Nanocrystalline materials have gained broad attention in microelectronic applications due to their excellent electrical conductivity and mechanical stability. In these materials, grain boundaries are of paramount importance controlling the microstructural stability and degradation. Typically, grain boundaries are seen as weak points in a material, since the degree of atomic order is lower than in the bulk. However, already 50 years ago it has been realized that grain boundaries can exist in more than one distinct structure and can undergo phase transformations, similar to bulk phases. Understanding those interface transitions offers new tools for an interface-controlled materials design. The characterization of interfaces and their transitions at the atomic scale is a crucial step to establish novel interface design strategies.
Here, we present insights into grain boundary phase transitions in copper (Cu) by employing aberration-corrected scanning transmission electron microscopy (STEM) and molecular dynamics simulations (MD).
In a first example, the segregation induced formation of silver (Ag) rich nanofacets at asymmetric  tilt grain boundaries in Cu is presented. The clean asymmetric grain boundary exhibits a complex atomic structure, whereas the overall grain boundary remains flat and no pronounced faceting is observed. With the controlled addition of Ag, the formation of nanometer sized, symmetric (210)-type facets is confirmed by STEM. The segregation induced facet formation mechanism and the stability of the nanofacets will be discussed with the aid of MD simulations.
In a second example, grain boundary phase transitions in  tilt grain boundaries in Cu thin film materials are introduced. Long-period structural units are observed at symmetric  tilt boundaries that are dissociating into structural sub-units. These sub-units are also present at corresponding asymmetric tilt grain boundaries and a further extension of the structural units is observed. The coexistence of two grain boundary phases is established experimentally at an asymmetric  tilt grain boundary, separated by a one-dimensional defect. The atomistic origins of this congruent-type grain boundary transition will be discussed in detail.