New design approach identifies routes 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 is often a trade-off between two key attributes: strength and ductility. Stronger materials are often less ductile, and ductile materials are often mechanically weak.

Now, researchers at MIT, in collaboration with researchers at ATI Specialty Materials, have discovered an approach to creating new titanium alloys that can overcome this historical tradeoff. The result is new alloys with exceptional combinations of strength and ductility, which could lead to new applications.

The findings are described in the journal Advanced materialsin a paper 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 tweaking the alloy’s chemical composition and lattice structure, while also adjusting the processing techniques used to produce the material on an industrial scale.

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

“You can create all kinds of different structures, which creates a large playing field where you can make good combinations of properties, both for cryogenic and elevated temperatures,” Tasan said.

But that wide range of possibilities, in turn, requires a way to guide the selections to produce a material that meets the specific needs of a given application. The analysis and experimental results described in the new study provide that guidance.

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

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

In addition to choosing the right alloy materials and ratios, steps in the processing played an important role. A technique called cross-rolling is another key to achieving the exceptional combination of strength and ductility, the team discovered.

Working with ATI researchers, the team tested several alloys under a scanning electron microscope while they were being deformed, revealing details about how their microstructures respond to external mechanical loading. They found that there was a particular set of parameters—of composition, proportions, and processing method—that produced a structure where the alpha and beta phases shared the deformation evenly, reducing the tendency for cracking that is likely to occur between the phases when they respond differently.

“The phases deform in harmony,” Tasan says. This cooperative response to deformation can produce a superior material, they discovered.

“We looked at the structure of the material to understand these two phases and their morphologies, and we looked at their chemistry by doing local chemical analyses at the atomic scale. We used a wide range of techniques to quantify different properties of the material across multiple length scales,” said Tasan, who is the POSCO Professor of Materials Science and Engineering and an associate professor of metallurgy.

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

According to Tasan, the research was industry-supported academic research aimed at proving design principles for alloys that can be commercially produced on a large scale.

“What we are doing in this collaboration is really focused on a fundamental understanding of crystal plasticity. We are showing that this design strategy has been validated and we are showing scientifically how it works,” he adds, noting that there is still much 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, these types of inventions open up new possibilities.”

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that reports on research, innovation, and education at MIT.