Grain boundary evolution in nanograined metals: from relaxation to Schwarzation

High density of grain boundaries (GBs) in nanograined metals may become unstable and evolve in various ways such as migration, sliding, or relaxation under thermal or mechanical stimuli. In this report, we will present our recent studies on structural evolution of GBs in nanograined Cu during intensive plastic deformation. As the grain size decreased below 200 nm, GB migration become obvious assisted by dislocation activities. Below 70 nm, accompanied by a transition in dominant deformation mechanism from full dislocations to partials, GB migration decays gradually, while the GB structures relax into lower energy states through their interactions with partials. The population of low-energy GBs increases with a decreasing grain size. Below 10 nm, further refinement of grains results in a violent structural evolution of the GB network into a 3-dimensional periodic minimal surface structure, Schwarz-D, through a phase-transformation-like process, which is termed as Schwarzation.