An alloy of chromium, cobalt and nickel has just given us the highest fracture resistance ever measured in a material on Earth.
It has exceptionally high strength and ductility, leading to what a team of scientists has called “exceptional damage tolerance.”
Furthermore, and counterintuitively, these properties increase as the material cools, suggesting an interesting potential for applications in extreme cryogenic environments.
“When you design structural materials, you want them to be strong, but also ductile and resistant to fracture,” says metallurgist Easo George, the Governor’s Chair in Advanced Alloy Theory and Development at Oak Ridge National Laboratory and the University from tennessee
“Typically, it’s a compromise between those properties. But this material is both, and instead of becoming brittle at low temperatures, it becomes harder.”
Strength, ductility and toughness are three properties that determine the durability of a material. Strength describes resistance to deformation. And ductility describes how malleable a material is. These two properties contribute to its overall toughness: fracture resistance. Fracture toughness is the resistance to further fracture in an already fractured material.
George and his lead author, mechanical engineer Robert Richie of Berkeley National Laboratory and the University of California, Berkeley, have spent some time working on a class of materials known as high-entropy alloys, or HEAs. Most alloys are dominated by one element, with small proportions of others mixed in. HEAs contain mixed elements in equal proportions.
One such alloy, CrMnFeCoNi (chromium, manganese, iron, cobalt, and nickel), has been the subject of intense study after scientists noticed that its strength and ductility increase at the temperature of liquid nitrogen without compromising hardness.
A derivative of this alloy, CrCoNi (chromium, cobalt and nickel), showed even more exceptional properties. So George and Ritchie and their team cracked their knuckles and set about pushing it to its limits.
The crystal lattice and grain structures of CrMnFeCoNi and CrCoNi. (Robert Ritchie/Berkeley Lab)
Previous experiments on CrMnFeCoNi and CrCoNi had been performed at liquid nitrogen temperatures, down to 77 Kelvin (-196 °C, -321 °F). The team pushed it even further, to liquid helium temperatures.
The results were more than surprising.
“The hardness of this material near the temperatures of liquid helium (20 Kelvin, [-253°C, -424°F]) is as high as 500 megapascals per square meter,” explains Ritchie.
“In the same units, the hardness of a piece of silicon is one, the aluminum fuselage of passenger aircraft is about 35, and the hardness of some of the best steels is about 100. So , 500, is an amazing number.”
To find out how it works, the team used neutron diffraction, electron backscatter diffraction and transmission electron microscopy to study CrCoNi down to the atomic level as it fractures at room temperature and in extreme cold.
This involved breaking the material and measuring the stress required to grow the fracture and then observing the crystal structure of the samples.
The atoms of metals are arranged in a repeating pattern in three-dimensional space. This pattern is known as the crystal lattice. The repeating components in the network are known as unit cells.
Boundaries are sometimes created between unit cells that are deformed and those that are not. These boundaries are called dislocations, and when force is applied to the metal, they move, allowing the metal to change shape. The more dislocations a metal has, the more malleable it is.
Scanning electron microscopy images of fractures in CrCoNi at 293 Kelvin (left) and 20 Kelvin (right). (Robert Ritchie/Berkeley Lab)
Irregularities in the metal can prevent dislocations from moving; this is what makes a material strong. But if dislocations are blocked, rather than deformed, a material can break, so high strength can often mean high brittleness. In CrCoNi, the researchers identified a particular sequence of three dislocation blocks.
The first thing that occurs is slip, which is when the parallel parts of the crystal lattice slide off each other. This causes the unit cells to no longer match perpendicular to the sliding direction.
The continued force produces nanotwinning, where the crystal lattices form a mirror arrangement on either side of a boundary. If still more force is applied, this energy is used to rearrange the shape of the unit cells, from a cubic to a hexagonal lattice.
“As you pull it, the first mechanism starts, and then the second one starts, and then the third one starts, and then the fourth one starts,” Ritchie says.
“Now, a lot of people will say, well, we’ve seen nanowiring in normal materials, we’ve seen slippage in normal materials. That’s true. There’s nothing new about it, but it’s the fact that they all occur in this magical sequence. it gives us these really tremendous properties.”
The researchers also tested CrMnFeCoNi at liquid helium temperatures, but it didn’t perform as well as its simpler derivative.
The next step will be to investigate the potential applications of this material, as well as to find other HEAs with similar properties.
The research has been published in Science.