponding change from a dislocation structure dominated by edge dipoles to a structure dominated by locked screw dipoles is caused by the transition from octahedral cross-slip to cube cross-slip. The measurements based on weak-beam dark-field TEM images show that stable configurations of screw dipoles occur above a minimum dipole height and within a certain range of dipole angles. In agreement with the calculations the stability region of the screw dipoles is limited since locked screws can become mobile in the vicinity of a screw of opposite sign. Remobilized screws can annihilate leading to the formation of antiphase boundary tubes or can pass each other generating glissile edge segments that contribute to plastic flow. The screw dipoles showing stable configurations are considered as strong locks and lead to an autocatalytic increase of their density with increasing deformation. (C) 2008 Elsevier Ltd. All rights reserved.