Efficient Calculation Methods for the Diffusion Coefficient of Interstitial Solutes in Dilute Alloys.

Xiaoshuang Wang,Jürgen Faßbender,Matthias Posselt

doi:10.3390/ma12091491

Abstract

In the example of oxygen diffusion in dilute ferritic iron alloys it is shown that the calculation of the diffusion coefficient can be separated into a contribution related to the migration in the interaction region between oxygen and the substitutional solute and a part related to diffusion in pure body centered cubic (bcc) Fe. The corresponding diffusion times are determined by analytical expressions using Density-Functional-Theory (DFT) data for the respective binding energies. The diffusion coefficient in the interaction region must be determined by atomistic kinetic Monte Carlo (AKMC) simulations with DFT values for the migration barriers as input data. In contrast to previous calculations, AKMC simulation must only be performed for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations in a very efficient manner. This leads to a tremendous decrease of computational efforts. Under certain conditions it is even possible to use analytical expressions where merely DFT data for the binding energies are needed. The limits of applicability of the presented calculation procedures are discussed in detail. The methods presented in this work can be generalized to interstitial diffusion in other host materials with small concentrations of substitutional solutes.

Highlights

Solutes with an atomic size smaller than that of the host atoms migrate via the interstitial mechanism [1]
The diffusion via the vacancy and the interstitialcy mechanism is much slower than that of the interstitial solutes, since the concentration of the native point defects is rather low under the conditions of thermal equilibrium
In a dilute alloy the migration of the diffusing interstitial atom cannot be influenced at the same time by more than one substitutional solute

Summary

Introduction

Solutes with an atomic size smaller than that of the host atoms migrate via the interstitial mechanism [1]. In this case the most stable site of the solute and the saddle point for the migration are highly symmetric interstitial positions in the lattice, e.g., octahedral and tetrahedral sites. The diffusion via the vacancy and the interstitialcy (or indirect interstitial) mechanism is much slower than that of the interstitial solutes, since the concentration of the native point defects is rather low under the conditions of thermal equilibrium. In a dilute alloy the migration of the diffusing interstitial atom cannot be influenced at the same time by more than one substitutional solute

Methods

Results

Conclusion