A map of the crystal structure of the alloy made by electron backscatter diffraction in a scanning electron microscope. Each color represents a part of the crystal where the repeating structure changes its 3D orientation. Credit: Berkeley Lab
Researchers have discovered an extraordinary metal alloy which will not crack at extreme temperatures due to buckling or bending of crystals in the alloy at the atomic level.
A metal alloy composed of niobium, tantalum, titanium and hafnium has shocked materials scientists with its impressive strength and toughness at both extremely hot and cold temperatures, a combination of properties that until now seemed virtually impossible to achieve. In this context, strength is defined as the amount of force a material can withstand before it is permanently deformed from its original shape, and toughness is the resistance to fracture (cracking). The alloy’s resilience to bending and breaking under a huge range of conditions could open the door to a new class of materials for next-generation engines that can operate at higher efficiency.
The team, led by Robert Ritchie of Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, working with groups led by professors Diran Apelian of UC Irvine and Enrique Lavernia of Texas A&M University, discovered the surprising properties of the alloy and then discovered how they arise from interactions in the atomic structure. Their work is described in a study recently published in the journal Science.
“The efficiency of converting heat into electricity or thrust is determined by the temperature at which fuel is burned – the hotter the better. However, the operating temperature is limited by the structural materials that must withstand it,” said first author David Cook, a Ph.D. student in Ritchie’s laboratory. “We have exhausted the possibilities to further optimize the materials we currently use at high temperatures, and there is a great need for new metallic materials. That is where this alloy is promising.”
The alloy in this study comes from a new class of metals known as high- or medium-entropy refractory alloys (RHEAs/RMEAs). Most metals we see in commercial or industrial applications are alloys made of one main metal mixed with small amounts of other elements, but RHEAs and RMEAs are made by mixing nearly equal amounts of metallic elements with very high melting temperatures, giving them unique properties that scientists are still unraveling. Ritchie’s group has been researching these alloys for several years because of their potential for high-temperature applications.
This material structure map shows kink bands formed near a crack tip during crack propagation (from left to right) in the alloy at 25 C, room temperature. Taken with an electron backscatter diffraction detector in a scanning electron microscope. Credit: Berkeley Lab
“Our team has done previous work on RHEAs and RMEAs and we found that these materials are very strong, but generally have extremely low fracture toughness. Therefore, we were shocked when this alloy exhibited exceptionally high toughness,” said co-correspondent author Punit Kumar, a postdoctoral researcher in the group.
According to Cook, most RMEAs have fracture toughness less than 10 MPa√m, making them among the most brittle metals ever known. The best cryogenic steels, specifically designed to withstand fracture, are approximately 20 times stronger than these materials. Yet niobium, tantalum, titanium and hafnium (Nb45Ta25Ti15Hf15) The RMEA alloy was even able to beat the cryogenic steel and was over 25 times stronger than typical RMEAs at room temperature.
But engines don’t work at room temperature. The scientists evaluated strength and toughness at five temperatures in total: -196°C (the temperature of liquid nitrogen), 25°C (room temperature), 800°C, 950°C and 1200°C. The latter temperature is about 1/5 of the sun’s surface temperature.
The team found that the alloy had the highest strength in the cold, weakening slightly as temperatures rose, but still having impressive numbers across its wide range. Fracture toughness, which is calculated based on the amount of force required to propagate an existing crack in a material, was high at all temperatures.
Unraveling the atomic arrangements
Nearly all metal alloys are crystalline, meaning the atoms in the material are arranged in repeating units. However, no crystal is perfect, they all contain flaws. The most prominent defect that moves is called the dislocation, an unfinished plane of atoms in the crystal. When force is applied to a metal, it causes many dislocations to move to accommodate the change in shape.
For example, when you bend a paper clip made of aluminum, the movement of the dislocations in the paper clip causes the change in shape. However, the movement of dislocations becomes more difficult at lower temperatures and, as a result, many materials become brittle at low temperatures because dislocations cannot move. This is why the Titanic’s steel hull broke when it struck an iceberg. Elements with high melting temperatures and their alloys take this to the extreme, with many elements remaining brittle up to even 800°C. However, this RMEA bucks the trend and withstands even temperatures as low as liquid nitrogen (-196°C).
This map shows kink bands formed near a crack tip during crack propagation testing (left to right) in the alloy at -196 C. Credit: Berkeley Lab
To understand what was happening inside the remarkable metal, co-researcher Andrew Minor and his team analyzed the strained samples, in addition to the unbent and uncracked control samples, using four-dimensional scanning transmission electron microscopy (4D-STEM) and scanning transmission electron microscopy. electron microscopy (STEM). ) at the National Center for Electron Microscopy, part of Berkeley Lab’s Molecular Foundry.
The electron microscopy data showed that the alloy’s unusual toughness results from an unexpected side effect of a rare defect called a kink band. Buckling bonds form in a crystal when an applied force causes strips of the crystal to collapse on themselves and bend abruptly. The direction in which the crystal bends in these strips increases the force that dislocations feel, making them move more easily. At the bulk level, this phenomenon softens the material (meaning less force needs to be applied to the material when it is deformed). The team knew from previous research that kink bands can easily form in RMEAs, but assumed the softening effect would make the material less tough by making it easier for a crack to propagate through the lattice. But in reality this is not the case.
“We show for the first time that buckling bonds, in the presence of a sharp crack between atoms, actually inhibit the propagation of a crack by spreading the damage away from it, preventing fracture and leading to exceptionally high fracture toughness,” says Cook.
The Nb45Ta25Ti15Hf15 alloy will have to undergo much more basic research and engineering testing before creating something like a jet turbine or a jet turbine SpaceX It is made into rocket nozzles, Ritchie says, because mechanical engineers rightly need an in-depth understanding of the performance of their materials before they can use them in the real world. However, this study indicates that the metal has the potential to build the engines of the future.
Reference: “Kink Bands Promote Exceptional Fracture Resistance in a NbTaTiHf Refractory Medium Entropy Alloy” by David H. Cook, Punit Kumar, Madelyn I. Payne, Calvin H. Belcher, Pedro Borges, Wenqing Wang, Flynn Walsh, Zehao Li, Arun Devaraj, Mingwei Zhang, Mark Asta, Andrew M. Minor, Enrique J. Lavernia, Diran Apelian and Robert O. Ritchie, April 11, 2024, Science.
DOI: 10.1126/science.adn2428
This research was conducted by David H. Cook, Punit Kumar, Madelyn I. Payne, Calvin H. Belcher, Pedro Borges, Wenqing Wang, Flynn Walsh, Zehao Li, Arun Devaraj, Mingwei Zhang, Mark Asta, Andrew M. Minor, Enrique J. Lavernia, Diran Apelian, and Robert O. Ritchie, scientists at Berkeley Lab, UC Berkeley, Pacific Northwest National Laboratory, and UC Irvine, with funding from the Department of Energy (DOE) Office of Science. Experimental and computational analyzes were conducted at the Molecular Foundry and the National Energy Research Scientific Computing Center – both are user facilities of the DOE Office of Science.