Single crystals of the L12 ordered alloy were deformed in compression at room temperature or 400C. These temperatures were below, and within, the well-known anomalous regime. Transmission electron microscopy was used to analyze the dislocation structures. It was found that, at room temperature, edge dipoles predominated (as in face-centered cubic metals). At 400C, locked screw dislocations, screw dislocation dipoles and near-screw dislocations which were bowed out on the (010) cube cross-slip plane predominated. Their formation was attributed to a gradual transition from so-called normal octahedral cross-slip to thermally activated cube cross-slip. By comparing weak-beam transmission electron microscopic images with computer simulations, and using anisotropic elasticity theory, the stacking-fault energy was deduced to be 235mJ/m2. The antiphase boundary energies were equal to 175mJ/m2 for (111) and 104mJ/m2 for (010). The self-interstitial stacking-fault energy was equal to 6mJ/m2. These values could be used to explain a shift to higher temperatures, of the anomalous increase in yield stress, that was observed in plain Ni3Al as compared with samples that contained 1at%Ta. The ratio of the (111) to (010) antiphase boundary energies governed the driving force for cube cross-slip. An opposite direction of the above shift was to be expected upon comparing this ratio for the mentioned alloys. The value of the stacking-fault energy was deemed to be the decisive parameter. It determined the dissociation width, and therefore the constriction energy, of the Shockley partials of the screw dislocations. A low value of the stacking-fault energy reduced the thermal activation that was necessary for the formation of Kear-Wilsdorf locks and of screw dislocations which bowed out on the (010) plane.

The Influence of the Fault Energies on the Anomalous Mechanical Behaviour of Ni3Al Alloys. H.P.Karnthaler, E.T.Mühlbacher, C.Rentenberger: Acta Materialia, 1996, 44[2], 547-60