Tem Studies of the Influence of the Orientation on the Dislocation Structures in Plastically Deformed Ni 3 Al Single Cry
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TEM STUDIES OF THE INFLUENCE OF THE ORIENTATION ON THE DISLOCATION STRUCTURES IN PLASTICALLY DEFORMED Ni 3AI SINGLE CRYSTALS H. P. Karnthaler, C. Rentenberger and E. Mifbibbacher University of Vienna, Inst. flir Festkorperphysik, Boltzmanngasse 5, A-1090 Vienna, Austria. ABSTRACT Ni 3AI shows like many other L1 2 alloys an anomalous increase of the yield stress up to a 0 peak temperature of about 800 C. This increase is 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 on this process two orientations were selected: compression axis [123] and [001]. They show a large difference of the Schmid factors for cube cross slip. Single crystals of Ni 3AI were deformed at RT and 400°C, and TEM foils cut parallel to (111) and (010) were studied. At RT which is below the anomalous temperature regime the dislocation structures are in both cases similar to those of fcc structures without an order. At 0 400 C the TEM results show a distinct difference: dislocations bowing out on (010) are a typical feature for the [123] axis; whereas in the case of the [001] axis dislocation reactions and long straight screws are predominant. They occur in the form of KW locks and contain frequently SISF converted KW locks. Therefore, despite of the different appearance, in both cases strong obstacles are contained in the observed dislocation structures. To overcome these strongly locked segments an Orowan type source model is proposed. When the stress is sufficiently high the dislocation can bow out between obstacles, move on and leave the locked segments behind, which are stored. INTRODUCTION Ni 3A1 shows like many other L1 2 alloys an anomalous increase of the yield stress up to a certain peak temperature which is about 800*C in the case of binary Ni 3Al. In this temperature regime below the peak the present studies were carried out. The thermal activated cross slip of superlattice screw dislocations (Burgers vector b=a) from their (111) glide plane onto the cube cross slip plane is a major ingredient of all the theories that try to explain the anomalous yield strength behaviour (e.g. [1], [2], [3]). On a macroscopic scale the experimental results show that from the observation of the slip lines there is no indication of the cube cross slip system being activated. The microscopical results (yielded by transmission electron microscopy, TEM) seem to be conflicting; some authors [4], [5] report the bowing out of dislocations on the cube cross slip planes whereas the results of Dimiduk [6] show the occurrence of straight screws (in a Kear Wilsdorf lock configuration). Since different orientations of the compression axis were used in these studies it is the purpose of this paper to carry out a systematical investigation. Therefore the dislocation structures observed after plastic deformation in crystals with single slip orientation ([123] axis) are compared with those occurring in crystals with multiple slip orientation ([001] axis). Th
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