Scientists have developed a new means to strengthen metal in extreme conditions. Whilst it has been long understood that heating a metal softens it, counterintuitively, heat strengthens pure metal in extreme conditions. This could lead to better designed metals for applications in extreme conditions, including outer space.
The findings come from Northwestern University engineers, and this challenges long-held assumptions of how metals behave. The researchers found that if you heat a pure metal and attempt to deform it at extremely high speeds, it flips. The opposite happens and the metal strengthens, resisting the deformation.
Pummelling metals with tiny particles
At everyday speeds, metals deform — meaning they bend, stretch or dent — in ways that scientists understand well. Heat helps atoms move, making metals softer and easier to shape. But when deformation happens extremely fast — in millionths or billionths of a second — those same rules no longer apply.
Since conventional tests cannot reach these extreme conditions the scientists turned to an unconventional approach. They used a specialized technique that blasts hard, microscopic particles at speeds up to hundreds of meters per second. At these speeds, the tiny particles ballistically impact the metal, stretching the metal to 100 million percent of its original length in one second.
The researchers also performed the experiment with metal samples ranging from high purity to slightly alloyed versions of nickel, titanium, gold and copper and from temperatures ranging from room temperature up to 155 degrees Celsius.
Strengthening pure metals with heat
The results revealed a stark divide. As temperatures increased, pure metals became stronger and harder. Alloyed metals, however, behaved typically — becoming softer when heated.
For decades, engineers have added impurities (or alloying elements) to metals to make them stronger. Pure iron, for example, is soft and bends easily. But adding carbon transforms iron into steel — a metal strong enough to support the world’s tallest skyscrapers and bridges that can hold millions of tons of weight across their lifetimes.
It would appear atomic vibrations are responsible for this counterintuitive behaviour. If a particle slams into a pure metal at an extreme speed, it meets resistance from the metal’s vibrating atoms. At any given moment, some atoms are vibrating in a direction that opposes the deformation. As the temperature increases, those vibrations intensify, making it harder for the fast-moving particle to deform the metal’s surface. So, the metal becomes stronger.
In alloys, impurities act as roadblocks that also resist deformations. In that case, heating the metal gives defects the energy to overcome these obstacles, restoring the typical hotter-is-softer behaviour. Adding just 0.3% alloying elements was enough to completely reverse the metal’s counterintuitive response.
Purity as a materials design parameter
These findings have implications for technologies that operate under intense heat and extreme strain rates. By heating a pure metal, it could become more resistant to sandblasting, ballistics and hypersonic speeds. Engineers also could tune a metal’s response to high temperatures by adjusting its purity.
The research appears in the journal Physical Review Letters, titled “At extreme rates, pure metals thermally harden while alloys thermally soften”.
