Stable nanofacets in [111] tilt grain boundaries of face-centered cubic metals

TL;DR

This study demonstrates the existence of stable nanometer-sized facets in [111] tilt grain boundaries of Cu, supported by atomistic simulations and microscopy, revealing a new stable facet configuration in face-centered cubic metals.

Contribution

It provides the first evidence of energetically stable, finite-sized nanofacets in grain boundaries, explaining their stability through unique junction properties.

Findings

Nanometer-sized facets are stable in Cu grain boundaries.

Facet junctions lack Burgers vector components, reducing energy costs.

Similar stability is predicted for Al and Ag.

Abstract

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.