Study of the Mechanical Behavior of BCC Transition Metals Using Bond-Order Potentials
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ABSTRACT Deformation properties of body-centered-cubic transition metals are controlled by the core structure of screw dislocations and their studies involve extensive computer simulations. In this paper we present the recently constructed bond-order potentials (BOP) that are based on the realspace parametrized tight-binding method. In order to examine the applicability of the potentials we have evaluated the energy differences of alternative structures, investigated several transformation paths leading to large distortions and calculated phonon dispersions. Using these potentials we have calculated y-surfaces that relate to the dislocation core structures and discuss then the importance of directional bonding in studies of dislocations in transition metals. INTRODUCTION The low temperature and high strain-rate plastic deformation of body-centered-cubic (bcc) transition metals is dominated by the structure of the cores of 1/2 screw dislocations [1-3]. Consequently, theoretical investigation of mechanical properties of these materials habitually involves extensive atomistic simulations [2-4]. The essential precursor of such calculations is an appropriate description of atomic interactions. The fact that it is the level of the filling of the dband that controls the stability of the bcc lattice relative to alternate structures (see e. g. [5]) suggests that the angular character of bonding originating from d-bonds should be properly accounted for. Nevertheless, the bulk of atomistic studies of dislocations and other extended defects in bcc metals have been made using central-force potentials; in the early studies pairpotentials [6, 7] and more recently many-body potentials of the embedded atom type [4, 8]. The first calculations that include the non-central character of bonding in transition metals are those employing potentials derived from first-principles generalized pseudopotential theory (MGPT potentials) [9-11]. Although they confirmed the general characteristics of the screw dislocation cores found in central-force studies, they also indicate that structural features specific to different bcc metals may not be appropriately captured by central force schemes. While a full analysis of the effect of non-central interactions on dislocation cores has not yet been made, recent theoretical and experimental investigation of the structure of the 15 (310) symmetrical tilt grain boundary in Nb and Mo clearly demonstrates the inability of central forces to differentiate between these two transition metals. Specifically, the electron microscopic studies reveal that this grain boundary is symmetric in Nb [12] while in Mo the symmetry is broken due to the relative displacement of the grains parallel to the tilt axis [13]. This difference is not found in studies employing central force potentials [12, 14] but it is revealed in calculations in which the covalent character of bonding has been included [12-14], comprising calculations employing the bond-order potentials discussed in this paper. All aspects of interatomic bonding a
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