Dislocation Structure and Deformation Behavior of Intermetallic Compounds

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ABSTRACT The fine structure of dislocations in intermetallic compounds can have a profound influence on their macroscopic mechanical properties. The development of appropriate models of deformation requires consideration of dislocation core structure, possible dissociation or decomposition reactions, overall dislocation morphology and relevant dislocation interactions based on detailed transmission electron microscopy study. This empirical information may be rationalized based on both atomistic and continuum-level dislocation modeling. The specific cases ofjogged 1/2 superdislocations in NiAI are discussed.

INTRODUCTION For more than two decades, research efforts have been directed toward developing an improved, fundamental understanding of the mechanical properties of intermetallic compounds. Interest in this class of materials has been driven in part by their potential for high temperature, structural applications. From the standpoint of dislocation-level modeling, these compounds also offer a remarkably rich and interesting field of study. A number of important and unique characteristics exhibited by many of these compounds, including the anomalous temperature dependence of the yield and flow strength, appear to be related directly to the intrinsic properties of dislocations. In contrast, the deformation behavior of pure metals, which one might expect to be simpler, are in many respects far more complicated than in many intermetallics. For example, the low temperature deformation of pure FCC metals is clearly dominated by extremely complex, collective interactions of large numbers of dislocations. The properties of individual dislocations are therefore of lesser importance. For intermetallics on the other hand, the study of dislocation fine structure and mobility, both from an experimental and theoretical basis, has provided significant insight into the behavior of these compounds. A classic example is the issue of cross-slip pinning/locking of superdislocations in LI 2 compounds, such as NiAl. Numerous TEM studies have shown that screw superdislocations tend not to be dissociated on the octahedral glide planes, but rather are dissociated with the APB lying on the cross-slip (010) plane [for reviews see 1-2]. HRTEM observations [3-4] and atomistic simulations (see [5] for a review) indicate however that the individual superpartials dissociate in octahedral planes. Thus, this configuration of the screw dislocation, originally hypothesized by Kear and Wilsdorf [6], is sessile to easy motion in either the octahedral or cube planes. Active debate in the literature has revolved around whether complete KW locks actually form dynamically during deformation, or whether only partial cross-slip occurs to the (010) plane [2]. Seemingly certain however is that the cross-slip process is thermally activated (thereby increasing the number of locks or pinning points as a function of temperature), and that the cross-slip process is analogous to the Escaig model of cross slip for dislocations in FCC metals [7]. The "pinning-poin

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