An alloy of chromium, cobalt, and nickel yielded the highest fracture toughness ever measured for a terrestrial material.
It has extremely high strength and ductility, leading to what a team of scientists call “excellent damage resistance.”
Moreover, counterintuitively, these properties increase as the material cools, suggesting interesting potential applications in extreme cryogenic environments.
“When designing structural materials, they need to be strong, but also ductile and fracture-resistant.” Metallurgist Eso George saysGovernor’s Chair of Advanced Alloy Theory and Development at Oak Ridge National Laboratory and the University of Tennessee.
“Usually it’s a compromise between these properties. But this material is both, making it tougher instead of becoming brittle at low temperatures.”
Strength, ductility, and toughness are three properties that determine the durability of a material. Strength describes the resistance to deformation. And ductility describes how malleable a material is. These two properties contribute to the overall toughness, or fracture resistance. Fracture toughness is the resistance of an already fractured material to further fracture.
George and senior author Robert Ritchie, a mechanical engineer at Berkeley National Laboratory and the University of California, Berkeley, have spent time studying a class of materials known as high-entropy alloys (HEA). Most alloys he is dominated by one element, mixed with small amounts of other elements. HEA contains elements mixed in equal proportions.
One such alloy, CrMnFeCoNi (chromium, manganese, iron, cobalt, and nickel), has since been the subject of intense research. scientists have noticed Its strength and ductility increase at liquid nitrogen temperatures without loss of toughness.
One derivative of this alloy, CrCoNi (chromium, cobalt, and nickel), showed even better properties. So George and Richie and their team cracked their knuckles and tried to push it to its limits.
Previous experiments with CrMnFeCoNi and CrCoNi were performed at 77 Kelvin (-196°C-321°F)The team took it even further, raising it to the temperature of liquid helium.
The results were beyond amazing.
“Liquid helium temperature (20 Kelvin, [-253°C, -424°F]) is also 500 megapascal square root meters. ” Richie explains.
“In the same units, a piece of silicon has a toughness of 1, an aluminum fuselage on an airliner is about 35, and the finest steels have a toughness of about 100. So 500 is a staggering figure.”
To understand how it works, the team used neutron diffraction, electron backscatter diffraction, and transmission electron microscopy to study CrCoNi down to the atomic level when fractured at room temperature and cryogenic temperatures. Did.
This involved cracking the material, measuring the stress required for fracture growth, and examining the crystal structure of the sample.
Atoms in metals are arranged in a repeating pattern in three-dimensional space. This pattern is known as a crystal lattice. A repeating component in a lattice is known as a unit cell.
Boundaries may be created between deformed and undeformed unit cells. These boundaries, called dislocations, move when a force is applied to the metal, changing the shape of the metal. The more dislocations, the more malleable the metal.
Metal irregularities can impede dislocation movement. This makes the material stronger. However, higher strength often means higher brittleness, as the material can crack if dislocations are blocked instead of deformed. In CrCoNi, the researchers identified specific sequences of three dislocation blocks.
The first thing that happens is slip. This is the sliding of parallel parts of the crystal lattice away from each other. This causes the unit cell to be misaligned perpendicular to the slip direction.
Creates a continuous force Nano twinning, where the crystal lattice forms a mirrored arrangement on either side of the boundary. When further force is applied, the energy rearranges the shape of the unit cell from cubic to hexagonal lattice.
“When you pull, the first mechanism starts, then the second mechanism starts, then the third mechanism starts, then the fourth mechanism starts.” richie says.
“Now a lot of people would say they’ve seen nano-twinning in regular materials and they’ve seen slip in regular materials. It’s true. There’s nothing new about it, but they’re all this magic It is the fact that it occurs in a sequence of
The researchers also tested CrMnFeCoNi at liquid helium temperatures, but it did not perform as well as the simple derivative.
The next step is to explore potential applications for such materials and find other HEAs with similar properties.
This research chemistry.
