Disordered materials could be the toughest, most heat-resistant carbides


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

Materials scientists at Duke University and UC San Diego have discovered a new class of carbides, which should be among the hardest and most efficient materials at the melting point. Made from inexpensive metals, the new materials could soon be used in a wide range of industries, from machinery and equipment to aerospace.

A carbide is traditionally a compound made of carbon and another element. When it is associated 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 coating the surface of cutting tools or spacecraft parts.

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

A team of materials scientists from Duke University and UC San Diego announced the discovery of a new class of carbides associating carbon with five different metal elements at a time. The results appear online November 27 in the newspaper. Nature Communications.

Researchers at Duke University predicted the existence of calculations for this material, which ensured the stability of the chaotic mixture of their atoms rather than their ordered atomic structure. They were then synthesized successfully at the University of San Diego.

"These materials are harder and lighter than today's 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 inexpensive material blends.This combination of attributes should make them very useful for a wide range of industries."

When students study molecular structures, they see crystals as salt, which resembles a three-dimensional checkerboard. These materials acquire their stability and resistance through regular and ordered atomic bonds where atoms interlock like puzzle pieces.

However, imperfections of a crystalline structure can often strengthen the material. If the cracks begin to propagate along a line of molecular bonds, for example, a group of misaligned structures may stop it. The heating and tempering process called annealing hardens solid metals by creating the perfect amount of mess.

The new five-metal carbide class propels this idea to the next level. Removing all confidence in the stability of structures and crystalline bonds, these materials rest entirely on disorder. Although a bunch of baseballs are not stand-alone, a bunch of baseballs, shoes, poles, hats and gloves might be enough.

The image on the left shows metallic elements forming large blocks of similar structures to each other, which does not allow to obtain a stable material. The right-hand picture elements, however, form many different structures, all mixed, giving one of the new materials of study. Credit: Kenneth Vecchio, UC San Diego

The difficulty lies in predicting the combination of elements that will remain firm. Trying to make new materials is expensive and time consuming. The calculation of atomic interactions by means of first-principle simulations is even more so. And with five slots for metal elements and a choice of 91, the number of potential receipts quickly becomes discouraging.

"To determine which combinations will blend well, you must perform spectral analysis based on entropy," said Pranab Sarker, a postdoctoral fellow at Curtarolo 's lab and one of the first authors of the study. article. "Entropy takes a lot of time and is difficult to calculate by building a model atom by atom, so we tried something different."

The team first restricted the field of known eight-metal ingredients to create carbide compounds with high hardness and melting temperature. They then calculated the amount of energy required for a five-metal carbide potential to form a large set of random configurations.

If the results were very far apart, this would indicate that the combination would likely favor a unique configuration and collapse – as if the number of baseballs was too high. But if there were many closely clustered configurations, it indicated that the material would probably form several different structures at the same time, creating the disorder necessary for structural stability.

The group then tested his theory by asking his colleague Kenneth Vecchio, professor of Nanoengineering at UC San Diego, to try to actually make nine of the compounds. This was done by combining the elements of each recipe in a finely pulverized form, squeezing them at temperatures up to 4000 degrees Fahrenheit and passing 2,000 amps of current directly through them.

"Learning to process these materials was a difficult task," said Tyler Harrington, Ph.D. student in Vecchio's lab and co-lead author of the article. "They behave differently from all the materials we've ever processed, even the traditional carbides."

They chose the three recipes that their system deemed most likely to form a stable material, the two least likely and the four random combinations marked between the two. As expected, the three most likely candidates were successful while the two least likely were not. Three of the four intermediate markers also formed stable structures. Although the new carbides all likely have desirable industrial properties, an unlikely combination has emerged: a combination of molybdenum, niobium, tantalum, vanadium and tungsten, called MoNbTaVWC5.

"Getting this set of elements to combine is basically like trying to collect a set of squares and hexagons," said Cormac Toher, an assistant research professor at Curtarolo's lab. "By pursuing intuition alone, you would never think 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," Curtarolo said. "But once we'll have it in the lab over the next couple of months, I would not be surprised that it's the hardest material with the highest melting point ever created. "

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

Explore further:
Materials scientists take a big step towards tougher ductile ceramic

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

Journal reference:
Nature Communications

Provided by:
Duke University

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