Transition Metal Impurity-Dislocation Interactions in NiAl: Dislocation Friction and Dislocation Locking.
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Transition Metal Impurity-Dislocation Interactions in NiAl: Dislocation Friction and Dislocation Locking. O.Yu. Kontsevoi1, Yu.N. Gornostyrev1, 2, and A.J. Freeman1 1 Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208-3112, U.S.A. 2 Institute of Metal Physics, Ekaterinburg, Russia. ABSTRACT The energetics of the interaction of the {010} edge dislocation in NiAl with early 3d transition metal (TM) impurities was studied using the ab initio real-space tight-binding LMTO-recursion method with 20,000 atom clusters and up to 1,000 non-equivalent atoms in the dislocation core. The coordinates of the atoms in the core were determined within the Peierls-Nabarro (PN) model with restoring forces determined from full-potential LMTO total energy calculations. TM impurities were then placed in different substitutional positions near the dislocation core. For most positions studied, the interaction between impurities and the dislocation is found to be repulsive (dislocation friction). However, when the impurity is in the position close to the central atom of the dislocation core, the interaction becomes strongly attractive, thus causing dislocation locking. Since the size misfit between the Al atom and the substituting TM atom is very small, this locking cannot be explained by elastic (or size misfit) mechanisms; it has an electronic nature and is caused by the formation of the preferred bonding between the electronic states of the impurity atom and the localized electronic states appearing on the central atom of the dislocation core. The calculated results are then discussed in the scope of experimental data on solid solution hardening in NiAl. INTRODUCTION The improvement of the strength of materials due to doping by ternary additions has become a traditional alloy design approach. At impurity concentrations below the solubility limit and at low temperatures, solid solution hardening (SSH) [1], which is determined by the interactions of impurities with dislocations, is the main reason for an increase of the yield stress. Hence, the nature of elementary impurity-dislocation interactions is one of the key questions in the physics of the strength and plasticity of solid solutions. Although impurity-dislocation interactions depend on many factors, according to the prevailing point of view [1, 2], the size misfit between the impurity and host atoms appears to make the main contribution in a majority of alloys. On the other hand, there are a number of cases, when the electronic structure of impurities plays a more important role than size misfit. In particular, it was found that sp-impurities and transition metal (d-) impurities with the same size misfit give very different contributions to SSH in Ni3Al [3, 4] (the so-called "extra" solution hardening effect [3]). In NiAl [4, 5] alloyed with transition metal impurities, SSH differs significantly for the elements with similar atomic radii and correlates with electronic structure features rather than with size misfit [6]. In addition to parelastic interact
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