High-strength metals are important for many mechanical applications such as wind energy turbines. Niels Hansen from the Department of Wind Energy at the Technical University of Denmark in Roskilde studies the microscopic phenomena and the atomic mechanical processes that determine or limit possible further strengthening of metals.

Refinement of the material microstructure is crucial for obtaining high-strength metals and is typically achieved by plastic deformation (e.g., cold-rolling). However, this is counteracted by a process of dynamic recovery. New high strain deformation processes have enabled further structural refinement, resulting in nanoscaled Cu with boundary spacing down to an experimental limit of 10–20 nm. This was thought to be a theoretical barrier as well, as molecular dynamic simulations predicted a change from dislocation to grain-boundary sliding on that scale.

Now, as reported in the April 4 issue of Physical Review Letters (DOI: 10.1103/PhysRevLett.112.135504), Hansen and his collaborator, D.A. Hughes from Sandia National Laboratories in Livermore, Calif., describe an experimental method to surpass this limit. By alloying Cu with Fe, they show how the microstructure of Cu can be refined to a record low of 5 nm between boundaries. To achieve this refinement, an extremely high strain (>150) was applied at a low sliding rate in liquid nitrogen.

The researchers also studied the evolution of the microstructure, and demonstrated that dislocation glide is still the limiting factor at this scale. With this observation, they then removed the expected theoretical limit. The researchers suggest that the presence of Fe, as well as a large density of dislocations, reduced the mobility of the boundaries and any possible dynamic recovery. The expected high strength suggests new potential applications in particular for hard wear-resistance materials—components that could make up the windmills of the future.