Gradient structures exist ubiquitously in natural materials such as shells, bones and trees, and exhibit high strength and high toughness to help creatures survive from natural damages. Strength and ductility are mutual repulsive in traditional strengthening strategies. Gradient structures have been therefore increasingly applied to engineering materials for better mechanical performances. It has been a big challenge to understand structural gradient–related mechanical behaviors in gradient structures.

A recent study published in Science shows how simultaneous enhancement in strength and work hardening can be achieved by solely increasing the structural gradient in pure Cu.

Prof. LU Lei’s group at Institute of Metal Research, Chinese Academy of Sciences (IMR, CAS), collaborated with Prof. Huajian Gao’s group at Brown University, USA, designed and synthesized gradient nanotwinned structures with large structural gradients in pure Cu and revealed the unique strengthening behaviors and mechanisms.

It is found that as the structural gradient increases, the strength and work hardening of gradient nanotwinned Cu are simultaneously enhanced with almost constant ductility and can even exceed those of the strongest component of the gradient microstructure when the structural gradient is sufficiently large. Such a strengthening behavior is never found in metals with homogenous or other inhomogeneous microstructures.

Microstructural characterization and massively parallel atomistic simulations were combined to find that the extra strengthening is attributed to the unique patterning of geometrically necessary dislocations in the form of bundles of concentrated dislocations uniformly distributed in grain interiors. The bundles of concentrated dislocation appear at small strains and lie along the gradient direction, and are fundamentally different from randomly distributed, statistically stored dislocations in homogeneous structures. The bundles of concentrated dislocations with ultrahigh density of dislocations act as strong obstacles to slip, help to delocalize plastic deformation inside the grains, and accelerate the work- hardening process.

The extra strengthening of gradient nanotwinned structures provides a guide for optimizing properties of metal through gradient structures.

This work is supported by the National Natural Science Foundation, the Ministry of Science and Technology of China and the Key Research Program of Frontier Science, Chinese Academy of Sciences.