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Mechanical properties-physical properties that a material exhibits upon the application of forces. Indentation-a common method of testing the mechanical properties of materials by pressing the materials using an indentor tip. Forcedisplacement curve-a curve specifying the displacement of an object that results from applied forces. Biodistribution-a method of tracking where nanoparticles of interest

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.