Disordered materials could be the hardest and most heat-tolerant carbides.


A computer model of the atomic structure of one of the new carbides. The disordered carbon disorder and five metal elements give stability to the overall structure. Credit: Pranab Sarker, Duke University

Material scientists at Duke University and UC San Diego have discovered a new clbad of carbides that are expected to be among the hardest materials with the highest melting points that exist. Manufactured with low-cost metals, the new materials will soon be able to be used in a wide range of industries, from machinery and hardware to the aerospace industry.

A carbide is traditionally a compound consisting 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 the carbides ideal for applications such as coating the surface of cutting tools or parts of a spacecraft.

There is also a small amount of complex carbides that contain three or more elements, but are not commonly found outside the laboratory or in industrial applications. This is mainly due to the difficulties in determining which combinations can form stable structures, let alone having desirable properties.

A team of material scientists from Duke University and UC San Diego has now announced the discovery of a new clbad of carbides that bind carbon with five different metal elements at once. The results appear online on November 27 in the magazine. Communications of nature.

By achieving the stability of the chaotic mixture of their atoms instead of the ordered atomic structure, researchers at Duke University computationally predicted that these materials would exist and then be synthesized successfully 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 are made from relatively cheap material blends, and this combination of attributes should make them very useful for a wide range of industries."

When students learn about molecular structures, they are shown crystals like salt, which resembles a 3-D chessboard. These materials obtain their stability and strength through regular ordered atomic bonds in which the atoms unite as pieces of a puzzle.

However, imperfections in a crystalline structure 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 in its tracks. The hardening of solid metals by creating the perfect amount of clutter is achieved through a heating and cooling process called annealing.

The new clbad of five-metal carbides takes this idea to the next level. Discarding any dependence on crystalline structures and bonds for their stability, these materials depend completely on the disorder. While a lot of baseballs will not stand on its own, a lot of baseballs, shoes, bats, hats and gloves could.

The image on the left shows metallic elements that form large blocks of structures similar to each other, 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 make new materials is expensive and time consuming. Calculating atomic interactions through simulations of first principle is even more. And with five slots for metal items and 91 to choose from, the number of potential recipes quickly becomes discouraging.

"To discover which combinations will mix well, you have to do a spectral badysis based on entropy," said Pranab Sarker, a postdoctoral badociate in Curtarolo's laboratory and one of the first authors of the article. "Entropy is incredibly slow and difficult to calculate when building an atom-by-atom model, so we try something different."

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

If the results were far apart, it indicated that the combination would probably favor a single configuration and crumble, like having too many baseballs in the mix. But if there were many closely grouped configurations, it would indicate that the material would probably form many different structures at the same time, providing the necessary clutter for structural stability.

The group then tested their theory by having Kenneth Vecchio, a professor at NanoEngineering at UC San Diego, try to make nine of the compounds. This was done by combining the elements of each recipe in a finely powdered form, pressing them to temperatures of up to 4,000 degrees Fahrenheit and running 2000 amperes of current directly through them.

"Learning to process these materials was a difficult task," said Tyler Harrington, Ph.D. Student in Vecchio's laboratory and co-first author of the article. "They behave differently from any material we have dealt with, including traditional carbides."

They chose the three recipes that their system considered most likely to form a stable material, the two least likely and four random combinations that obtained an intermediate score. As predicted, the three most likely candidates were successful, while the two least likely candidates were not. Three of the four intermediate scorekeepers also formed stable structures. While it is likely that all new carbides have desirable industrial properties, an unlikely combination was highlighted: a combination of molybdenum, niobium, tantalum, vanadium and tungsten, called MoNbTaVWC5, for short.

"Getting this set of elements to combine is basically like trying to put together a lot of squares and hexagons," said Cormac Toher, badistant research professor in Curtarolo's lab. "Following intuition alone, you would never think that such a combination would be feasible, but it turns out that the best candidates are actually contrary to intuition."

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

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

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

Journal reference:
Communications of nature

Provided by:
Duke University

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