Abstract
AbstractSingle crystalline specimens of L12 ordered binary Ni3A1 and Ni3(A1,B) with single slip [123] and multiple slip [001] orientations were deformed in compression within the anomalous temperature regime. In all cases the TEM investigations show that dipoles of locked screws are a dominant feature of the dislocation structure. This is of special interest since screw dipoles are not observed in disordered fee structures. The results indicate that screws interact and form long sessile dipoles even when their height is much larger (= factor of 10) than that given by the passing stress. The minimum height of the screw dipoles is about 16±4 nm and seems to be given when the interaction stress corresponds to the saturation stress. In the dipoles the locked screws are very difficult to unlock since the external stress is screened to a large extent. Therefore the deformation has to occur by the movement of edge segments. It is suggested that based on this exhaustion of the screws steady state conditions do not always apply on a local scale.
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