By using a combination of long columns of coarse grains in a patchwork of small grains, researchers have designed a metal that is both strong and ductile.

AsianScientist (Nov. 23, 2015) – Researchers have made titanium stronger without sacrificing its ductility—a combination that no one has achieved before. Their results are reported in the Proceedings of the National Academy of Sciences.

The researchers, from the Institute of Mechanics of the Chinese Academy of Sciences and North Carolina State University, believe this approach 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. However, the researchers managed to achieve the best of both worlds. The key 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 mm-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.

The first step allowed some—but not all—of the small grains to consume each other and form large grains, and this second process creates a patchwork 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 tend to deform at different rates. But the different grain sizes have to coordinate with each other, much like traffic has to adjust to 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 have not been used in this way. For this reason, the researchers suggest that this technique should be fairly easy to scale up.

The article can be found at: Wu et al. (2015) Heterogeneous Lamella Structure Unites Ultrafine-Grain Strength with Coarse-Grain Ductility.

Source: Chinese Academy of Sciences; Photo: David DeHetre/Flickr/CC. Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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