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Disordered materials could be hardest, most heat-tolerant carbides

A computer model of the atomic structure of one of the new carbides. The confused chaos of carbon and five metallic elements gives stability to the general structure. Credit: Pranab Sarker, Duke University

Material scientists at Duke University and UC San Diego have discovered a new class of carbides expected to be among the hardest materials with the highest melting points in existence. Made from inexpensive metals, the new materials could soon be used in a wide range of industries, from machinery and hardware to aerospace.

A carbide is traditionally a compound made up of carbon and another element. When combined with a metal such as titanium or tungsten, the resulting material is extremely hard and difficult to melt. This makes carbides ideal for applications such as surface coating of cutting tools or parts of a spacecraft.

There is also a small number of complex carbides containing three or more elements, but they are not commonly found outside the laboratory or in industrial applications. This is mainly due to the difficulty of determining which combinations can form stable structures, not to mention the desirable properties.

A team of materials scientists at Duke University and UC San Diego announced the discovery of a new class of carbides that combine carbon with five different metal elements simultaneously. The results appear online on November 27 in the journal Nature Communications .

By obtaining stability from the chaotic mixture of their atoms rather than the ordered atomic structure, these materials were predicted to be computationally existed by Duke University researchers and then successfully synthesized at UC San Diego.

"These materials are harder and lighter than current carbides," said Stefano Curtarolo, professor of mechanical engineering and materials science at Duke. "They also have very high melting points and consist of relatively inexpensive mixtures of materials. This combination of attributes should make them very useful for a wide range of industries."

When students learn about molecular structures, crystals like salt are shown, which resembles a three-dimensional chess board. These materials obtain stability and strength through regular and orderly atomic bonds, in which atoms fit together like pieces of a puzzle.

Imperfections in a crystalline structure, however, can often add strength to a material. If cracks begin to propagate along a line of molecular bonds, for example, a group of misaligned structures can stop it on its tracks. The hardening of solid metals creating the perfect amount of disorder is achieved through a process of heating and extinguishing called annealing.

The new class of five-metal carbides brings this idea to the next level. By discharging any dependence on crystalline structures and bonds due to their stability, these materials are completely based on disorder. While a bunch of baseballs can not stand alone, a bunch of baseballs, shoes, bats, hats and gloves could just.

The image on the left shows metal elements that form large blocks of similar structures, which do not produce a stable material. The elements in the image on the right, however, form many different structures all mixed together, producing one of the new materials in the study. Credit: Kenneth Vecchio, UC San Diego

The difficulty lies in predicting which combination of elements will remain firm. Trying to create new materials is expensive and takes a long time. Calculating atomic interactions through principle simulations is even more important. And with five slots for metallic items and 91 to choose from, the number of potential recipes quickly becomes daunting.

"To understand which combinations blend well, you need to do a spectral analysis based on entropy," said Pranab Sarker, a postdoctoral associate in the Curtarolo laboratory and one of the first authors of the paper. "The entropy is incredibly long and hard to calculate by building an atomic model per atom, so we tried something different."

The team first restricted the field of ingredients to eight known metals to create carbide compounds with high hardness and melting temperatures. Then they calculated the amount of energy needed for a potential five-metal carbide to form a large set of random configurations.

If the results were far away, he indicated that the combination would probably favor a single configuration and disintegrate -How to have too many baseballs in the mix. But if there were many configurations tightly grouped together, it indicated that the material would probably have formed many different structures simultaneously, providing the disorder necessary for structural stability.

The group then tested its theory by meeting with colleague Kenneth Vecchio, a Nano Engineering professor at UC San Diego, to attempt to actually do nine of the compounds. This was achieved by combining the elements of each recipe in a finely pulverized form, pressing them to temperatures up to 4,000 degrees Fahrenheit and using 2000 current amps directly through them.

"Learning to process these materials was a difficult task," said Tyler Harrington, a Ph.D. student in the Old Lab and co-author of the paper. "They behave differently than any other material we've ever treated, even traditional carbides."

They chose the three recipes that their system considered most likely to form a stable material, the two least probable and four random combinations that marked in the middle. As expected, the three most likely candidates were successful while the two least likely candidates were not. Three of the four intermediate markers also formed stable structures. While it is likely that the new carbides have desirable industrial properties, an unlikely combination has emerged – a combination of molybdenum, niobium, tantalum, vanadium and tungsten called MoNbTaVWC5 for short

"Getting this set of elements to combine is basically similar to trying to squeeze a bunch of squares and hexagons, "said Cormac Toher, an assistant researcher in Curtarolo's lab. "By continuing with intuition alone, you would never think that this combination would be feasible, but it turns out that the best candidates are actually counterintuitive."

"We do not yet know its exact properties because it has not been fully tested," said Curtarolo. "But once we enter the lab in the next two months, I would not be surprised if it turned out to be the toughest material with the highest melting point ever made."

"This collaboration is a team of researchers focused on demonstrating the unique and potentially changing implications of the paradigm of this new approach," said Vecchio. "We are using innovative approaches to modeling the first principles combined with synthesis tools and state-of-the-art characterization to provide the integrated" closed loop "methodology necessary for advanced material discovery."

Explore further:
Material scientists take a big step towards the toughest ductile ceramics

Further information:
Pranab Sarker et al., High hardness high entropy metal carbides discovered by entropy descriptors, Nature Communications (2018). DOI: 10.1038 / s41467-018-07160-7

Reference to the magazine:
Communications of nature

Provided by:
Duke University

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