The new design approach identifies routes to stronger titanium alloys

Titanium alloys are essential structural materials for a wide variety 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 main characteristics: strength and ductility. Stronger materials tend to be less deformable, and deformable materials tend to be mechanically weak.

Now, researchers at MIT, collaborating 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 extraordinary combinations of strength and ductility. which may 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, arise 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 have been important due to their exceptional mechanical properties, corrosion resistance and light weight when compared to steels for example. Through the careful selection of alloy elements and their relative proportions, as well as the way the material is processed.

“You can create many different structures, and it creates a playground for you to get good combinations of properties, both for cryogenic temperatures and elevated temperatures,” Tasan says.

But this great variety of possibilities in turn requires a way to guide selections 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 guidance.

The structure of titanium alloys, down to the atomic scale, governs 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 the individual crystal. For example, by carefully choosing the alloying elements, you can have a more ideal crystal structure of the alpha phase that enables special deformation mechanisms. The other scale is the polycrystalline scale, which involves the interactions of the alpha phases and beta So the approach taken here includes design considerations for both.”

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

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

“Phases warp in harmony,” says Tasan. This cooperative response to deformation can yield a superior material, they found.

“We looked at the structure of the material to understand these two phases and their morphologies, 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. with multiple lengths,” says Tasan, who is a professor of Materials Science and Engineering at POSCO and an associate professor of metallurgy.

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

This was industry-supported academic research aimed at proving design principles for connections that could be produced commercially at scale, according to Tasan.

“What we do in this collaboration is really toward a fundamental understanding of crystal plasticity. We show that this design strategy is validated and we show scientifically how it works,” he adds, noting that there is important for further improvements.

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

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