A new design approach identifies pathways to stronger titanium alloys

Titanium alloys are essential structural materials for a wide range of applications, from aerospace and energy infrastructure to biomedical devices. But like most metals, optimizing their properties tends to involve a trade-off between two key properties: strength and ductility. Stronger materials tend to be less deformable and deformable materials tend to be mechanically weak.

Now, MIT researchers, working with researchers at ATI Specialty Materials, have discovered an approach to creating new titanium alloys that can overcome this historic trade-off, leading to new alloys with exceptional combinations of strength and ductility that may lead to new applications.

The findings are described in the journal Advanced materialsin an article by Shaolou Wei ScD, Professor C. Cem Tasan, Postdoc Kyung-Shik Kim, and John Foltz of ATI Inc. The improvements, the team says, come from tailoring the chemical composition and lattice structure of the alloy, while also adjusting the processing techniques used to produce the material on an industrial scale.

Titanium alloys were important for their exceptional mechanical properties, corrosion resistance and low weight compared to, for example, steel. By careful selection of alloying elements and their relative ratios and the way the material is processed.

“You can create a variety of different structures, and that gives you a big playing field to get good combinations of properties, both for cryogenic and elevated temperatures,” Tasan says.

But this large assortment of options in turn requires a way to guide the selection to produce a material that meets the specific needs of a particular application. The analysis and experimental results described in the new study provide this clue.

The structure of titanium alloys, down to the atomic scale, controls their properties, Tasan explains. And in some titanium alloys, this structure is even more complex, consisting of two different intermixed phases, known as the alpha and beta phases.

“A key strategy in this design approach is to consider different scales,” he says. “One scale is the structure of the single crystal. For example, by carefully choosing the alloying elements, you can have a more ideal crystal structure of the alpha phase that allows for particular deformation mechanisms. The other scale is the polycrystalline scale, which includes interactions of the alpha and beta phases. So the approach followed here involves design considerations for both.”

In addition to choosing the right alloy materials and proportions, processing steps have been shown to play an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team found.

Working with ATI researchers, the team tested various alloys under a scanning electron microscope as they were deformed, revealing details of how their microstructures respond to external mechanical loads. They found that there was a certain set of parameters—composition, proportions, and processing method—that provided a structure in which the alpha and beta phases shared deformation equally, mitigating the tendency for cracking likely to occur between the phases as they reacted. otherwise.

“The phases distort harmonically,” says Tasan. This cooperative response to deformation may yield a better material, they found.

“We looked at the structure of the material to understand the two phases and their morphology, and we looked at their chemistry by performing local chemical analysis at the atomic scale. We adopted a wide range of techniques to quantify different properties of the material across multiple length scales,” says Tasan , who is the POSCO Professor of Materials Science and Engineering and Associate Professor of Metallurgy.

“When we look at the overall properties” of the titanium alloys produced by their system, “the properties are really much better than comparable alloys.”

According to Tasan, this was industry-supported academic research aimed at demonstrating the design principles of alloys that could be produced commercially on a large scale.

“What we’re doing in this collaboration is really going towards a fundamental understanding of crystal plasticity. We’re showing that this design strategy is validated and we’re showing scientifically how it works,” he adds, adding that there is still considerable room for further improvement.

As for the potential applications of these findings, he says, “for any aerospace application where an improved combination of strength and ductility is useful, this kind of invention provides new opportunities.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.