Self-diffusion in a triple-defect A-B binary system: Monte Carlo simulation

J Betlej,P Sowa,R Kozubski,G.E Murch,I.V Belova

doi:10.1016/j.commatsci.2019.109316

J Betlej, P Sowa + Show 3 more

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https://doi.org/10.1016/j.commatsci.2019.109316

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Abstract

In this comprehensive and detailed study, vacancy-mediated self-diffusion of A- and B-elements in triple-defect B2-ordered ASB1-S binaries is simulated by means of a kinetic Monte Carlo (KMC) algorithm involving atomic jumps to nearest-neighbour (nn) and next-nearest-neighbour (nnn) vacancies. The systems are modelled with an Ising-type Hamiltonian with nn and nnn pair interactions completed with migration barriers dependent on local configurations. Self-diffusion is simulated at equilibrium and temperature-dependent vacancy concentrations are generated by means of a Semi Grand Canonical MC (SGCMC) code. The KMC simulations reproduced the phenomena observed experimentally in Ni-Al intermetallics being typical representatives of the 'triple-defect' binaries. In particular, they yielded the characteristic ‘V’-shapes of the isothermal concentration dependencies of A- and B-atom diffusivities, as well as the strong enhancement of the B-atom diffusivity in B-rich systems. The atomistic origins of the phenomenon, as well as other features of the simulated self-diffusion such as temperature and composition dependences of tracer correlation factors and activation energies are analyzed in depth in terms of a number of nanoscopic parameters that are able to be tuned and monitored exclusively with atomistic simulations. The roles of equilibrium and kinetic factors in the generation of the observed features are clearly distinguished and elucidated.

Highlights

The notion of the ‘triple defect’ was introduced by Wasilewski [1] who originally defined it as a complex of a single A- or B-antisite defect and two nn vacancies in a stoichiometric A-50 at% B system with the B2 superstructure (Fig. 1)
In nonstoichiometric binaries the tendency for triple-defect disordering’ (TDD) – i.e. lower formation energy for A-antisites, means that while A-antisites compensate for the deficit of B atoms in A-rich systems, the B-atoms in B-rich systems remain on the β-sublattice and the departure from stoichiometry is compensated by ‘structural’ α-vacancies
Supercells were composed of 25 × 25 × 25 unit cells of the B2 superstructure (Fig. 1) – i.e. containing N = 31250 lattice sites belonging to equi-numerous α- and β-sublattices and populated with NA A-atoms, NB B-atoms and NV vacancies. 3D periodic boundary condition (PBC) were imposed upon the supercells

Summary

Introduction

The notion of the ‘triple defect’ was introduced by Wasilewski [1] who originally defined it as a complex of a single A- or B-antisite defect and two nn vacancies in a stoichiometric A-50 at% B system with the B2 superstructure (Fig. 1). Generation of ‘triple defects’ stems from a substantial difference between the formation energies for A- and Bantisite defects whose consequence is that the system disorders (e.g. due to increasing temperature) by preferentially creating the antisites with lower formation energy. Such a phenomenon is called ‘triple-defect disordering’ (TDD). The tendency for TDD implies: (i) a large difference between the Aand B-antisite concentrations; (ii) a large difference between the concentrations of vacancies residing on α- and β-sublattices (the ‘home’ sublattices of A and B atoms); (iii) an increase of vacancy concentration with decreasing degree of chemical long-range order – i.e. with an increasing concentration of antisite defects. The process of TDD should be contrasted from the so called triple-defect mechanism of diffusion which means atomic migration via correlated atomic jumps mediated by vacancy-pairs [2]

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