Scientists have discovered an impressive metal alloy that is resistant to cracking at extreme temperatures because the crystals in the alloy bend at the atomic level. This unique alloy maintains its shape and prevents cracking at both high and low temperatures, making it a potential candidate for challenging applications such as high-efficiency aerospace engines.
fnium has surprised materials scientists with its remarkable durability and strength in both extreme hot and cold temperatures. This combination of properties was previously thought to be nearly impossible to achieve. Strength refers to how much force a material can withstand before it is permanently deformed, while toughness is its resistance to fracturing. The alloy’s ability to resist bending and fracture across a wide range of conditions could lead to the development of a new class of materials for next-generation engines that can operate at higher efficiencies.m, led by Robert Ritchie at Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, in collaboration with the groups led by professors Diran Apelian at UC Irvine and Enrique Lavernia at Texas A&M University, found surprising properties of the alloy and how they arise from interactions in the atomic structure. Their work is detailed in a study published on April 11, 2024 in Science.
“The efficiency of converting heat to 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 structuralmaterials which need to endure high temperatures,” stated David Cook, a Ph.D. student in Ritchie’s lab and the first author of the study. “We have reached the limit of how much we can improve the materials we currently use at high temperatures, and there is a great need for new metallic materials. That’s where this alloy shows potential.”
The alloy studied belongs to a new category of metals called refractory high or medium entropy alloys (RHEAs/RMEAs). While most commercial or industrial metals are alloys made of one main metal mixed with small amounts of other elements, RHEAs and RMEAs are created by mixing nearly equal amounts of metallic eleThe team has been studying alloys with high melting temperatures for a while, and they have found that these materials are very strong but usually have low fracture toughness. However, a specific alloy they have been investigating displayed unexpectedly high toughness. This has puzzled the researchers because it goes against the typical characteristics of most high-temperature alloys.The niobium, tantalum, titanium, and hafnium (Nb45Ta25Ti15Hf15) RMEA alloy is extremely brittle, with a strength of less than 10 MPa√m. In comparison, the best cryogenic steels, specifically designed to resist fracture, are about 20 times tougher. However, the RMEA alloy was found to be over 25 times tougher than typical RMEAs at room temperature, even outperforming cryogenic steel.
When evaluating the performance of engines, it’s important to consider a range of temperatures. The scientists tested the strength and toughness of the alloy at five temperatures: -196°C (the temperature of liquid nitrogen), 25°C (room temperature), and 800°C.
The alloy was tested at extreme temperatures of 25°C, 950°C, and 1200°C. The highest temperature is equivalent to about 1/5 of the sun’s surface temperature.
Researchers discovered that the strength of the alloy was at its peak in cold temperatures and slightly decreased as the temperature increased. However, it still maintained impressive strength across the entire temperature range. The fracture toughness, which measures the force required to propagate an existing crack in a material, remained high at all temperatures.
Revealing the atomic structure
Most metallic alloys are crystalline, indicating that the atoms are arranged in a repeating unit inside the material.s. However, no crystal is perfect, as they all contain imperfections. The primary imperfection that occurs is known as a dislocation, which is an incomplete plane of atoms within the crystal. When force is exerted on a metal, it causes numerous dislocations to relocate in order to adjust to the change in shape. For instance, when you bend an aluminum paper clip, the movement of dislocations inside the clip accommodates the change in shape. Nevertheless, the movement of dislocations becomes more challenging at lower temperatures, causing many materials to become brittle in cold conditions due to the inability of dislocations to relocate. This is why the steel hull of the Titanic, for example, became brittle in the cold temperatures of the ocean.The Titanic broke when it struck an iceberg. Substances with high melting points and their mixtures take this to an extreme, with many remaining fragile even at temperatures as high as 800°C. However, this RMEA goes against the norm, enduring breaking even at temperatures as low as liquid nitrogen (-196°C).
To comprehend what was occurring inside the exceptional metal, co-researcher Andrew Minor and his team analyzed the strained samples, along with unbent and uncracked control samples, using four-dimensional scanning transmission electron microscopy (4D-STEM) and scanning transmission electron microscopy (STEM) at the National Center for Electron Microscopy.The research was a part of Berkeley Lab’s Molecular Foundry. The electron microscopy data showed that the alloy’s unique toughness is due to an unexpected side effect of a rare defect known as a kink band. Kink bands form in a crystal when an applied force causes strips of the crystal to collapse on themselves and abruptly bend. The direction in which the crystal bends in these strips increases the force that dislocations feel, causing them to move more easily. This phenomenon causes the material to soften on a bulk level, meaning that less force is required to deform the material. The team had prior knowledge from past research that kink bands…The formation of RMEAs was believed to make the material less tough by softening it, but research has shown that this is not the case. In fact, kink bands resist crack propagation by distributing damage away from it, preventing fracture and leading to high fracture toughness. According to Cook, “We show, for the first time, that in the presence of a sharp crack between atoms, kink bands actually resist the propagation of a crack by distributing damage away from it, preventing fracture and leading to extraordinarily high fracture toughness.” Further research and engineering testing will be necessary for the Nb45Ta25Ti15Hf15 alloy before it can be used in jet propulsion.Ritchie stated that titanium is a popular choice for building turbine blades and SpaceX rocket nozzles, as mechanical engineers need a deep understanding of the material’s performance before using it in real-world applications. However, this study suggests that titanium has the potential to be used in the construction of future engines.
The research involved scientists from various institutions, including Berkeley Lab, UC Berkeley, and the Pacific Northwest National Laboratory, such as 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.The research was a collaborative effort between researchers from Purdue University, UC Berkeley, and UC Irvine, funded by the Department of Energy (DOE) Office of Science. The Molecular Foundry and the National Energy Research Scientific Computing Center, both of which are DOE Office of Science user facilities, were used for experimental and computational analysis.
Journal Reference:
- 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, Robert O. Ritchie. “Kink bands promote exceptional fracture resistance”
- Title: “Enhanced mechanical properties in a NbTaTiHf refractory medium-entropy alloy”. Published in Science in 2024; 384 (6692): 178. DOI: 10.1126/science.adn2428