In an effort to make alloys stronger, researchers at North Carolina State University took a cue from bamboo: They increased the grain size gradually, going from the skin toward the center of the metal part. This so-called gradient structure mimics the cell structure of both bamboo and bone, two resilient biological materials with high strength-to-weight ratios.

Parts composed of small-grained metals are generally hard but not ductile, meaning they cannot stretch much without breaking. But if the grain size gradually gets larger with depth from the surface, the material becomes stronger and more ductile. Testing their theory of gradient structure on copper, iron, nickel, and stainless-steel alloys showed that it does increase strength and ductility.

Alloy grains

Interstitial-free steel, for example, can ordinarily be made strong enough to withstand 450 Mpa of stress. But it ends up having low ductility, breaking before it can be stretched to 105% of its original length. The team gave the alloy a gradient structure, which then became strong enough to withstand 500 Mpa and ductile enough to stretch to 120% of its length before breaking.

Researcher says the gradient approach to alloys could result in materials more resistant to corrosion, wear, and fatigue.