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Even minute amounts of one solute atom per one million bulk atoms may give rise to qualitative changes in the mechanical response and fracture resistance of modern structural materials. These changes are commonly related to enrichment by several orders of magnitude of the solutes at structural defects in the host lattice. The underlying concept—segregation—is thus fundamental in materials science. To include it in modern strategies of materials design, accurate and realistic computational modelling tools are necessary. However, the enormous number of defect configurations as well as sites solutes can occupy requires models which rely on severe approximations. In the present study we combine a high-throughput study containing more than 1 million data points with machine learning to derive a computationally highly efficient framework which opens the opportunity to model this important mechanism on a routine basis.

We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5f210g segments and Ag-depleted f230g=f100g asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag.

We studied the migration behavior of mixed tilt and twist grain boundaries in the vicinity of a symmetric tilt 111 7 grain boundary in aluminum. We show that these grain boundaries fall into two main categories of stepped and kinked grain boundaries around the atomically flat symmetric tilt boundary. Using these structures together with size converged molecular dynamics simulations and investigating snapshots of the boundaries during migration, we obtain an intuitive and quantitative description of the kinetic and atomistic mechanisms of the migration of general mixed grain boundaries. This description is closely related to well-known concepts in surface growth such as step and kink-flow mechanisms and allows us to derive analytical kinetic models that explain the dependence of the migration barrier on the driving force. Using this insight we are able to extract energy barrier data for the experimentally relevant case of vanishing driving forces that are not accessible from direct molecular dynamics simulations and to classify arbitrary boundaries based on their mesoscopic structures. DOI: 10.1103/PhysRevB.94.165413 I. MOTIVATIONThe migration of grain boundaries plays a pivotal role in the evolution of materials microstructures, which strongly impacts the mechanical, chemical, or electronic response of a material. Thus, for designing optimized materials with tailored microstructures, a detailed understanding of grain boundary kinetics and of the underlying atomistic mechanisms is crucial. Identifying these fundamental mechanisms and understanding their impact on grain boundary migration has been a key topic both from an experimental as well as a theoretical perspective for several decades.In 1948 Mott [1] proposed that the migration is a thermally activated process. In his model for migration, the energy barrier that the boundary must overcome is related to an island nucleation mechanism where atoms melt on one side of the boundary and solidify on the other crystalline side. In 1969 Gleiter portrayed a general grain boundary as a 3D stepped structure by interpreting diffraction contrast microscopy images [2,3]. He proposed that general nonsymmetric grain boundaries are not atomically flat but are rather comprised of atomic steps and kinks, similar to the well-known structural elements on surfaces. He also pointed out that the boundary moves via the emission and absorption of atoms at step kinks.The picture of both Mott [1] and Gleiter [2,3] that grain boundaries consist of an intermediate layer changed with the advent of high-resolution electron microscopy which showed that the interface region is rather sharp [4]. In the late 1970s several electron microscopic studies analyzed grain boundary migration in terms of the motion of secondary grain boundary dislocations [5][6][7]. However a more recent in situ electron microscopy work by Babcock and Balluffi [8] in 1989 showed the contribution from the dislocation motion to be negligible. In their study [8] of curvature-driven grain boundary migration in near 5 bound...