Ni3Al shows like many other L12 alloys an anomalous increase of the yield stress up to a peak temperature of about 800C. This increase was generally explained by a thermal activated cross-slip process of superlattice screw dislocations from (111) to (010). To study the influence of the external stress upon this process two orientations were selected: compression axis [¯123] and [001]. They exhibited a large difference in the Schmid factors for cube cross-slip. Single crystals of Ni3Al were deformed at room temperature and 400C, and transmission electron microscopy foils cut parallel to (111) and (010) were studied. At room temperature, which was below the anomalous temperature regime, the dislocation structures were in both cases similar to those of face-centered cubic structures without order. At 400C the transmission electron microscopy revealed show a distinct difference: dislocations bowing out on (010) were a typical feature for the [¯123] axis; whereas in the case of the [001] axis dislocation reactions and long straight screws were predominant. They occur in the form of Kear-Wilsdorf locks and contain frequently superlattice interstitial stacking-fault converted Kear-Wilsdorf locks. Therefore, despite of the different appearance, in both cases strong obstacles were contained in the observed dislocation structures. To overcome these strongly locked segments an Orowan type source model was proposed. When the stress was sufficiently high the dislocation could bow out between obstacles, move on and leave the locked segments behind, which were stored.

TEM Studies of the Influence of the Orientation on the Dislocation Structures in Plastically Deformed Ni3Al Single Crystals. Karnthaler, H.P., Rentenberger, C., Muhlbacher, E.: Materials Research Society Symposium – Proceedings, 1993, 288, 293-8