Stability of nanocrystalline metals: The role of grain-boundary chemistry and structure

Nanocrystalline metals are transitioning from laboratory curiosities to engineering materials, in large part due to advances in improving their stability, making their exceptional properties more predictable and accessible. Nanoscale grains typically have a very strong innate tendency to coarsen, but the grain-boundary structure can be designed and tuned to lower its excess energy, reducing both the driving force for coarsening and the grain-boundary mobility. This article reviews two major strategies for achieving low-energy grain boundaries in nanocrystalline structures. First, grain-boundary alloying is discussed, including grain-boundary segregation and its energetic competition with the formation of second phases; with sufficient grain-boundary segregation tendency it is possible to stabilize nanostructures to high temperatures. Second, methods of achieving low-energy crystallographic grain-boundary structures are discussed, including the formation of nanotwinned structures and relaxing grain boundaries into low-energy structures through their interactions with partial dislocations. Both of these strategies have led to effective and implementable stable nanocrystalline materials, and point to many directions for future advancements.