The conventional view of metals is that increased heat makes them softer and more malleable, like the classic image of a swordsmith shaping a sword by hammering it under heat.

That belief isn’t always true, according to a study co-authored by McCormick Dean Christopher Schuh. The study, titled “At Extreme Strain Rates, Pure Metals Thermally Harden while Alloys Thermally Soften,” was published Feb. 17 in the journal “Physical Review Letters.”

“What we found was that pure metals exhibit a counterintuitive behavior,” said Ian Dowding, a materials scientist and the study’s first author. “It actually gets stronger as you increase the temperature.” 

During the time of the study, which was supported by the Department of Energy, Dowding was a Ph.D. student at the Massachusetts Institute of Technology.

Dowding said pure metals have just one element in them, such as the nickel used in the study, while alloys like steel have two or more elements. He also said alloys are typically used in most engineering applications. 

Pure metals get stronger and alloys get weaker with increased temperature, Dowding said, because the vibrations of the atoms in pure metals all belong to one element and “resist the deformation” when exposed to heat. 

The study used Laser-Induced Particle Impact Tests, which involved the group tracking the flight of small particles toward metals to determine the metals’ properties based on how the particles bounced, Dowding said. 

Mostafa Hassani, an assistant professor at Cornell University who was not connected to the study, uses LIPIT in his work. Hassani said the method uses a laser pulse to accelerate tiny projectiles of roughly 10, 20 or 30 micrometers, which are then launched at the target. 

“The projectile can be accelerated to supersonic speeds, so we’re talking about a kilometer per second,” Hassani said. “That’s basically three times faster than the speed of sound. Then we can use other pulses and high speed cameras to record the interaction between the projectile and a target, which is usually the material we study.”

Suhas Eswarappa Prameela, an assistant professor in the University of Utah’s Department of Materials Science and Engineering who was also not connected to the study, noted the project’s main limitation. He said that the strengthening under heat is only possible in ultrapure metals, which have a “lower absolute strength” than alloys. 

“It’s not that we have discovered a much, much stronger alloy,” Prameela said. “What we have discovered is just a new material design rule, in a way.”

Another limitation, which Prameela said he wants to see addressed in future studies on properties of metals, was the effect of even higher temperatures. He said he wants a future study to observe whether the discovered effect holds true at even higher levels, as the NU study only went up to 155 degrees Celsius.

Dowding said he thinks the study will change the way engineers design materials for extreme applications, such as missiles and rockets. 

“Being able to better understand how these materials will behave in their operation, but being able to do it on a lab scale in a room probably no bigger than the one you’re sitting in, is really important,” Dowding said. “Then, you can go and design the materials for the future and actually understand how they’re going to behave.”

Email: [email protected] 

Related Stories:

— Northwestern advises researchers to delay staffing changes, reduce spending following funding freeze 

— Christopher Schuh named McCormick School of Engineering Dean 

— Experts talk at Northwestern Sustainability Lecture Series