Ab initio calculations have become a very powerful tool for RIS simulations. They have been shown to be able to provide not only the variation of the atomic jump frequencies with local concentration,125 but also new diffusion mechanisms.7 From a precise knowledge of the atomic jump frequencies and the recent develop­ment of diffusion models,98 a quantitative description of the flux coupling is expected to be feasible even in concentrated alloys. A unified description of flux coupling in dilute and concentrated alloys would allow the simultaneous prediction of two different mechanisms leading to RIS: solute drag by vacancies, and an IK effect involving the major elements.

An RIS segregation profile spreads over nan­ometers. Cell sizes of RIS continuous models are then too small for the theory of TIP to be valid. A mesoscale approach that includes interface energy between cells, such as the Cahn-Hilliard method, would be more appropriate. A derivation of quantita­tive phase field equations with fluctuations has recently been published.62 The resulting kinetic equations are dependent on the local concentration and also cell-size dependent. However, the diffusion mechanism involved direct exchanges between atoms. The same method needs to be developed for a system with point defect diffusion mechanisms.

Although it has been suggested since the first publications30 that the vacancy flux opposing the set up of RIS could slow down the void swelling, the change of microstructure and its coupling with RIS have almost never been modeled. Only very recently, phase field methods have tackled the kinetics of a concentration field and its interaction with a cavity population (see Chapter 1.15, Phase

Field Methods). The development of a simulation tool able to predict the mutual interaction between the point defect microstructure and the flux coupling is quite a challenge.