A multiscale atomistic QM/MM approach for the investigation of phase boundaries in metallic alloys
Although Density Functional Theory (DFT) and Molecular Dynamics (MD) proved to be tools of choice in the field of atomistic modelling, when accurately modelling extended planar defects in metals (e.g grain boundaries, stacking faults or phase boundaries), both methods suffer from certain limitations. For low symmetry defects the system size if prohibitive for DFT, while investigating a diverse local chemical environment (e.g., segregation study) severly limits the use of MD.
In the present contribution we will describe our developments for linking quantum mechanical (QM, VASP implementation) calculations with molecular mechanics (MM in LAMMPS) within the pyiron framework. While MM is applied to (chemically and structurally simpler) matrix regions, QM is used to treat chemical impurities and extended defects such as interfaces or grain boundaries for which interatomic potentials are not available. After introducing the methodology its implementation, we will show a benchmark study against pure expensive DFT calculations for a Mo Σ3 grain-boundary. Furthermore we will present predictions of segregation to phase boundaries in TiAl intermetallics (γ/γ and γ/α2) corroborated by APT data. Finally, by having accurate physical properties at the boundaries from the QM region, we propose a method of etimating the impact of the segregants on the local elastic properties.