## Metadynamics simulations to determine the free energy of grain boundaries during shear

Grain boundaries in metals and alloys influence the deformability and strength of the material. They interact with dislocations and represent weak links in the microstructure at which cracks can nucleate. Thus, to understand and to predict macroscopic strength and plastic deformation of polycrystalline metals at finite temperatures, knowledge of sliding barriers for, and the shear strength of grain boundaries in the microstructure is necessary.

This knowledge can be gained from calculations of the generalized-stackingfault-energy surface (gamma-surface), which also give insight into the interaction of grain boundaries with dislocations. Gamma-surfaces can be determined with high accuracy by means of ab-initio electronic structure calculations. However, in the calculations of this energy hypersurface, the relaxation of atomic positions have to be constrained, thus energy barriers and the derivatives of energy vs. displacement, i.e. the stress, are overestimated. In this project we improve the conventional gamma-surface method and extend it to finite temperatures by deriving the free energy surface for shear in aluminium single crystals and at grain boundaries from atomistic simulations employing the metadynamics method.

This project is funded by the DFG, GZ JA 1079/5-1.