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