Under extreme impacts, metals become stronger with heat

May 23. () –

Scientists at MIT have seen that when metals are extremely deformed by an object moving at high speeds, High temperatures make the metal stronger, not weaker.

This unexpected finding could be important for the design of spacecraft shielding, hypersonic aircraft or in high speed machining applications.

The findings are described in an article that appears in the Nature magazineby Ian Dowding, an MIT graduate student, and Christopher Schuh, former head of the Department of Materials Science and Engineering at MIT.

The new discovery, the authors write, “is contradictory and contradicts decades of studies in less extreme conditions.” The unexpected results could affect a variety of applications because the extreme velocities involved in these impacts routinely occur in meteorite impacts on orbiting spacecraft and in high-speed machining operations used in manufacturing, sandblasting, and some processes. additive manufacturing (3D printing).

The experiments the researchers used to find this effect involved shooting tiny sapphire particles, just a millionth of a meter wide, at flat sheets of metal. Driven by laser beams, the particles reached high speeds, on the order of a few hundred meters per second.

While other researchers have occasionally performed experiments at similar speeds, they have tended to use larger impactors, on the scale of centimeters or more. Because these larger impacts were dominated by impact effects, there was no way to separate the mechanical and thermal effects.

The small particles in the new study do not create a significant pressure wave when they hit the target. But it has taken a decade of research at MIT to develop methods to propel microscopic particles to such high speeds. “We’ve taken advantage of that,” says Schuh it’s a statementalong with other new techniques to observe the high-speed impact itself.

The team used extremely high-speed cameras “to watch the particles as they came in and out,” he says. As the particles bounce off the surface, the difference between the entry and exit velocities “tells how much energy was deposited” in the target, which is an indicator of surface resistance.

The tiny particles they used were made of alumina or sapphire and are “very hard,” Dowding says. With a diameter of 10 to 20 microns (millionths of a meter), they represent between one tenth and one fifth of the thickness of a human hair. When the launch pad behind those particles is hit by a laser beam, some of the material vaporizes, creating a jet of vapor that propels the particle in the opposite direction.

FIRST DIRECT OBSERVATION OF THIS ANOMALOUS EFFECT

The researchers shot the particles at samples of copper, titanium and gold, and hope their results will apply to other metals as well. They say their data provide the first direct experimental evidence of this anomalous thermal effect of higher resistance with higher heat, although indications of such an effect had been reported before.

According to the researchers’ analysis, the surprising effect appears to be the result of the way the ordered sets of atoms that make up the crystal structure of metals move under different conditions. They show that there are three separate effects which govern how metal deforms under stressand while two of them follow the predicted path of increasing strain at higher temperatures, it is the third effect, called drag strengthening, that reverses its effect when the strain rate crosses a certain threshold.