The toughest material on Earth has just been found and the structure is just grains
The metallic alloy, made of chromium, cobalt, and nickel, is called CrCoNi.
Researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and Oak Ridge National Laboratory just measured the highest toughness ever recorded, of any material, while investigating a metallic alloy made of chromium, cobalt, and nickel,
called CrCoNi. The material was found to be highly malleable with impressive resistance to permanent deformation.
"When you design structural materials, you want them to be strong but also ductile and resistant to fracture," project co-lead Easo George, the Governor’s Chair for Advanced Alloy Theory and Development at ORNL and the University of Tennessee,
said in a statement. "Typically, it’s a compromise between these properties. But this material is both, and instead of becoming brittle at low temperatures, it gets tougher."
The record-breaking findings were published in
Science on December 2.
What is CrCoNi?
The alloy is a subset of a
class of metals called high entropy alloys (HEAs). While alloys today contain a high proportion of one element with lower amounts of additional elements added, HEAs are made of an equal mix of each constituent element.
This recipe gives the material a high combination of strength and ductility when stressed.
"The toughness of this material near liquid helium temperatures (20 kelvin, -424 Fahrenheit) is as high as 500 megapascals square root meters. In the same units, the toughness of a piece of silicon is one, the aluminum airframe in passenger airplanes is about 35, and the toughness of some of the best steels is around 100. So, 500, it’s a staggering number," said research co-leader Robert Ritchie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and the Chua Professor of Engineering at UC Berkeley.
Multiple techniques were required to define 'toughness'
The scientists used neutron diffraction,
electron backscatter diffraction, and transmission electron microscopy to examine the lattice structures of CrCoNi samples that had been fractured at room temperature and 20 K.
As per the release, "the images and atomic maps generated from these techniques revealed that the alloy’s toughness is due to a trio of dislocation obstacles that come into effect in a particular order when force is applied to the material".
"It’s amusing because metallurgists say that the structure of a material defines its properties, but the structure of the NiCoCr is the simplest you can imagine – it’s just grains," said Ritchie.
"However, when you deform it, the structure becomes very complicated, and this shift helps explain its exceptional resistance to fracture," added co-author Andrew Minor, director of the National Center of Electron Microscopy facility of the Molecular Foundry at Berkeley Lab and Professor of Materials Science and Engineering at UC Berkeley.
Real-life applications are further away
CrCoNi and other HEAs are closer to being implemented for special applications. However, as these materials are not the easiest to create, George states that they could someday be used in environmental extremes that could destroy standard metallic alloys, such as in the frigid temperatures of deep space.
However, real-world applications need time.
"When you are flying on an airplane, would you like to know that what saves you from falling 40,000 feet is an airframe alloy that was only developed a few months ago? Or would you want the materials to be mature and well-understood? That’s why structural materials can take many years, even decades, to get into real use," said Ritchie.
Study Abstract:
Finding structural materials that have good fracture properties at very low temperatures is challenging but is important for fields such as space exploration. Liu et al. discovered a high-entropy chromium-cobalt-nickel alloy that has an incredibly high fracture toughness at 20 kelvin (see the Perspective by Zhang and Zhang). This behavior is caused by an unexpected phase transformation that, when combined with other microstructures, prevents crack formation and propagation. The fracture toughness of this alloy makes it potentially useful for a range of cryogenic applications.
From the study:
High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements..
Mechanical properties and failure characteristics of the CrCoNi medium-entropy alloy. (a) Tensile tests show a significant increase in yield strength, σy, ultimate tensile strength, σUTS and strain to failure, εf, with decreasing temperature from room temperature, 293 K, to cryogenic temperatures, 198 and 77 K. In the same temperature range, the work of fracture increases from 3.5 MJ m−2 to 6.4 MJ m−2. (b) Fracture toughness tests on compact-tension, C(T), specimens show an increasing fracture resistance with crack extension and crack initiation, KJIc, values of 208, 265 and 273 MPa m1/2 at 293, 198 and 77 K, respectively. (c) Stereo microscopy and scanning electron microscopy images show a clear transition from the notch to the pre-crack and a pronounced stretch-zone between the pre-crack and the fully ductile fracture region of a sample that was tested at 198 K. (d) The fracture surface shows ductile dimpled fracture and Cr-rich particles that act as void initiation sides. (Data points shown are mean±s.d.; see Supplementary Table 1 for exact values.) The scale bars in c and d, and the insets of d are 75, 5 and 2 μm, respectively.
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