Metals typically soften when heated, allowing for easier shaping by blacksmiths. However, researchers at MIT have discovered that when metal is struck by an object moving at a high velocity, the opposite occurs: the hotter the metal, the stronger it becomes. Copper, traditionally more pliable than steel, can become as strong as steel under these extreme conditions. This finding could revolutionize the design of materials for high-stress environments such as shields for spacecraft or equipment for high-speed manufacturing.
The study, published in the journal Nature, was carried out by Ian Dowding and Christopher Schuh. When tiny particles of sapphire were shot at metal sheets at high speeds using laser beams, the researchers observed that the incoming and outgoing velocities of the particles indicated the amount of energy deposited into the metal samples. This marked the first direct experimental evidence of the anomalous thermal effect of increased strength with higher temperatures in metals.
The surprising effect arises from the movement of atoms in the crystalline structure of metals under various conditions. While two effects predict increased deformation at higher temperatures, the drag strengthening effect reverses its impact beyond a certain threshold. This results in increased strength due to interactions between heat waves and dislocations in the metal lattice, limiting its ability to deform. The effect intensifies with higher impact speeds and temperatures until the metal starts to melt.
This discovery could lead to the use of different materials in designing devices that experience extreme stresses. Weaker metals that are more affordable or easier to process may now be considered for situations where their strength can be enhanced by these thermal effects. The findings are applicable not only to spacecraft or high-speed manufacturing but also to everyday situations like flying a helicopter in a sandstorm, where particles can reach high velocities and temperatures.
The techniques developed by the researchers could be applied to a wide range of materials and situations, preventing erroneous assumptions about material behavior under extreme conditions. Designing materials for extreme environments based on properties at milder conditions may no longer be accurate. Further research could lead to the identification of the ultimate limit to the strength-increasing effect before the metal starts to melt under extreme conditions.
Overall, the study highlights the potential for unexpected strengthening effects in metals subjected to extreme velocities and temperatures. This novel approach to material design could open up new possibilities for creating robust materials for use in challenging environments. The research was supported by the U.S. Department of Energy, indicating the significance of these findings for various applications in high-stress scenarios.