A new migration mechanism for oversized solutes in cubic lattices: correlation effects

TL;DR

This paper introduces a novel migration mechanism for oversized solutes in cubic lattices, accounting for correlation effects and macrojumps, revealing that Y diffuses faster than iron due to high macrojump frequencies.

Contribution

It develops a new formalism to accurately calculate diffusion coefficients and correlation factors for oversized solutes considering complex vacancy interactions.

Findings

Y diffuses faster than iron in BCC and FCC lattices.

Correlation factor is small due to vacancy jump frequencies near the solute.

Large macrojump frequency compensates for small correlation factor, enhancing diffusion.

Abstract

Recent ab-initio calculations showed that oversized solute atoms (OSA) in BCC and FCC iron exhibit a very strong attraction with a nearby vacancy (V) at first neighbour distance (1NN). The attraction it is so large that the 1NN pair OSA+V is no longer stable and relaxes spontaneously towards a new configuration where the OSA sits in the middle of the bond, the two ends of which are decorated with two half-vacancies (V/2). Taking into account the formation of this complex V/2+OSA+V/2 in the migration process has never been done before: it requires a new formulation of correlation effects. This is the aim of the present contribution which establishes the expression of the diffusion coefficient of the OSA and evaluates exactly the correlation factor in BCC and FCC lattices, thanks to the introduction of macrojumps. The formalism is applied to the case of Y in BCC iron and its results are compared to those obtained by previous authors. It is shown that the smallness of the correlation factor is not due to the transport mechanism of the OSA but only to the jump frequencies of the vacancy in the vicinity of the OSA. This smallness of the correlation factor is however overcompensated by a large macrojump frequency, and, as a result, the Y atom diffuses more rapidly than iron at all temperatures by orders of magnitude in the two structures.