A computer model of the atomic structure of one of the new carbides. The jumbled touch of carbon and five…
A computer model of the atomic structure of one of the new carbides. The jumbled touch of carbon and five metal elements provides stability to the overall structure. Credit: Pranab Sarker, Duke University
Material scientists at Duke University and UC San Diego have discovered a new class of carbides that are expected to be among the toughest materials with the highest melting points available. Made of cheap metals, the new materials can soon be used in a wide range of industries from machinery and hardware to the aviation industry.
A carbide is traditionally a compound consisting of coal and another element. When paired with a metal such as titanium or tungsten, the resulting material is extremely difficult and difficult to melt. This makes carbides ideal for applications such as cutting tools or parts of a spacecraft.
A small number of complex carbides containing three or more elements are also present, but are usually not found outside the laboratory or industrial applications. This is mainly due to the difficulty in determining which combinations can form stable structures, even less desirable characteristics.
A team of material scientists at Duke University and UC San Diego has now announced the discovery of a new class of carbides that carry coal with five different metal elements at once. The results will be displayed online on November 27th in the journal Nature Communications .
Achieve stability from the chaotic mixture of their atoms in place of orderly atomic structure, these were estimated to exist by the researchers at Duke University and then successfully synthesized at UC San Diego.
“These materials are harder and lighter than today’s carbide,” says Stefano Curtarolo, professor of mechanical engineering and material science at Duke. “They also have very high melting points and are made of relatively inexpensive materials. This combination of attributes should make them very useful for a wide range of industries.”
When students learn about molecular structures, they display crystals like salt, similar to a 3-D control board. These materials gain their stability and strength through regular, arranged atomic ligations where atoms fit together as parts of a puzzle.
Incompetences in a crystalline structure can, however, often increase the strength of a material. For example, if cracks begin to spread along a series of molecular bonds, a group of wrong structures can stop it in their tracks. Hardening of solid metals by creating the perfect amount of disturbance is achieved by a process of heating and extinguishing known as annealing.
The new class of five-metal carbide takes this idea to the next level. To iron all dependence on crystalline structures and bindings for stability, these materials are completely dependent on disturbance. While a pile of baseballs will not stand on its own, a pile of baseball balls, shoes, bats, hats and gloves can only be played.
The image on the left shows metallic elements that form large blocks of similar structures, which do not provide stable material. However, the elements in the image to the right form many different structures, all mixed, giving one of the new materials in the study. Credit: Kenneth Vecchio, UC San Diego
The difficulty lies in predicting which combination of elements will be fixed. Attempting to make new materials is expensive and time consuming. The computer’s interactions through first principle simulations are even more. And with five slits for metallic elements and 91 to choose from, the number of potential recipes is rapidly scary.
“To find out which combinations to mix well, you need to do a spectral analysis based on entropy,” says Pranab Sarker, a postdoctoral co-worker in Curtarologos lab and one of the first authors of the paper. “Entropy is extremely time consuming and difficult to calculate by building an atomic atom, so we tried something different.”
The team was first confined to the eight-metal ingredients that were known to create high carbide carbide compounds and melt temperatures. They then calculated how much energy it would take for a potential five-metal carbide to form a large set of random configurations.
If the results were spread far apart, it was indicated that the combination would probably benefit from a single configuration and fall apart – Like having too many baseballs in the mix. However, if there were many configurations closely clumped together, it indicated that the material would likely form many different structures at once, giving the disturbance necessary for structural stability.
The group then tested their theory by getting colleague Kenneth Vecchio, professor of NanoEngineering at UC San Diego, to actually try to make nine of the associations. This was done by combining the elements of each recipe in fine powder form, pressing them at temperatures up to 4000 degrees Fahrenheit and running 2000 Amps of power directly through them.
“Learning how to process these materials was a difficult task,” said Tyler Harrington, a Ph.D. student in Vecchio’s lab and co-author of the paper. “They behave differently from materials we have ever taken care of, including the traditional carbides.”
They chose the three recipes as their system probably formed a stable material, the two least likely and four random combinations that made in between. As predicted, the three most likely candidates were successful while the two least likely were not. Three of the four intermediate scores also formed stable structures. While the new carbides are likely to have desirable industrial properties, a probable combination stood out – a combination of molybdenum, niobium, tantalum, vanadium and tungsten called MoNbTaVWC5 for short.
“Getting this set of elements to combine is basically trying to squeeze a lot of squares and hexagons,” said Cormac Toher, an assistant research professor at Curtarolo’s laboratory. “If you only think of intuition, you would never think the combination would be possible. But it turns out that the best candidates are actually counterintuitive.”
“We do not know its exact characteristics yet, because it has not been fully tested,” said Curtarolo. “But when we get into the laboratory in the next few months, I would not be surprised if it proved to be the hardest material with the highest melting point ever made.”
“This collaboration is a team of researchers focused on showing the unique and potentially paradigm-altering consequences of this new approach,” Vecchio says. “We use innovative methods for first-class modeling combined with state-of-the-art synthesis and characterization tools to provide the integrated” closed loop “method necessary for advanced material detection.”
Material scientists take great steps towards harder ductile ceramics
Pranab Sarker et al. High entropy high hardness metal carbides detected by entropy descriptors, Nature Communications (2018). DOI: 10,1038 / s41467-018-07160-7