Thermally Activated Unpinning of Screw Dislocations in the Anomalous Regime in L1 2 Compounds
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THERMALLY ACTIVATED UNPINNING OF SCREW DISLOCATIONS IN THE ANOMALOUS REGIME IN L1 2 COMPOUNDS M. Khantha, J. Cserti and V. Vitek Department of Materials Science and Engineering University of Pennsylvania Philadelphia, PA 19104-6272. ABSTRACT We present a model for the anomalous increase of the yield stress exhibited by many LI 2 compounds. It is based on two thermally activated processes that describe respectively the pinning and unpinning of [T01] screw dislocations in the (11) plane. The model explains all the important characteristic features observed in the anomalous regime. We discuss the applications of the model to Ni 3Ga and Ni 3 (AI,Ta). INTRODUCTION The anomalous increase of the yield stress with increase in temperature is well known in several L1 2 compounds and has been studied extensively. It is now recognized that the factors causing this behavior are also responsible for the many characteristic features in the anomalous regime such as (i) Strong orientation dependence of the yield stress and tension/compression (T/C) asymmetry [1-3]; (ii) A very low strain-rate sensitivity in the anomalous regime extending up to the peak temperature (Tp) [1, 4]; and (iii) A large discontinuity in the activation volume (v,) at a temperature (Ta) well below Tt [4, 5]. While it has been known that a thermally activated mechanism obstructing the motion of screw dislocations in the (111) plane is the primary cause for the anomalous behavior, different approaches have been proposed to describe the unpinning or release of screw dislocations from the 'obstacles' that impede its motion. Many of the earlier models [1, 6] were based on a thermally activated process that 'pins' the screw dislocations but assumed an athermal process for the release. Such an approach could explain the feature (i) listed above as it is related to the details of the pinning mechanism but could not explain features (ii) and (iii) as they are related specifically to the thermally activated aspects of the release mechanism [7]. Both the pinning and unpinning mechanisms are assumed to be thermally activated in some recent models [8, 9] and agreement between theory and experiment has been demonstrated at certain selected temperatures and stresses. The starting point for all the models is common and is based on the thermally activated pinning mechanism proposed in the PPV model [6]. However, the models differ in the final dislocation configuration in the pinned state which then leads to differences in the unpinning mechanism as well. Recently, we have proposed a model [10, 11] which is also based on two thermally activated mechanisms and includes a simple unpinning process based on the concept of major breakaway that accounts for all the macroscopic characteristic features associated with the yield stress anomaly. There are three main parts in our approach. The first describes a thermally activated process that results in the local pinning of screw dislocations in the (111) plane at random positions such that their movement in the glide plane is impeded.
