Some metals actually grow more resilient when hot

Heating metals can sometimes make them stronger, despite the common conception that higher temperatures just make them pliable. This surprising phenomenon could lead to a better understanding of important industrial processes and make for tougher aircraft.

“It was just so unexpected or backwards of what you might conventionally see,” says Ian Dowding at the Massachusetts Institute of Technology. Together with Christopher Schuh at Northwestern University in Illinois, he uncovered the odd effect by bombarding metals with tiny projectiles.

The researchers used a laser to launch microscopic aluminium oxide particles towards heated samples of the metals copper, gold and titanium at velocities of thousands of kilometres per hour.

A high-speed camera recorded the impact and rebound of these tiny projectiles as they hit each metal sample, a process illuminated by another laser. Based on the particles’ trajectories and the size of the craters they left on the metals, Dowding and Schuh calculated the strength of each metal and determined how it changed at rising temperatures.

The copper grew roughly 30 per cent stronger after the team increased its temperature by 157°C. Most strikingly, at 177°C (350°F) this typically soft material proved as sturdy as some types of steel.

Usually, heat softens metals because it loosens some of the bonds between metal atoms, Schuh says. So when you put pressure on the metal, some atoms “sloppily” slide around and reconnect elsewhere within it, deforming the material and making it pliable.

After diving into other researchers’ calculations on metals’ properties under extreme conditions, Schuh says he and Dowding learned that the microparticles hit the metals too quickly for this sloppy sliding to occur. And at higher temperatures, more waves of heat or sound passed through the metal and made it harder for bond-breaking to spread across the metal.

Although this outcome had been predicted before, “this research now provides experimental evidence for the concept”, says Mostafa Hassani at Cornell University in New York.

While the “hotter is stronger” phenomenon occurred under carefully controlled laboratory conditions, Schuh says it may happen undetected in a range of real-world industrial processes. For instance, cutting and smoothing processes, which involve blasting materials with fast particles of sand or jets of water, may be inadvertently changing the materials’ strength. The effect could also come into play in some types of 3D printing where “ink” particles move very quickly.

Still, some of the physics behind this finding remain unclear. Researchers know that turning up the heat will eventually warm the metal to its melting point, but future experiments must pinpoint the highest possible temperatures for this strengthening effect to occur, says Schuh.