Transmission electron microscopy methods were used to study the dislocation structures in L12 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 was caused by the transition from octahedral cross-slip to cube cross-slip. The measurements based on weak-beam dark-field transmission electron microscopic images showed 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 was limited since locked screws could become mobile in the vicinity of a screw of opposite sign. Remobilized screws could annihilate leading to the formation of antiphase boundary tubes or could pass each other generating glissile edge segments that contribute to plastic flow. The screw dipoles showing stable configurations were considered as strong locks and lead to an autocatalytic increase of their density with increasing deformation.

The Stability of Screw Dipoles in Ni3Al Studied by TEM. Rentenberger, C. , Karnthaler, H.P.: Intermetallics, 2008, 16[4], 571-9