Certain metals exhibit increased elasticity at elevated temperatures

Heating a metal can make it stronger, despite the common belief that higher temperatures only make the metal softer. This surprising phenomenon could lead to a deeper understanding of critical industrial processes and create more robust aircraft.

“It was completely unexpected or the opposite of what we traditionally see,” he says. Ian Dowding At Massachusetts Institute of Technology. Christopher Shue At Northwestern University in Illinois, he discovered the strange effect by bombarding metal with small projectiles.

The researchers used a laser to fire microscopic aluminum oxide particles at heated copper, gold, and titanium samples at speeds of thousands of kilometers per hour.

High-speed cameras recorded the impact and rebound as these tiny projectiles hit each metal sample, and another laser illuminated the process. Dowding and Xu calculated the strength of each metal based on the particle trajectories and the size of the craters left on the metal, determining how the metal changes as temperature increases. did.

After the team increased the temperature to 157°C, the copper became about 30% stronger. What's most surprising is that this normally soft material proved to be as tough as some types of steel at 177°C (350°F).

Heat typically softens metals by loosening some of the bonds between metal atoms, Xu says. So when you apply pressure to a metal, some atoms slide “sloppily” and recombine elsewhere within the metal, causing the material to deform and become flexible.

Xu said that after poring over calculations made by other researchers about the properties of metals in extreme conditions, he and Dowding found that particles are hitting the metals too fast, causing this unnatural slippage. He said he learned that it would not happen. Additionally, the higher the temperature, the more heat waves and sound waves will pass through the metal, making it harder for bond failure to spread throughout the metal.

Although this result had been predicted for some time, “this study provides experimental proof of concept.” Mostafa Hasani At Cornell University in New York.

While the “hotter is stronger” phenomenon occurred in a carefully controlled laboratory environment, Xu says it could be happening unnoticed in a variety of real-world industrial processes: For example, cutting or smoothing processes that involve blasting material with high-velocity sand or water streams can unintentionally change a material's strength.

This effect could also improve the durability of helicopters and airplanes. Building materials previously thought to be too soft for these applications may actually become stronger when exposed to fine particles from hot climates, such as desert sand.

Still, some of the physics behind this discovery remains unclear. Researchers know that increasing the heat will eventually warm the metal to its melting point, but future experiments will need to pinpoint the highest temperature at which this strengthening effect occurs, Xu said. says.