Research Article

Extra strengthening and work hardening in gradient nanotwinned metals

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Science  02 Nov 2018:
Vol. 362, Issue 6414, eaau1925
DOI: 10.1126/science.aau1925

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Stronger copper through twin power

Materials with structural gradients often have unique combinations of properties. Gradient-structured materials are found in nature and can be engineered. Cheng et al. made a structural gradient by introducing gradients of crystallographic twins into copper. This strategy creates bundles of dislocations in the crystal interiors, which makes the metal stronger than any of the individual components. This method offers promise for developing high-performance metals.

Science, this issue p. eaau1925

Structured Abstract

INTRODUCTION

Gradient structures ubiquitously exist in natural materials such as bone, shells, and trees. Microstructural gradients are increasingly being introduced into a wide range of engineering materials, providing them with enhanced strength, hardness, work hardening, ductility, and fatigue resistance through deformation mechanisms that are distinct from those operating in gradient-free counterparts. However, understanding structural gradient–related mechanical behaviors in all gradient structures, including those in engineering materials, has been challenging.

Although control of the structural gradient is essential to achieving optimized mechanical behaviors, existing manufacturing approaches are limiting for bulk gradient materials. For instance, surface tooling and mechanical treatments generate either limited volume fractions of gradient layers localized only near the surface or a negligible degree of structural gradient along the gradient direction. All of these limit our ability to tailor the mechanical properties and understand the deformation mechanisms of gradient-structured metals.

RATIONALE

We used a direct-current electrodeposition method to synthesize bulk gradient nanotwinned pure Cu samples with controllable patterning of homogeneous nanotwinned components. The individual components are composed of high-density, preferentially oriented nanometer-scale twin boundaries embedded within micrometer-scale columnar-shaped grains. We observed gradient-induced enhancements in both tensile strength and work-hardening rate through a wide range of structural gradients (in both twin boundary spacing and grain size) that span across the entire thickness of the sample. We combined scanning electron microscopy, a two-beam diffraction technique in transmission electron microscopy, and massively parallel atomistic simulations to identify the underlying strengthening mechanism in gradient nanotwinned Cu. We also prepared homogeneous nanotwinned samples as controls to demonstrate the importance of the structural gradient.

RESULTS

Our findings indicate that simultaneous enhancement in strength and work hardening can be achieved by solely increasing the structural gradient in pure Cu. The maximum structural gradient led to an improved work-hardening rate and tensile strength that can exceed even the strongest component of the gradient microstructure—an unusual phenomenon that appears to be absent from the existing literature on metals and alloys with gradient nanograined or any other heterogeneous microstructures. We attribute the extra strengthening and work hardening of gradient nanotwinned structures to the unique patterning of geometrically necessary dislocations in the form of bundles of concentrated dislocations uniformly distributed in grain interiors. Such dislocation patterns in grain interiors 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.

CONCLUSION

In our simple bottom-up approach to create gradient nanotwinned structure in pure Cu, a large structural gradient allows for an exceptional combination of high strength and work hardening that can exceed even the strongest component of the gradient structure. Both experimental and computational evidence suggest the importance of covering the whole structure with tunable structural gradients for the development of high densities of dislocations in grain interiors. The gradient nanotwinned 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 creating the next generation of metals with both high strength and high ductility.

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.

Abstract

Gradient structures exist ubiquitously in nature and are increasingly being introduced in engineering. However, understanding structural gradient–related mechanical behaviors in all gradient structures, including those in engineering materials, has been challenging. We explored the mechanical performance of a gradient nanotwinned structure with highly tunable structural gradients in pure copper. A large structural gradient allows for superior work hardening and strength that can exceed those of the strongest component of the gradient structure. We found through systematic experiments and atomistic simulations that this unusual behavior is afforded by a unique patterning of ultrahigh densities of dislocations in the grain interiors. These observations not only shed light on gradient structures, but may also indicate a promising route for improving the mechanical properties of materials through gradient design.

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