Electronic Effects on Grain Boundary Structure in BCC Metals
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INTRODUCTION Atomistic simulations are an increasingly important means of understanding the behavior of materials under a variety of conditions. With this technique, an assembly of thousands, or up to billions, of atoms is defined in a computer simulation and allowed to interact according to certain rules and boundary conditions. The boundary conditions include temperature and states of stress, allowing the calculation of such properties as the equation of state or unstable stacking fault energy. The structure and properties of crystal defects can also be predicted, such as the configuration of a dislocation core or the formation and migration energies of interstitials and vacancies (see e.g. [I ]). The rules of interaction are often very simple in order to speed computation. This simplification requires approximations to be made about the physics of the interacting atoms. Hence, in the development of models of interatomic interactions, an evaluation is necessary of whether the essential physics have been incorporated in the model. The models are validated by comparing their predictions with experimental observations. Recently developed models of interatomic interactions incorporate angularly dependent contributions to model materials with directional bonding [2-7], such as the body centered cubic transition metals in which the d - bands participate in bonding. The strength of the directional component of the bonding has a major influence on the structure of crystal defects. The model of interatomic interactions with angular dependence that we use is the Model Generalized Psuedopotential Theory [5]. We have applied it to modeling the 15 (310)/[001] symmetric tilt grain boundary (STGB) in niobium [8), molybdenum [9], and now tantalum as a critical test of its accuracy. We report here on the new results for Ta. Grain boundaries are a particularly good test case for atomistic simulations. Perhaps the biggest reason is that high quality experimental data on the atomic structure of grain boundaries can be obtained through high resolution transmission electron microscopy (HREM). The limited resolution of the electron microscope, however, does place some stringent constraints on precisely which 347 Mat. Res. Soc. Symp. Proc. Vol. 589 0 2001 Materials Research Society
grain boundaries can be imaged at atomic resolution [10]. This limitation can be overcome by first choosing an amenable boundary to study both theoretically and experimentally, in this case the Y5(3 10)/[001] STGB, and then fabricating that very boundary for experimental study. The capability that enables this approach to the problem is the ultra-high vacuum (UHV) Diffusion Bonding Machine [11]. ATOMISTIC SIMULATIONS The Model Generalized Pseudopotential Theory (MGPT) [5, 12, 13] of interatomic bonding in BCC transition metals has been shown to represent both bulk and defect properties in the case of Mo [1, 9, 13]. Useful MGPT potentials have now also been determined for Ta over a wide volume range and preliminary results for selected mechanical properties ha
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