Metal becomes soft when heated, which is why blacksmiths heat iron to red-hot iron and shape it into complex shapes. And anyone who compares copper wire to a steel coat hanger will quickly see that copper is much more flexible than steel.
But scientists at MIT have discovered that the opposite happens when metal is struck by an object moving at very high speeds. In other words, the hotter the metal, the stronger it becomes. Under these conditions, which place extreme stress on the metal, copper can actually be as strong as steel. The new discovery could lead to new approaches to designing materials for extreme environments, such as shields protecting spacecraft or hypersonic aircraft, or equipment for high-speed manufacturing processes.
The findings are described in a paper published today in the journal natureIt was written by MIT graduate student Ian Dowding and Christopher Schuh, former chair of MIT's Department of Materials Science and Engineering and currently dean of engineering at Northwestern University and visiting professor at MIT.
The new findings are “counterintuitive and conflict with decades of research under less extreme conditions,” the authors wrote. Because the extreme velocities associated with these collisions occur routinely in meteorite strikes against spacecraft in orbit and in the high-speed machining operations used in manufacturing, sandblasting, and some additive manufacturing (3D printing) processes, the unexpected results have a wide range of applications. may affect.
The experiment the researchers used to discover this effect involved shooting tiny sapphire particles just a millionth of a meter in diameter onto a flat metal plate. Particles propelled by the laser beam reached speeds as high as hundreds of meters per second. Other researchers have sometimes performed experiments at similarly high velocities, but they have tended to use larger impactors on the scale of centimeters or more. Because these larger impacts were dominated by the effects of the impact shock, there was no way to separate the mechanical and thermal effects.
New research shows that small particles do not create significant pressure waves when they hit a target. But it took MIT a decade to develop a way to propel such tiny particles at such high speeds. Schuh said he utilized this along with other new techniques to observe the high-speed impact itself.
The team used ultra-fast cameras “to observe particles coming in and flying out,” he said. When a particle bounces off a surface, the difference between its incoming and outgoing speeds “tells us how much energy has been deposited” in the object, an indicator of surface strength.
The tiny particles they used are made of alumina or sapphire and are “very hard,” Dowding said. With a diameter of 10 to 20 microns (one millionth of a meter), it is about one-tenth to one-fifth the thickness of a human hair. When the launcher behind those particles is hit by the laser beam, some of the material is vaporized, creating a steam jet that propels the particles in the opposite direction.
The researchers fired particles at samples of copper, titanium and gold, and expect the results to apply to other metals as well. They say their data provides the first direct experimental evidence for an anomalous thermal effect whose intensity increases with greater heat. However, hints of such an effect have been reported previously.
The researchers' analysis suggests that this surprising effect stems from the way the regular arrangement of atoms that make up the metal's crystal structure moves under different conditions. They show that there are three separate effects that control how a metal deforms under stress, two of which follow the predicted trajectory of increasing strain at high temperatures, but whose effects are reversed when the strain rate is The third effect is called drag enhancement. Specific threshold.
Beyond this crossing point, higher temperatures increase the activity of acoustic waves (waves of sound or heat) within the material, and these phonons interact with the dislocations of the crystal lattice in ways that limit their ability to slip or deform. The effect increases as impact speed and temperature increase, so “the faster you move, the less likely the dislocation will react,” Dowding says.
Of course, at some point the temperature rises and the metal begins to melt, at which point the effect reverses again and softens. “There will be limits” to this strengthening effect, Dowding said. “But we don’t know what it is.”
The findings could lead to different material choices when designing devices that could face these extreme stresses, Schuh said. For example, a metal that may normally be much weaker but is cheaper or easier to machine may be useful in situations where no one would have thought of using it before.
The extreme conditions studied by researchers are not limited to spacecraft or extreme manufacturing methods. “If you are flying a helicopter in a sandstorm, many of these sand particles will reach high velocities as they hit the blades,” Dowding said. In desert conditions, high temperatures can be reached at which this hardening effect begins.
The techniques the researchers used to uncover this phenomenon can be applied to a variety of other materials and situations, including other metals and alloys. They say that designing materials for use in extreme conditions by simply extrapolating properties known from less extreme conditions can lead to seriously incorrect predictions about how the materials will perform under extreme stresses.
This research was supported by the U.S. Department of Energy.