Direct Molecular Dynamics Simulations of Diffusion Mechanisms in NiAl
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Direct Molecular Dynamics Simulations of Diffusion Mechanisms in NiAl D. Farkas and B. Soulé de Bas Dept. of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA ABSTRACT Molecular dynamics simulations of the diffusion process in ordered B2 NiAl at high temperature were performed using an embedded atom interatomic potential. Diffusion occurs through a variety of cyclic mechanisms that accomplish the motion of the vacancy through nearest neighbor jumps restoring order to the alloy at the end of the cycle. The traditionally postulated 6-jump cycle is only one of the various cycles observed and some of these are quite complex. A detailed sequential analysis of the observed 6-jump cycles was performed and the results are analyzed in terms of the activation energies for individual jumps calculated using molecular statics simulations. INTRODUCTION Contrary to the case of pure metals, where the self-diffusion mechanism is well established and consists of a nearest neighbor jumps (NN), the diffusion in B2 compounds is much more complex. Two main categories of mechanisms have been postulated to characterize the diffusion in B2 compounds: the mechanisms involving next nearest neighbor jumps (NNN), where the order is maintained at all times, and cyclic mechanisms involving nearest neighbor jumps, that destroy order temporarily. The next nearest neighbor jump mechanism can be expected as energetically favorable based on the fact that there is no disorder created during the process. Thus, Donaldson and Rawlings [1], based on a study of the diffusion tracer, suggested a NNN mechanism for the Ni atoms in the NiGa B2 compound. Theoretical considerations and static computer simulation studies [2], performed for B2 NiAl also suggested the NNN mechanism may be energetically favorable. The nearest neighbor jump mechanisms can be argued to be less favorable because the atoms jump initially to a site in the wrong sublattice, creating partial disorder in the crystal in the form of antisites. Several mechanisms have been suggested where the partial order is recovered after a certain number of nearest neighbor jumps, constituting a diffusion cycle. The best known of these is the 6-jump cycle (Elcock and McCombie [3]) where the vacancy migrates along a definite path of 6 nearest neighbor jumps. Since this mechanism was first proposed in 1958, this has been widely accepted as a main diffusion mechanism in B2 ordered alloys. Wynblatt [4], in a study based on β-AgMg, found that the 6-jump cycles was energetically the most favorable. Investigations done with quasi-elastic Mössbauer spectroscopy [5] and nuclear resonant scattering [6] on FeAl showed that the diffusion of Fe in the B2 phase takes place via NN jumps. Similarly, studies done using nuclear neutron scattering [7] on NiGa showed that the Ni atoms diffuse via NN jumps. Other mechanisms have also been proposed, such as the anti-structure bridge [8], where the vacancy migrates through a “bridge” created by an existing antisite,
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