New view at

Metals deform under applied stresses because of the movement of dislocations — line defects in the crystal lattice. Many dislocations are produced during plastic fl ow. Th e accumulation and interaction of these crowded dislocations creates obstacles that make the propagation of dislocations diffi cult. Further deformation therefore requires higher stresses, leading to work hardening. Conversely, annealing, a heat treatment oft en given to deformed metals to rearrange dislocations and relieve stresses, reduces strength and improves ductility as the dislocation roadblocks annihilate. Th is well-established picture for conventional metals apparently no longer holds for nanostructured aluminium, as reported recently by Huang et al.1. In fact, what they found was just the opposite, “hardening by annealing and soft ening by deformation”