Mechano-chemical coupling during precipitate formation in Al-based alloys
Owing to their light weight combined with high strength, Al-based alloys have always been an interesting topic for research in materials community. The strengthening mechanism in such alloys is governed by the precipitate formation which in turn depends on the local environment chemistry and the morphology of the precipitates thereby indicating a thermo-mechanical coupling between the two. In the case of Al-Mg-Sc alloys it has been recently discovered that the formation of an ultra-fine-grained microstructure can significantly affect the size and morphology of second-phase precipitates, as well as their chemistry and distribution within the Al matrix. It is therefore the aim of the present project to understand and resolve the coupling between thermo-chemistry and thermo-mechanics underlying these processes.
To achieve this goal, ab initio based atomistic simulations, accompanied by dedicated and carefully selected experiments will be performed. The effect of the stress field caused by the microstructure (e.g. grain boundaries) and external loads on the local chemistry will on the one hand be accurately determined by performing fully temperature-dependent calculations within density functional theory, which also take anharmonic entropy contributions into account.
On the other hand, to simulate precipitate formation under strongly strained conditions a kinetic Monte-Carlo scheme that allows to include medium and long range elastic interactions will be developed. The proper treatment of the diffusion kinetics requires the accurate consideration of vacancies. Anharmonicity has been proven to affect the vacancy formation energies significantly. We have developed methods to study the temperature dependence of point defect energies  and will apply them in the present project. The results of the kinetic simulations will be evaluated in terms of TTT (time-temperature-transformation) diagrams.
The coupled thermodynamic-kinetic approach will not only allow a detailed analysis of how large local strain fields affect the formation and chemistry of precipitates, but also the opposite route, i.e. how the formation of a new chemical phase (precipitates) affects the mechanical strain fields.