Extra strengthening and work hardening in gradient nanotwinned metals

The gradient structures exist ubiquitously in the natural materials such as shells, bones and trees, and exhibit a high strength and a good toughness to help creatures survive from the natural damages. These ingenious gradient structures shed a light on developing the high performance metals through tailoring their microstructures. Such as, gradient nano-grained metals prepared by surface mechanical treatment techniques exhibit a high strength and a good ductility which are usually mutually repulsive through the conventional strengthening strategies, such as cold deformation, grain refinement and solid solution. However, due to the limitation of producing high structural gradient, understanding the structural gradient–related mechanical behaviors and deformation mechanism has been a big challenging.

In our study, the gradient nanotwinned (GNT) Cu samples with a wide range of structural gradients (in both the twin thickness and grain size that span across the entire thickness of the sample) were synthesized by means of direct-current electrodeposition. It is found that as the structural gradient increases, both the strength and work hardening are simultaneously enhanced, while keeping the ductility almost constant. The maximum structural gradient leads to an improved strength that can even exceed the strongest component of the gradient microstructure, which has never been reported in the literatures on any other gradient metals and alloys.

Microstructural characterization and massively parallel atomistic simulations were combined to undercover that the extra strengthening of GNT Cu is attributed to the unique patterning of high density of geometrically necessary dislocations (GND) in the form of bundles of concentrated dislocations uniformly distributed in grain interiors. The bundles of concentrated dislocation is spontaneously formed during gradient plastic deformation 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 GNDs act as strong obstacles to dislocation slip, helping to delocalize plastic deformation inside the grains, and accelerate the work- hardening process as well.

The GNT strengthening concept proposed in this work provides insights into combining structural gradients at different length scales in order to push forward the strength limit of materials and may be essential to create the next generation of high strength-ductility metals.

Figure 1 Highly tunable structural gradient for extra strengthening and ductility in metals. A gradient nanotwinned microstructure with a spatial gradient in both twin boundary (TB) spacing and grain size offers an unusual combination of strength, uniform elongation, and work hardening, which is superior to its strongest component and to existing heterogeneous strengthening approaches through gradient nanograined (GNG), homogeneous nanotwinned (NT), and multilayered microstructures.