  Grain boundaries can dissociate into facets if that reduces their excess
energy. This, however, introduces line defects at the facet junctions, which
present a driving force to grow the facets in order to reduce the total number
of junctions and thus the system's energy. Often, micrometer-sized facet
lengths are observed and facet growth only arrests for kinetic reasons. So far,
energetically stable, finite-sized facets have not been observed, even though
theoretical stability conditions have already been proposed. Here, we show a
case where nanometer-sized facets are indeed stable compared to longer facets
in [111] tilt grain boundaries in Cu by atomistic simulation and transmission
electron microscopy. The facet junctions lack a Burgers vector component, which
is unusual, but which removes the main energy cost of facet junctions. Only
attractive interactions via line forces remain, which result from a
discontinuity of grain boundary excess stress at the junction. Atomistic
simulations predict that the same phenomenon also occurs in at least Al and Ag.
