Gradient structures have evolved over millions of years through natural selection and optimization in many biological systems such as bones and plant stems, where the structures change gradually from the surface to interior. The advantage of gradient structures is their maximization of physical and mechanical performance while minimizing material cost. Here we report that the gradient structure in engineering materials such as metals renders a unique extra strain hardening, which leads to high ductility. The grain-size gradient under uniaxial tension induces a macroscopic strain gradient and converts the applied uniaxial stress to multiaxial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby the accumulation and interaction of dislocations are promoted, resulting in an extra strain hardening and an obvious strain hardening rate up-turn. Such extraordinary strain hardening, which is inherent to gradient structures and does not exist in homogeneous materials, provides a hitherto unknown strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.gradient structured metal | nanocrystalline metal M ankind has much to learn from nature on how to make engineering materials with novel and superior physical and mechanical properties (1, 2). For examples, the clay-polymer multilayers mimicking naturally grown seashells are found to have exceptional mechanical properties (3). Another example is the gradient structure, which exists in many biological systems such as teeth and bamboos. A typical gradient structure exhibits a systematic change in microstructure along the depth on a macroscopic scale. Gradient structures have been evolved and optimized over millions of years to make the biological systems strong and tough to survive nature. They are greatly superior to manmade engineering materials with homogeneous microstructures.Here we report the discovery of a hitherto unknown, to our knowledge, strain hardening mechanism, which is intrinsic to the gradient structure in an engineering material. The gradient structure shows a surprising extra strain hardening along with an up-turn and subsequent good retention of strain hardening rate. Strain hardening is critical for increasing the material ductility (4-6). We also show a superior ductility-strength combination in the gradient structure that is not accessible to conventional homogeneous microstructures. Microstructural Characterization of Gradient StructureWe demonstrate these behaviors in a grain-size gradient-structured (GS) sample, i.e., two GS surface layers sandwiching a coarse-grained (CG) core, produced by the surface mechanical attrition treatment (SMAT) (7) in a 1-mm-thick CG interstitial free (IF)-steel sheet (SI Materials and Methods). The GS layers on both sides have a gradual grain-size increase along the depth (Fig. 1A). In the outermost layer of ∼25-μm thickness are nearly equiaxial nanograins with a mean size of 96 nm (Fig. 1B). The grain size increases gradually to 0.5 and 1 μm at ...

Gradient structures are characterized with a systematic change in microstructures on a macroscopic scale. Here, we report that gradient structures in engineering materials such as metals produce an intrinsic synergetic strengthening, which is much higher than the sum of separate gradient layers. This is caused by macroscopic stress gradient and the bi-axial stress generated by mechanical incompatibility between different layers. This represents a new mechanism for strengthening that exploits the principles of both mechanics and materials science. It may provide for a novel strategy for designing material structures with superior properties.

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