An alloy of chromium, cobalt and nickel has just given us the highest fracture toughness ever measured of any material on Earth.
It has high strength and exceptionally high ductility, resulting in what a team of scientists called “outstanding damage tolerance.”
Furthermore – and unexpectedly – these properties increase as the material gets colder, indicating some interesting potential for applications in extremely cold environments.
“When you design structural materials, you want them to be strong but also ductile and break-resistant,” says metallurgist Izzo George, portfolio chair for advanced alloy theory and development at Oak Ridge National Laboratory and the University of Tennessee.
“Usually this is a compromise between these properties. But this material has both, and instead of becoming brittle at lower temperatures, it becomes tougher.”
Strength, ductility, and toughness are three properties that determine how durable a material is. Strength describes resistance to deformation. Plasticity describes how flexible a material is. These two properties contribute to its overall hardness: resistance to fracture. Fracture toughness is the resistance to further fractures in an already broken material.
George and fellow senior 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 percentages of other elements being mixed. HEAs contain elements mixed 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 a temperature of liquid nitrogen without compromising toughness.
A derivative of this alloy, CrCoNi (chromium cobalt nickel), showed even more exceptional properties. So George, Richie and their team crack their knuckles and proceed to push it to its limits.
Previous experiments with CrMnFeCoNi and CrCoNi were carried out at liquid nitrogen temperatures, up to 77 K (-196)Celsius-321degrees Fahrenheit). The team pushed it even further, to liquid helium temperatures.
The results were absolutely amazing.
The hardness of this material is near liquid helium temperatures (20 K, [-253°C, -424°F]) up to 500 MPa per square metre,” Ritchie explains.
“In the same units, the hardness of a piece of silicon is 1, aluminum airframes in passenger planes are about 35, and the toughness of some of the best steels is about 100. So, 500, it’s an amazing number.”
To find out how they work, the team used neutron diffraction, electron backscattering diffraction, and transmission electron microscopy to study CrCoNi down to the atomic level when cracked at room temperature and in extreme cold.
This involved fracturing the material, measuring the pressure required to cause the fracture to grow, and then looking at the crystal structure of the samples.
The atoms in minerals are arranged in a repeating pattern in three-dimensional space. This pattern is known as a crystal lattice. The repeating components in a network are known as unit cells.
Sometimes a boundary is created between the distorted unit cells and those that are not. These boundaries are called dislocations, and when force is applied to the mineral, they move, allowing the mineral to change its shape. The more dislocations in a metal, the more malleable it is.
Irregularities in the metal can prevent turbulence from moving; This is what makes the material strong. But if dislocations are blocked, instead of deforming, the material can crack, so high strength can often mean high brittleness. In CrCoNi, the researchers have identified a specific sequence of three dislocation blocks.
The first thing that happens is slip, which is when parallel parts of the crystal lattice slide away from each other. This causes the unit cells to not match perpendicular to the sliding direction.
The continuous force produces a nanotwinning process, in which crystal lattices form an inverse order on either side of the boundary. If more force is applied, that energy goes to rearrange the shape of the unit cells, from a hexagonal crystal cube.
“As you’re pulling it on, the first mechanism starts, then the second, then the third, and then the fourth,” says Ritchie.
“Now, a lot of people will say, well, we’ve seen nanofabrication in ordinary materials, we’ve seen slippage in ordinary materials. That’s right. There’s nothing new about that, but the fact that they all happen in this magical sequence gives us these really awesome properties.” .
The researchers also tested CrMnFeCoNi at liquid helium temperatures, but its simple derivative didn’t perform nearly as well as its simple derivative.
The next step will be to investigate potential applications for such materials, as well as to find other HEAs with similar properties.
Research published in Sciences.
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