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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 confused mess of carbon and five metal elements provides stability throughout the structure. Picture credits: Pranab Sarker, Duke University

Materials scientists at Duke University and UC San Diego have discovered a new class of carbides that are expected to be among the hardest materials with the highest melting points. The new materials made from cheap metals will soon be used in a variety of industries, from machinery and hardware to aerospace.

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

There are also a small number of complex carbides with three or more elements, but these are not found outside the laboratory or in industry applications. This is mainly due to the difficulties of determining which combinations can form stable structures, let alone have desirable properties.

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

The stability of the chaotic mixture of their atoms rather than a proper atomic structure is mathematically predicted by the researchers of Duke University and then successfully synthesized at the UC San Diego.

"These materials are harder and lighter than current carbides," said Stefano Curtarolo, professor of engineering and materials science at Duke. "They also have very high melting points and are made of relatively cheap mixtures of materials, and this combination of properties should make them very useful to 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 gain their stability and strength through regular, ordered atomic bonds in which the atoms are pieced together like puzzle pieces.

Imperfections in a crystalline structure, however, can often impart strength to a material. For example, when cracks propagate along a series of molecular bonds, a group of misaligned structures can trap them in their orbits. The hardening of solid metals by creating the perfect amount of disorder is achieved by a process of heating and quenching, the so-called tempering.

The new class of five-metal carbide takes this idea to the next level. Since these materials do not rely on crystalline structures and bonds because of their stability, they are completely dependent on disorder. While a heap of baseballs can not stand alone, a pile of baseballs, shoes, bats, hats, and gloves could only play a role.

The image on the left shows metallic elements that form large blocks of similar structures that do not yield a stable material. However, the elements in the right image form many different structures that are all mixed together, creating one of the new materials in the study. Picture credits: Kenneth Vecchio, UC San Diego

The difficulty lies in predicting which combination of elements will remain fixed. Trying to make new materials is expensive and time consuming. The calculation of atomic interactions by first-principle simulations is even more so. With five slots for metallic items and 91 to choose from, the number of possible recipes quickly becomes daunting.

"To figure out which combinations mix well, you need to perform an entropy-based spectral analysis," said Pranab Sarker, a postdoctoral fellow in Curtarolo's lab and one of the newspaper's first authors. "Entropy is incredibly time-consuming and hard to compute, building a model atom by atom, so we tried something different."

The team initially restricted itself to eight metals known to form high hardness carbide compounds and melting temperatures. They then calculated how much energy a potential five-metal carbide would need to form a large number of random configurations.

When the results are far apart, this suggests that the combination would probably favor a single configuration and fail – as if you had too many baseballs in the mix. If many configurations are tightly lumped together, this suggests that the material would likely form many different structures simultaneously to provide the disruption required for structural stability.

The group tested their theory by getting their colleague Kenneth Vecchio, a professor of NanoEngineering, at UC San Diego to try to actually make nine of the compounds. For this purpose, the elements in each recipe were combined in a finely powdered form, pressed at temperatures of up to 4,000 degrees Fahrenheit and passed through with a current of 2000 A directly.

"Learning to process these materials was a difficult task." said Tyler Harrington, a Ph.D. Student in Vecchio's laboratory and co-author of the newspaper. "They behave differently than any material we've ever studied, even the traditional carbides."

They chose the three recipes that most likely make up a stable material in their system, the least likely and four random combinations in between. As predicted, the three most likely candidates were successful, while the two were least likely. Three of the four intermediate seekers also formed stable structures. While the new carbides are likely to have all the desirable industrial properties, an unlikely combination – a combination of molybdenum, niobium, tantalum, vanadium, and tungsten, or MoNbTaVWC5 for short – "I'm trying to squeeze a few squares and hexagons," said Cormac Toher, scientific assistant in the laboratory of Curtarolo. "If you go for intuition, you would never think that a combination is possible, but it turns out that the best candidates are actually not catchy."

"We do not know the exact characteristics yet, as they were not fully tested," said Curtarolo. "But as soon as we got it to the lab in the next few months, I would not be surprised if it turned out to be the hardest, highest melting point material ever."

"This collaboration is a team of researchers focused on demonstrating the unique and potential impact of this new approach on the paradigm," said Vecchio. "We use innovative approaches to modeling the first principles in combination with state-of-the-art synthesis and characterization tools to provide the integrated closed-loop approach required for advanced materials research."

Explore further:
Materials scientists take a big step towards harder ductile ceramics

More information:
Pranab Sarker et al., High-entropy and High Hardness Metal Carbides Discovered by Entropy Descriptors, Nature Communications (2018). DOI: 10.1038 / s41467-018-07160-7

Magazine Reference:
Nature Communications

Provided by:
Duke University

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