A new study reveals the profound properties of a simple metal alloy
From: News From Berkeley Lab
By Aliyah Kovner
December 8, 2022 -- Scientists
have measured the highest toughness ever recorded, of any material, while
investigating a metallic alloy made of chromium, cobalt, and nickel (CrCoNi).
Not only is the metal extremely ductile – which, in materials science, means
highly malleable – and impressively strong (meaning it resists permanent
deformation), its strength and ductility improve as it gets colder. This runs
counter to most other materials in existence.
The team, led by
researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and Oak
Ridge National Laboratory, published a study describing their
record-breaking findings in Science on Dec. 2, 2022. “When you
design structural materials, you want them to be strong but also ductile and
resistant to fracture,” said project co-lead Easo George, the Governor’s Chair
for Advanced Alloy Theory and Development at ORNL and the University of
Tennessee. “Typically, it’s a compromise between these properties. But this
material is both, and instead of becoming brittle at low temperatures, it gets
tougher.”
CrCoNi is a subset of a
class of metals called high entropy alloys (HEAs). All the alloys in use today
contain a high proportion of one element with lower amounts of additional
elements added, but HEAs are made of an equal mix of each constituent element.
These balanced atomic recipes appear to bestow some of these materials with an
extraordinarily high combination of strength and ductility when stressed, which
together make up what is termed “toughness.” HEAs have been a hot area of research
since they were first developed about 20 years ago, but the technology required
to push the materials to their limits in extreme tests was not available until
recently.
“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.
Ritchie and George
began experimenting with CrCoNi and another alloy that also contains manganese
and iron (CrMnFeCoNi) nearly a decade ago. They created samples of the alloys
then lowered the materials to liquid nitrogen temperatures (around 77 kelvin,
or -321 F) and discovered impressive strength and toughness. They
immediately wanted to follow up their work with tests at liquid helium
temperature ranges, but finding facilities that would enable stress testing
samples in such a cold environment, and recruiting team members with the
analytical tools and experience needed to analyze what happens in the material
at an atomic level took the next 10 years. Thankfully, the results were worth
the wait.
Peering into the
crystal
Many solid substances,
including metals, exist in a crystalline form characterized by a repeating 3D
atomic pattern, called a unit cell, that makes up a larger structure called a
lattice. The material’s strength and toughness, or lack thereof, come from
physical properties of the lattice. No crystal is perfect, so the unit cells in
a material will inevitably contain “defects,” a prominent example being
dislocations – boundaries where undeformed lattice meets up with deformed
lattice. When force is applied to the material – think, for example, of bending
a metal spoon – the shape change is accomplished by the movement of
dislocations through the lattice. The easier it is for the dislocations to
move, the softer the material is. But if the movement of the dislocations is
blocked by obstacles in the form of lattice irregularities, then more force is
required to move the atoms within the dislocation, and the material becomes
stronger. On the flip side, obstacles usually make the material more brittle –
prone to cracking.
Using neutron
diffraction, electron backscatter diffraction, and transmission electron
microscopy, Ritchie, George, and their colleagues at Berkeley Lab, the
University of Bristol, Rutherford Appleton Laboratory, and the University of
New South Wales examined the lattice structures of CrCoNi samples that had been
fractured at room temperature and 20 K. (For measuring strength and ductility,
a pristine metal specimen is pulled until it fractures, whereas for fracture
toughness tests, a sharp crack is intentionally introduced into the sample
before it is pulled and the stress needed to grow the crack is then measured.)
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. First, moving dislocations cause areas
of the crystal to slide away from other areas that are on parallel planes. This
movement displaces layers of unit cells so that their pattern no longer matches
up in the direction perpendicular to the slipping movement, creating a type of
obstacle. Further force on the metal creates a phenomenon called nanotwinning,
wherein areas of the lattice form a mirrored symmetry with a boundary in
between. Finally, if forces continue to act on the metal, the energy being put
into the system changes the arrangement of the unit cells themselves, with the
CrCoNi atoms switching from a face-centered cubic crystal to another
arrangement known as hexagonal close packing.
This sequence of atomic
interactions ensures that the metal keeps flowing, but also keeps meeting new
resistance from obstacles far past the point that most materials snap from the
strain. “So as you are pulling it, the first mechanism starts and then the
second one starts, and then the third one starts, and then the fourth,”
explained Ritchie. “Now, a lot of people will say, well, we’ve seen
nanotwinning in regular materials, we’ve seen slip in regular materials. That’s
true. There’s nothing new about that, but it’s the fact they all occur in this
magical sequence that gives us these really tremendous properties.”
The team’s new
findings, taken with other recent work on HEAs, may force the materials science
community to reconsider long-held notions about how physical characteristics
give rise to performance. “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. “We were able to visualize this unexpected
transformation due to the development of fast electron detectors in our
electron microscopes, which allow us to discern between different types of
crystals and quantify the defects inside them at the resolution of a single
nanometer – the width of just a few atoms – which as it turns out, is about the
size of the defects in deformed NiCoCr structure.”
The CrMnFeCoNi alloy
was also tested at 20 kelvin and performed impressively, but didn’t achieve the
same toughness as the simpler CrCoNi alloy.
Forging new products
Now that the inner
workings of the CrCoNi alloy are better understood, it and other HEAs are one
step closer to adoption for special applications. Though these materials are
expensive to create, George foresees uses in situations where environmental
extremes could destroy standard metallic alloys, such as in in the frigid
temperatures of deep space. He and his team at Oak Ridge are also investigating
how alloys made of more abundant and less expensive elements – there is a
global shortage of cobalt and nickel due to their demand in the battery
industry – could be coaxed into having similar properties.
Though the progress is
exciting, Ritchie warns that real-world use could still be a ways off, for good
reason. “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.”
This research was
supported by the Department of Energy’s Office of Science. The low-temperature
mechanical testing and neutron diffraction was performed at the ENGIN-X ISIS
Facility at the Rutherford Appleton Laboratory, led by first author Dong Liu.
Microscopy was performed at the National Center for Electron Microscopy at the
Molecular Foundry, a DOE Office of Science user facility at Berkeley Lab. The
other authors on this project were Qin Yu, Saurabh Kabra, Ming Jiang,
Joachim-Paul Forna-Kreutzer, Ruopeng Zhang, Madelyn Payne, Flynn Walsh, Bernd
Gludovatz, and Mark Asta.
https://newscenter.lbl.gov/2022/12/08/say-hello-to-the-toughest-material-on-earth/
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