Atomistic structure and lattice effects of vacancies in Ni-Al intermetallics
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Atomistic computer simulation with embedded atom method potentials has been performed to study the energetics and structures of point defects in LI 2 Ni3Al and B2 NiAl. The large size of Al atoms plays a dominant role in the relaxed atomic configurations of vacancies and antisite defects in the system. In both Ni3Al and NiAl, it was found that excess Ni can be accommodated by Ni in Al sites. Accommodation of excess Al by locating Al in Ni sites is not energetically favorable in NiAl. The most stable di-vacancy configuration is two vacancies as first nearest neighbors in Ni sublattices for Ni3Al and as second nearest neighbors in Al sublattices for NiAl. General attraction of two vacancies in Ni3Al was found. On the contrary, vacancies in NiAl show repulsive interaction in several cases. This accounts for the existence of structural vacancies in B2 NiAl. The effects of different degrees of random disorder on the lattice parameters were evaluated, and it was found that these effects can be correlated with the local distortion around antisite defects using a simple model.
I. INTRODUCTION A knowledge of the energetics and the atomic displacements around vacancies and other point defects is important for the study of several physical and mechanical properties of crystals as well as for the interpretation of various experimental studies. The case of the Ni-Al system is of particular interest due to the practical importance of intermetallics in this system and the strong effects of point defects associated with stoichiometric deviations in these intermetallics. Ordered Ni3Al and NiAl have been the subject of extensive studies due to their excellent high-temperature properties and propensity to intergranular fracture. Structural vacancies have been experimentally observed in NiAl,1 as well as a very high rate of hardening with stoichiometric deviations in this alloy.2 Also, only Ni-rich Ni3Al can be ductilized by B additions.3 The study of point defects in this system is necessary in order to understand these types of behavior. Point defects are also important in the understanding of material behavior under irradiation and diffusion mechanisms. For the calculation of the lattice distortion it is essential to use a method that takes into account the discrete nature of the lattice. This has been done in the past by using a semidiscrete model, in which the lattice in a small region near the defect is treated as discrete while the rest of the lattice is replaced by the continuum.4'5 The accuracy of this method would obviously increase with the size of discrete region. Kanzaki6 J. Mater. Res., Vol. 9, No. 4, Apr 1994
proposed an approach to account for the overall discrete nature of the lattice in the static point defect problem based upon the Born-von Karman model of the lattice. A Green function method, basically equivalent to the Kanzaki approach, was further developed by Tewary.7 With the availability of increasing computer power and more accurate interatomic potentials, atomistic computer simulation is now a possible str
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