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Groundbreaking Study Finds Metals Actually Get Stronger When Hot

A blacksmith hammers a strip of metal against an iron anvil
Credit: Jonny Gios / Unsplash.
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In blacksmithing, metals are heated up to be pounded and stretched into other, more complex shapes. This works because metals get softer and more malleable when they are hot.

But this supposedly basic physics might not be the full story.

In a new study, researchers at the Massachusetts Institute of Technology (MIT) discovered that the opposite is true when metal is struck by a fast-moving projectile – the hotter the metal, the stronger it was at impact. If heated to high enough temperatures, this effect is so strong that copper, ordinarily a very soft metal, can become as strong as steel.

The study findings, published in Nature, could lead to new approaches in designing materials for extreme environments, the researchers say. This could include new shields for protecting spacecraft or hypersonic aircraft as well as advanced equipment for high-speed manufacturing operations.

Hot metals hold up under impact

In their new experiments, the MIT researchers used laser power to shoot tiny fragments of sapphire – measuring just 10 to 20 microns across – at flat metal sheets. Using extremely high-speed cameras, the team was able to study how the particles behaved as they collided with and ricocheted off the metal surfaces.

By measuring the change in velocities between the particles’ approach and subsequent bounce back from the metal surfaces, the researchers could calculate how much energy was transferred into the metal targets – a good indicator of surface strength. In choosing to fire very tiny particles at the metal, the researchers were also able to negate any complications arising from large pressure shockwaves.

Three different metal sheets were used in the study – copper, titanium and gold – which were impacted at room temperature, 100 °C and 177 °C. The researchers found that as the test temperatures increased, the sapphire particles began to rebound more aggressively, suggesting that the surface strength of the metals increased with the rising temperatures.

According to the researchers, this is the first direct experimental evidence for this counterintuitive thermal effect.

Why do extreme conditions increase metal strength?

This unusual thermal effect is likely due to the way that the orderly rows of atoms behave within a metal’s crystalline structure under pressure.

Three main effects govern how metals deform under stress. The first two – thermal and athermal – are of less interest here, as they follow the traditional relationship where deformation is increased at higher temperatures. But the third factor, known as drag strengthening, appears to deviate from this when the deformation rate crosses a certain threshold – in this case, brought about by the ultra-fast impact with a very small particle.

Beyond this crossover point, the higher temperature increases the activity of phonons – a quasiparticle associated with vibrations in a crystal lattice – within the metal. These phonons can interact with dislocations in the metals’ crystalline lattice, limiting their ability to move and cause deformation. “The faster you go, the less the dislocations are able to respond,” said Ian Dowding, an MIT graduate student and first author of the study.

The researchers know there must be an upper temperature limit to this effect, but where exactly this occurs before the metal’s melting point is still unknown.

Improving our knowledge of how this effect works could be very important when designing new parts and devices that are likely to be exposed to extreme conditions, they say.

“If you are flying a helicopter in a sandstorm, a lot of these sand particles will reach high velocities as they hit the blades,” Dowding remarked, pointing out that deep desert temperatures could be high enough to trigger this strange drag-strengthening behavior.

While this phenomenon could be of interest in developing new technical innovations to do with metal parts, the researchers also see it as a sign that other seemingly “obvious” material behaviors need to be studied more closely in extreme conditions. If these properties are simply extrapolated from what we currently know, they warn that this could lead to seriously flawed expectations about how parts and materials might behave under extreme stresses.

Reference: Dowding I, Schuh CA. Metals strengthen with increasing temperature at extreme strain rates. Nature. 2024:1-5. doi: 10.1038/s41586-024-07420-1

This article is a rework of a press release issued by the Massachusetts Institute of Technology. Material has been edited for length and content.