New Age Alloys
By Cindy Moffett
As carbon emissions choke our rapidly warming planet, researchers are scrambling to find cleaner ways to provide energy. Almost 40 percent of US electricity currently comes from coal, which is burned to drive steam turbines. But to reduce coal’s carbon footprint, turbines will need to operate at higher temperatures.
Enter the world of Peter Liaw, UT professor of materials science and engineering and Ivan Racheff Chair of Excellence. Building on decades of research in structural alloys, he is leading a team responsible for creating and testing better materials for gas and steam turbine components, which will allow fuels to burn more cleanly and efficiently.
“Higher efficiency reduces costs and waste per unit of electricity generated,” says UT materials science and engineering department head Kurt Sickafus.
For centuries, alloys such as steel were made of one principal element and tiny amounts of several other elements that added desirable characteristics such as strength, resilience, ductility, or flexibility. But what if high-temperature alloys could comprise a more balanced combination of each principal element?
A variety of multi-element alloys have been around for years, but long-term exposure to high temperatures caused them to form new compounds that were less plastic, structurally unstable, and easily corroded. These were not good candidates for the high-temperature, high-pressure environment of a steam turbine.
Nonetheless, as questioning researchers like Liaw will do, they kept experimenting and found a few simple microstructures that are not only workable, but also remarkable.
Known as high-entropy alloys (HEAs), they combine five or more elements in 5 to 35 percent atomic concentrations to create new materials with the potential to be far more than the sum of its parts: stronger, less brittle, longer lasting, and less expensive.
“This is a new era, different from the iron-based steels,” Liaw says. “Older alloys have been pretty thoroughly researched. These may be different.”
Together with doctoral students Michael Hemphill, Zhi Tang, Lou Santodonato, Zhinan An, and Haoyan Diao, among others, Liaw is creating, testing, and analyzing compositions of aluminum, chromium, copper, iron, manganese, and nickel. Their goal is to find HEAs that outperform conventional alloys.
Investigating materials at the atomic level is a painstaking process. First, a candidate alloy is evaluated based on its elements and composition using a formulation rule devised by Liaw and other scientists. Before this evaluation tool existed, potential alloy combinations were tackled by trial and error–a very expensive and inefficient process with no guarantee of success.
“Now, with the theoretical formula to set us on the right track, we don’t waste time up front,” says Hemphill. The formula identifies workable HEAs; the question is, how well.
To find out how well, the researchers take advantage of the nearby Spallation Neutron Source at ORNL. At this world-class facility, neutrons bombard the new material to measure the atomic lattice strain, making it possible to understand the material’s deformation behavior.
“Already, one HEA containing only a very small (9.1 atomic percentage) amount of aluminum looks promising under high stress and a temperature well above that of conventional turbines,” Liaw says. “In studies of fatigue, the HEA material really stands out.”
“We’re finding information in the infant stages of an era,” Hemphill says. “This new alloy has many potential uses and promising industrial applications. Because these new materials may be less expensive and last longer, they’ll be more cost effective, especially for high-temperature applications.”
As promising as the alloy is, a great deal of meticulous research remains to be done. The team will continue adjusting the amounts of aluminum in their formula until they come up with a winning combination of strength, ductility, and resistance to change in shape.
The HEA investigation is funded by a $300,000 award from the US Department of Energy. UT is one of only nine universities to receive support in this new area.
“Liaw’s research will benefit the world with the promise of increasing the efficiency of coal-fired power plants,” says Wayne Davis, dean of the College of Engineering.
Liaw says that this DOE project, his fourth, is the result of having his former professor Morris E. Fine from Northwestern University come and talk at UT almost a decade ago. “Maybe we should have him come here more often,” he jokes.
The new alloy has already drawn interest from big companies such as airplane manufacturer Boeing because of the material’s light weight and high temperature tolerance. The road to industrial application might take ten to twenty years, but HEAs may one day also have applications in advanced nuclear reactors, an area of particular interest to Zhi Tang. “This is an exciting new area,” Tang says. “Each small discovery makes a big difference in the knowledge of this developing field.”
In a field this new and promising, it seems, no discovery will be truly small.