Researchers at the Chinese Academy of Sciences, Institute of Mechanics, and North Carolina State University have proposed a novel strategy to make titanium much stronger without sacrificing any of the metal’s ductility – a combination that no one has achieved before. The researchers believe this strategy could also be used for other metals, and the advance has potential applications for creating more energy-efficient vehicles.
A metal is either strong or ductile, but almost never both simultaneously. They have managed to get the best of both worlds. The key idea here is 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. But metals with a small grain size are also less ductile, which means they can withstand less strain before breaking. Metals that aren’t ductile won’t bend or stretch much – they just snap. Conversely, metals with a large grain size are more ductile, but have lower strength. The new technique manipulates the grain size to give the metal the strength of ultrafine-grained titanium but the ductility of coarse-grained titanium.
The researchers began with a two-millimeter thick sheet of titanium, and processed it using asymmetric rolling. This effectively creates small grains in the material. The researchers repeated the rolling process until the metal was 0.3 mm thick, then exposed the sheet to 475 degrees Celsius 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 strains are 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.
The processes used in the new technique are already in widespread industrial use. They just haven’t been used in this way, so this should be fairly easy to scale up.
The paper, “Heterogeneous Lamella Structure Unites Ultrafine-Grain Strength with Coarse-Grain Ductility,” has been published on November 9, 2015 in the Proceedings of the National Academy of Sciences of the United States of America.
Wu’s work was supported by the National Natural Science Foundation of China and China’s Ministry of Science and Technology.