Researchers at North Carolina State University and the Chinese Academy of Sciences have developed a technique to make titanium stronger without sacrificing the metal’s ductility. The approach involves manipulating the grain size to give the metal the strength of ultra-fine-grained titanium, but the ductility of coarse-grained titanium.

Researchers Yuntian Zhu, a professor of materials science and engineering at North Carolina State University, and X.L. Wu, who is based at the Chinese Academy of Sciences’ Institute of Mechanics, use asymmetric rolling to process a two-millimeter-thick sheet of titanium. In asymmetric rolling, the sheet passes between two rollers that apply pressure to each side of the sheet. One of the rollers rotates more quickly than the other. This not only presses the sheet thinner but, because of the different roller speeds, also creates a sheer strain in the metal.

That means the crystal structure within the titanium moves forward faster on the side of the fast roller than on the side of the slow roller. This effectively distorts and breaks down the crystalline structure, creating small grains in the material.

The researchers repeat the asymmetric rolling process until the metal is 0.3 millimeters thick, then expose the sheet to 475 degrees Celsius for five minutes. This allows some, but not all, of the small grains to consume each other and form large grains.

This second process creates a patchwork quilt of small and large grains. The large grains are laid out in long, narrow columns, with each column surrounded by a layer of small grains. The resulting material is as strong as the small-grained titanium because the surrounding layer of small grains makes it difficult for the large grains to deform.

Embedding ductile, large-grained columns in a harder, ultrafine-grained matrix, researchers improved titanium's strength without impairing its ductility. Image credit: Yuntian Zhu.The material also retains the ductility of the large grains because once enough strain is applied, the small and large grains want to deform at different rates. But the different grain sizes have to coordinate with each other, much like traffic has to adjust to account for slower cars on a road. The differential in grain sizes creates a phenomenon called “strain hardening” in which the more the material is stretched, the harder it becomes.

The key is grain size, or the size of the crystals in the metal. Metals with a small grain size are stronger, meaning they can withstand more force before they start to deform. However, metals with a small grain size are also less ductile, which means they can withstand less strain before breaking.

Materials that are not ductile would not bend or stretch much, they just snap. Conversely, metals with a large grain size are more ductile but have lower strength.

Wu and Zhu are working on projects to confirm that this technique can work for other metals and alloys. If so, the advance has potential uses across a range of applications.