A team from North Carolina State University, Raleigh, and the Chinese Academy of Sciences announces that titanium strength and ductility are both increased when large grains are added to an ultrafine-grain matrix. The matrix is shown in the image as black, and the large grains as various colors.
Metals with a small grain size are stronger but less ductile, while metals with a large grain size are more ductile but less strong. The new technique manipulates the grain size to give the metal the strength of ultrafine-grained titanium and the ductility of large-grained titanium.
The researchers began by using asymmetric rolling to process a 2-mm thick sheet of titanium. In asymmetric rolling, the sheet passes between two rollers that apply pressure to each side of the sheet, but 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.
In other words, 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 repeated the asymmetric rolling process until the metal was 0.3 mm thick, then exposed the sheet to 475°C for five minutes. This allowed 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 completely 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.
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 the slower cars on the road. The differential in grain sizes creates a phenomenon called strain hardening, in which the more the material is stretched, the harder it becomes.
“In addition to creating a metal with an unprecedented combination of strength and ductility, this material has higher strain hardening than coarse-grained titanium – which was thought impossible,” says X.L. Wu, co-corresponding and first author of the paper, who is based at the Chinese Academy of Sciences Institute of Mechanics.
