Atomistic Modeling of Extended Defects in Metalic Alloys: Dislocations and Grain Boundaries in L1 2 Compounds
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ATOMISTIC MODELING OF EXTENDED DEFECTS IN METALIC ALLOYS: DISLOCATIONS AND GRAIN BOUNDARIES IN L1 2 COMPOUNDS V. Vitek*, G. J. Ackland** and J. Cserti* *Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.; ** Department of Physics, University of Edinburgh, Edinburgh, U.K. ABSTRACT Extended defects, such as dislocations and grain boundaries, control a wide variety of material properties and their atomic structure is often a governing factor. A necessary precursor for modeling of these structures is a suitable description of atomic interactions. We present here empirical many-body potentials for alloys which represent a very simple scheme for the evaluation of total energies and yet reflect correctly the basic physical features of the alloy systems modeled. As examples of atomistic studies we show results of calculations of the core structures of screw dislocations in Li 2 compounds. The same potentials have also been used to calculate structures of grain boundaries in these compounds. The deformation and fracture behavior of L1 2 alloys is then discussed in the light of grain boundary and dislocation core studies.
INTRODUCTION Many physical properties of crystalline solids, in particular their mechanical behavior, are controlled by extended defects such as dislocations, grain boundaries and interfaces between different phases and materials. This is the reason why studies of the structure and behavior of such defects have been in the forefront of fundamental research in materials science for many years. While in some cases more macroscopic approaches, such as continuum mechanics analyses, suffice, in others it is the atomic structure and atomic level behavior of the defects which need to be understood. Examples of the former are the continuum theory of dislocations and fracture mechanics in the framework of which many important features of the mechanical behavior can be analyzed and understood. On the other hand, a wide variety of properties cannot be comprehended without studying the atomic structure of the defects concerned. The prime example are properties of interfaces which are controlled by the atomic structure in the very narrow region of their cores (see e.g. reviews [1,2] and proceedings of several recent symposia [3-5]). In the case of plastic deformation examples of phenomena which cannot be analyzed using the continuum theory of dislocations are the invalidity of the Schmid law observed in many materials other than f.c.c. metals, non-compact slip in h.c.p. materials, anomalous temperature dependences of the yield stress in intermetallic compounds etc. (see e.g. reviews [6-8]). Most of the atomistic studies of extended defects have been made for pure metals, ionic crystals and semiconductors (see e.g. [9-14]) although understanding of the atomic structure of crystal defects in alloys is even more important. Two examples of atomic level phenomena crucial for microscopic understanding of the mechanical properties of materials are segregation to grain b
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