Gradient structure by SMAT. (A) Variation of average grain sizes along the depth. The error bars represent the SD of the grain sizes. The GS sample was produced by means of SMAT for 5 min on both sides of a CG IF-steel sheet. (B) Cross-sectional TEM bright-field image of the nanograins with a mean grain size of 96 nm at the depth of ?10 ?m. (C) Electron back-scatter diffraction image of coarse grains with a mean grain size of 35 ?m. Wu et al. (2014b) Click to enlarge.

Drawing inspiration from the structure of bones and bamboo, researchers at the Chinese Academy of Sciences (CAS) and North Carolina State University have found that by gradually changing the internal structure of metals to a gradient structure (GS), they can make stronger, tougher materials that can be customized for a wide variety of applications—from body armor to automobile parts.In a pair of open access papers, one published in the Proceedings of the National Academy of Sciences (PNAS), the other in Materials Research Letters, the researchers, led by Yuntian Zhu at NC State and Xiaolei Wu at CAS, 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; the gradient structure 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 multi-axial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby accumulation and interaction of dislocations are promoted, resulting in an extra 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 novel strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.

This finding represents a new mechanism for strengthening that exploits the principles of both mechanics and materials science, and may provide for a new strategy for designing material structures with superior properties.

The bio-inspired aspect of this is that 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 gradually change from the surface to interior.

The advantage of such gradient structures is their maximization of physical and mechanical performance while minimizing material cost, the researchers noted.

Metal is composed of millions of closely-packed grains, explains Yuntian Zhu, senior author of the two papers; the size and disposition of those grains affect the metal’s physical characteristics.

Having small grains on the surface makes the metal harder, but also makes it less ductile—meaning it can’t be stretched very far without breaking. But if we gradually increase the size of the grains lower down in the material, we can make the metal more ductile. You see similar variation in the size and distribution of structures in a cross-section of bone or a bamboo stalk.

In short, the gradual interface of the large and small grains makes the overall material stronger and more ductile, which is a combination of characteristics that is unattainable in conventional materials. We call this a “gradient structure,” and you can use this technique to customize a metal’s characteristics.

Wu and Zhu collaborated on research that tested the gradient structure concept in a variety of metals, including copper, iron, nickel and stainless steel. The technique improved the metal’s properties in all of them.

The research team also tested the new approach in interstitial free (IF) steel, which is used in some industrial applications. They produced the GS using a surface mechanical attrition treatment (SMAT) technique.

The samples were pre-annealed at 1173 K (900 ?C) for 1 h to obtain a coarse-grained (CG) microstructure witha mean grain size around 35 µm. All samples were processed by SMAT (i.e., rapid impacts with small steel balls) for 5 minutes on both sides.

The resulting GS layers on both sides had a gradual grain size increase along the depth, with the sizes of grains, subgrains, and dislocation cells ranging from sub-micrometers to micrometers, producing two gradient layers of ~ 120 µm thick each.

If conventional IF steel is made strong enough to withstand 450 megapascals (MPa) of stress, it has very low ductility—the steel can only be stretched to less than 5% of its length without breaking. That makes it unsafe. Low ductility means a material is susceptible to catastrophic failure, such as suddenly snapping in half. Highly ductile materials can stretch, meaning they’re more likely to give people time to respond to a problem before total failure.

By comparison, the IF steel with a gradient structure was strong enough to handle 500 MPa and ductile enough to stretch to 20% of its length before failing.

The researchers are also interested in using the gradient structure approach to make materials more resistant to corrosion, wear and fatigue.

The work was supported by the US Army Research Office under grants W911NF-09-1-0427 and W911QX-08-C-0083.