Dislocation nucleation governed softening and maximum strength in nano-twinned metals

Conventional metals gain much of their strength through the interaction of dislocations with obstacles such as grain boundaries, whereas the geometrical constraints prevailing in nanostructured materials limit such effects. Huajian Gao and colleagues now report molecular dynamics simulations which reveal that the strength of ultrafine grained copper containing twin boundaries can be controlled by a dislocation nucleation mechanism activated below a critical twin thickness. The motion of the new dislocations leads to the migration of twin planes, and as a result the material becomes softer. The smaller the grains, the smaller the twin-boundary spacing and the higher the maximum strength of the material.