An alloy of chromium, cobalt and nickel has just given us the highest fracture toughness ever measured in a material on Earth.
It has exceptionally high strength and ductility, leading to what a team of scientists called “exceptional damage tolerance”.
Additionally – and counterintuitively – these properties increase as the material cools, suggesting exciting potential for applications in extreme cryogenic environments.
“When you design structural materials, you want them to be strong but also ductile and resistant to breaking,” says metallurgist Easo GeorgeGovernor’s Chair for 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, 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. Force describes the resistance to deformation. And ductility describes the malleability of a material. These two properties contribute to its overall toughness: breaking strength. Fracture toughness is the resistance to further fracture in an already fractured material.
George and his fellow lead author, mechanical engineer Robert Richie of Berkeley National Laboratory and the University of California, Berkeley, spent time working on a class of materials called high-entropy alloys, or HEAs. Most alloys are dominated by one element, with small proportions of others mixed in. HEAs contain elements mixed in equal proportions.
One of these alloys, CrMnFeCoNi (chromium, manganese, iron, cobalt and nickel), has been the subject of extensive studies after scientists have noticed that its strength and ductility increase at the temperature of liquid nitrogen without compromising toughness.
A derivative of this alloy, CrCoNi (chromium, cobalt and nickel), displays even more exceptional properties. So George and Ritchie and their team cracked their fingers and set about pushing him to his limits.
Previous experiments on CrMnFeCoNi and CrCoNi had been conducted at liquid nitrogen temperatures, down to 77 Kelvin (-196°C-321°F). The team took it even further, to liquid helium temperatures.
The results were more than striking.
“The toughness of this material at temperatures close to liquid helium (20 Kelvin, [-253°C, -424°F]) is as high as 500 square root meters megapascals”, Ritchie explains.
“In the same units, the toughness of a piece of silicon is one, the aluminum airframe of passenger aircraft is about 35, and the toughness of some of the best steel is about 100. So 500, that’s a staggering number.”
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 extreme cold.
This involved cracking the material and measuring the stress needed to cause the fracture to grow, then examining the crystal structure of the samples.
Atoms in metals are arranged in a repeating pattern in three-dimensional space. This pattern is known as the crystal lattice. The repeating components of the network are called unit cells.
Sometimes boundaries are created between unit cells that are distorted and those that are not. These boundaries are called dislocations, and when a 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.
Irregularities in the metal can prevent dislocations from moving; this is what makes a material strong. But if dislocations are stuck, instead of deforming, a material can crack, so high strength can often mean high brittleness. In CrCoNi, the researchers identified a particular sequence of three dislocation blocks.
The first to occur is slippage, which is when parallel parts of the crystal lattice move away from each other. This causes the unit cells to no longer correspond perpendicular to the sliding direction.
The continuous force produces nanotwinning, where the crystal lattices form a mirrored arrangement on either side of a boundary. If even more force is applied, this energy serves to rearrange the shape of the unit cells from a cubic crystal lattice to a hexagonal lattice.
“As you pull it, the first mechanism starts, then the second starts, then the third starts, then the fourth”, Ritchie says.
“Now a lot of people will say, well, we’ve seen nanotwinning in ordinary materials, we’ve seen slippage in ordinary materials. That’s right. There’s nothing new about that, but it’s It’s the fact that they all happen in this magical sequence that gives us these really great 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 study the potential applications of such a material, as well as to find other HEAs with similar properties.
The research has been published in Science.