Copper is typically more pliable than steel. However, MIT researchers discovered that when metal is struck by an object moving at a super high velocity, the opposite occurs: the hotter the metal, the stronger it becomes, and copper can be just as strong as steel.
The discovery could lead to new materials for extreme environments, including spacecraft, hypersonic aircraft, and high-speed manufacturing process equipment. The researchers published their findings in the journal Nature.
The team agrees this is counterintuitive and that the results could affect various applications. The researchers’ experiments involved shooting tiny particles of sapphire at flat sheets of metal. The particles reached high velocities, on the order of a few hundred meters per second. They used extremely high-speed cameras to watch the particles as they came in and flew away. As particles bounce off the surface, the difference between the incoming and outgoing velocities indicates how much energy was deposited into the target – an indicator of the surface strength.
The researchers shot the particles at copper, titanium, and gold samples, and the data provides the first evidence for this strange thermal effect of increased strength. The effect results from the way the arrays of atoms that make up the crystalline structure of metals move under different conditions. There are three separate effects governing how metal deforms under stress. While two follow the predicted trajectory of increasing deformation at higher temperatures, the third effect, called “drag strengthening,” reverses its effect when the deformation rate crosses a certain threshold.
The findings could lead to different choices of materials for devices that may encounter extreme stresses.
