A Monte Carlo study of atomic migration by the interstitialcy mechanism in binary random alloys

Over the past decades, there have been numerous Monte Carlo (MC) studies on diffusion by the vacancy mechanism. These were motivated by the importance of vacancies for atomic transport in many materials, particularly in metallic systems, and further inspired by Manning’s random alloy model. However, MC investigations on diffusion involving interstitialcy defects have remained extremely sparse, although self-interstitials may play a crucial role in ionic crystals and semiconductor alloys. In this work, we explore by MC simulation some basic features of atomic (or ionic) movement mediated by interstitialcies in binary random alloys AB with a simple cubic structure [1,2]. After the introduction of a basic analytical and computational framework, we focus on the calculation of correlation factors governing the diffusivity of tracer atoms and the generalized interstitialcy defect (D_I) appearing in either A or B state. Apart from collinear interstitialcy exchange as a standard case, the effects of both non-collinear and direct interstitial jumps are examined as well. It is shown that physical correlation effects, which are due to the different jump frequencies of A and B atoms, have a dominating impact below the percolation threshold of the more mobile component (B). In particular, for compositions poor in B the B-to-A tracer diffusivity ratio is bounded by a maximum value. However, this bound is less restrictive compared to a corresponding bound for the vacancy mechanism under similar conditions [3]. We also discuss the results for the Haven ratio, which relates the tracer diffusion coefficients to the charge diffusivity as may be deduced from the ion conductivity through the Nernst-Einstein equation.

[1] F. Wilangowski and N. A. Stolwijk, J. Phys. Condens. Matter 27 (2015) 505401

[2] F. Wilangowski and N. A. Stolwijk, Phil. Mag. 97 (2017) 108-127.

[3] F. Wilangowski and N. A. Stolwijk, Phil. Mag. 95 (2015) 2277-2293