Possibilities of Slip Modification in B2 NiAl
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POSSIBILITIES OF SLIP MODIFICATION IN B2 NiAl Diana Farkas and Zhao Yang Xie Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
ABSTRACT We discuss the possibilities of stabilizing (111) slip in NiAI based on recent results of the atomistic structure of dislocation cores in this material. In previous work it was found that the (111) complete dislocation is stable with respect to partials of 1/2 (111) Burgers vector. However, split configurations were found with a very similar energy. In particular, split configurations that are nearly planar were found, indicating that if this type of dislocation could be stabilized they would possibly result in ductile alloys. We discuss the possible effects of local disorder and deviations from stoichiometry on core structure concentrating on the objective of stabilizing (111)slip. Atomistic simulations performed to study these effects show that local compositional changes and disorder may have significant effects on the core structure. These effects are most important in the regions of the core where the point defects are localized and it is unlikely that such local changes could be used to successfully stabilize (111) slip. Furthermore, it is not clear whether local changes in ordering and compositional states can follow the dislocation as it moves during the deformation process. We therefore propose that the alloying efforts should be directed towards changing the APB energy of the NiAl- base alloy to a lower value.
INTRODUCTION A large research effort has been devoted in recent years to the study of different possible methods to improve ductility in ordered intermetallic alloys, and in particular NiAl. These methods include second phase formation[l] and utilizing the role of surface and interface dislocations [2]. We recall that the basic reason for the lack of ductility in NiAl is usually believed to be the lack of sufficient slip systems, which is a consequence of the fact that NiAl deforms through (100) slip. Other compounds, such as CuZn with the same structure that deform through (111) slip do exhibit ductile behavior. It follows that in order to improve ductility one could investigate the possibilities of stabilizing (111) slip in this material, for example, through alloying. It has been shown that when the crystal orientation is such that the applied stress on the existing (100) dislocations is zero, the material indeed deforms through (111) slip [3]. As a matter of fact, the occurrence of (111) slip has also been proposed as an explanation for the limited ductility of NiAl observed at room temperature [4]. The instability of (111) slip is certainly connected to the fact that the ordering energy is high and to the fact that the (111) dislocations apparently do not split into partials [5]. In agreement with experimental observations, computer simulation also shows that the stable form for screw (111) dislocations is for these dislocations to remain as complete (111). In more ductile B2 materials t
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