This alloy is kinky

Researchers have uncovered a remarkable metal alloy that won’t crack at extreme temperatures due to kinking, 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 seemed so far to be nearly impossible to achieve. In this context, strength is defined as how much force a material can withstand before it is permanently deformed from its original shape, and toughness is its resistance to fracturing (cracking). The alloy's resilience to bending and fracture across an enormous range of conditions could open the door for a novel class of materials for next-generation engines that can operate at higher efficiencies.

The team, 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, discovered the alloy's surprising properties and then figured out how they arise from interactions in the atomic structure. Their work is described in a study that was published 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 structural materials which must withstand it," said first author David Cook, a Ph.D. student in Ritchie's lab. "We have exhausted the ability to further optimize the materials we currently use at high temperatures, and there's a big need for novel metallic materials. That's what this alloy shows promise in."

The alloy in this study is from a new class of metals known as refractory high or medium entropy alloys (RHEAs/RMEAs). Most of the metals we see in commercial or industrial applications are alloys made of one main metal mixed with small quantities of other elements, but RHEAs and RMEAs are made by mixing near-equal quantities of metallic elements with very high melting temperatures, which gives them unique properties that scientists are still unraveling. Ritchie's group has been investigating these alloys for several years because of their potential for high-temperature applications.

"Our team has done previous work on RHEAs and RMEAs and we have found that these materials are very strong, but generally possess extremely low fracture toughness, which is why we were shocked when this alloy displayed exceptionally high toughness," said co-corresponding author Punit Kumar, a postdoctoral researcher in the group.

According to Cook, most RMEAs have a fracture toughness less than 10 MPa√m, which makes them some of the most brittle metals on record. The best cryogenic steels, specially engineered to resist fracture, are about 20 times tougher than these materials. Yet the niobium, tantalum, titanium, and hafnium (Nb45Ta25Ti15Hf15) RMEA alloy was able to beat even the cryogenic steel, clocking in at over 25 times tougher than typical RMEAs at room temperature.

But engines don't operate at room temperature. The scientists evaluated strength and toughness at five temperatures total: -196°C (the temperature of liquid nitrogen), 25°C (room temperature), 800°C, 950°C, and 1200°C. The last temperature is about 1/5 the surface temperature of the sun.

The team found that the alloy had the highest strength in the cold and became slightly weaker as the temperature rose, but still boasted impressive figures throughout the wide range. The fracture toughness, which is calculated from how much force it takes to propagate an existing crack in a material, was high at all temperatures.

Unraveling the atomic arrangements

Almost all metallic alloys are crystalline, meaning that the atoms inside the material are arranged in repeating units. However, no crystal is perfect, they all contain defects. The most prominent defect that moves is called the dislocation, which is an unfinished plane of atoms in the crystal. When force is applied to a metal it causes many dislocations to move to accommodate the shape change. For example, when you bend a paper clip which is made of aluminum, the movement of dislocations inside the paper clip accommodates the shape change. 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 steel hull of the Titanic fractured when it hit an iceberg. Elements with high melting temperatures and their alloys take this to the extreme, with many remaining brittle up to even 800°C. However, this RMEA bucks the trend, withstanding snapping even at temperatures as low as liquid nitrogen (-196°C).

To understand what was happening inside the remarkable metal, co-investigator Andrew Minor and his team analyzed the stressed samples, alongside 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, part of Berkeley Lab's Molecular Foundry.

The electron microscopy data revealed that the alloy's unusual toughness comes from an unexpected side effect of a rare defect called 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. On the bulk level, this phenomenon causes the material to soften (meaning that less force has to be applied to the material as it is deformed). The team knew from past research that kink bands formed easily in RMEAs, but assumed that the softening effect would make the material less tough by making it easier for a crack to spread through the lattice. But in reality, this is not the case.

"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," said Cook.

The Nb45Ta25Ti15Hf15 alloy will need to undergo a lot more fundamental research and engineering testing before anything like a jet plane turbine or SpaceX rocket nozzle is made from it, said Ritchie, because mechanical engineers rightfully require a deep understanding of how their materials perform before they use them in the real world. However, this study indicates that the metal has potential to build the engines of the future.

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Sustainable energy for aviation: What are our options?

Scientists and industry leaders worldwide are looking for answers on how to make aviation sustainable by 2050 and choosing a viable sustainable fuel is a major sticking point. Phil Ansell, aerospace engineer at the University of Illinois Urbana-Champaign, took a full inventory of the options to make a data-driven assessment about how they stack up in comparison. He reviewed over 300 research projects from across different sectors, not just aerospace, to synthesize the ideas and draw conclusions to help direct the dialogue about sustainable aviation toward a permanent solution.

Ansell said several key energy carriers emerged, including bio jet fuel pathways for synthetic kerosene, power-to-liquid pathways for synthetic kerosene, liquid hydrogen, ammonia, liquid natural gas, ethanol, methanol, and battery electric systems. Ansell compared each of them to conventional fossil-derived aviation turbine fuel.

For each of the alternate fuels Ansell addressed factors such as how their material properties impact aircraft performance and fuel handling, emissions, cost and scalability, and resource and land requirements, as well as social impacts, which can be difficult to measure.

"Let's face it, if we want to do this at scale, we need all three pillars of the environmental, economic, and societal contributions, to make that energy carrier sustainable, and every stakeholder in the value chain sees the challenges differently," Ansell said. "Because the production and infrastructure costs required to adopt an alternative fuel source are significant, people think we can only pick one, the biggest contenders being bio jet fuel and hydrogen," Ansell said. "But the choice doesn't have to be mutually exclusive. For example, we can use hydrogen to produce synthetic aviation fuels like the power-to-liquid pathway or use biomass to produce hydrogen."

Ansell admitted this is not what he typically studies, but his research and teaching areas in aircraft design and aerodynamics must consider where the energy will come from to make flight possible. So, for any fuel associated with a bio aspect Ansell had to look at the stresses it might create for crops.

"I leaned on a lot of the observations from the community, especially for the land use change question," he said. "It is so driven case by case. Making a broad assessment doesn't do it justice, because land use changes depend on their location."

Ansell said he has been working with hydrogen for several years and battery/electric systems before that, so he needed to remain objective and all the data to drive the conclusion.

"About eight years ago, I realized that battery systems are a pie-in-the-sky solution. The technology challenge is insurmountable. The weight and volume required for batteries is too difficult to close. I think my biases were from the fact that I've been studying hydrogen for a long time, and I think it has real potential. That's one of the conclusions I arrived at from the data, and I think I would have learned that independently."

Ansell said hydrogen presents infrastructural and integration challenges, unique to the aircraft platform and unique to the cryogenic handling of fuel on aircraft.

"The technological challenges of hydrogen are very solvable. And I can say that with confidence because we've done it as a society." He referred to Tupolev 155, a commercial scale aircraft which was flown by the former Soviet Union with liquid hydrogen in the 1980s on a relevant airframe. Even earlier experimental studies were conducted by NASA. "It will take a bit longer to implement at scale, but it's doable."

In the study, Ansell examined numerous options to produce biofuel from just about anything, from municipal waste to seaweed and algae.

"Basically, anything that you can burn, create energy from, decompose, can be turned into a jet fuel. We've already been using corn to produce ethanol. But if you were to take corn, ferment it, then turn that ethanol into jet fuel, you now have lost the ability to feed people or animals that corn. This is one of the challenges of all first-generation biofuels."

He said people are trying to use the stover, the parts of a corn plant left on the ground after harvesting to make fuel. Corn stover is full of sugar but it's difficult to extract.

Why is an aerospace engineer studying feedstocks?

"I want to be able to know enough to interact with scientists who are tackling these options," Ansell said. "And it's important that the aviation community understands where the challenges exist. We need to draw the lines between the aircraft as a system, the aircraft operating in an airspace, and how that connects with energy. With a goal of net zero CO2 by 2050, I want the aviation community to recognize how big of a task this is.

"As a society, we are often attentive to challenges that are right in front of us, with limited foresight to plan for the future. So even as it pertains to the greenhouse gas effect, we still don't have an appreciation for the long-term sense of the damage that produces. We have other immediate concerns to spend money on. But without that foresight, we are going to struggle and regret decisions not to make investments and not to take seriously these aviation sustainability challenges when we still had time."Again, Ansell stressed that it might not need to be a one-fuel-fits-all solution. In fact, countries may need different strategies, different rates of implementation, and adoption of renewables, based on their own resources. For example, Denmark doesn't have as much land as the U.S. and so is making great use of offshore wind turbine platforms.

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Digital Twin technology a 'powerful tool' but requires significant investment, say experts

Healthcare and aerospace experts at King's College London, The Alan Turing Institute, the University of Cambridge, and the Oden Institute for Computational Engineering and Sciences at UT Austin in Texas have said advances in digital twin technology make it a powerful tool for facilitating predictive and precision medicine and enhancing decision-making for aerospace systems. Their opinion piece was published today in Nature Computational Science.

When applied to healthcare, the digital twin, a virtual version of real-life objects that can be used to predict how that object will perform, could predict how a patient's disease will develop and how patients are likely to respond to different therapies.

It is also of huge benefit in aerospace, where, for example, the technology will be needed to monitor and control thousands of drones, ensuring that they are maintained, have efficient and safe flight plans and can automatically adapt to changes in conditions, such as weather, without the need for human interaction.

However, current digital twins are largely the result of bespoke technical solutions that are difficult to scale.

The authors say that these use cases place new demands on the speed, robustness, validation, verification and uncertainty quantification in digital twin creation workflows.

Achieving digital twins at scale will require a drastic reduction in technical barriers to their adoption.

Lead author Professor Steven Niederer from the School of Biomedical Engineering & Imaging Sciences, King's College London said in medicine, the digital twin will allow testing of a large number of therapies on a patient to identify the best option for that individual with their unique disease.

"There is, however, a need to invest in the underlying theory for how to make models, how to run these models at speed and how to combine multiple models together to ensure that they run as expected," he said.

"We also need to further develop the mathematics of how we create digital twins from patient data, how we measure uncertainty in patient data, and how to account for uncertainty in the model in predictions. These are all things which need further investment."

"We are making more measurements in patients in the hospital and from remote monitoring and we need to develop methods for rapidly and robustly combining this patient information into a digital twin to provide a single representation of the patient."

"Another challenge is how do we get better at predicting how the heart will operate under extreme conditions. We often want to predict when the heart will fail, however, we only have information that is obtained from them under normal operating procedures."

Similarly from an aerospace perspective, Dr Karen Willcox, Director, Oden Institute for Computational Engineering and Sciences at The University of Texas at Austin, said in order to be useful, the digital twin must be predictive and quantify uncertainty.

"Digital twins must be able to analyze 'what if' scenarios and issue predictions about the future, in order to guide decision making to manage a physical asset. That means the digital twin cannot be built on data alone, it needs to include both data and predictive models," she said.

Though open challenges remain, the value of digital twins is clear. And they don't have to be perfect to be valuable. "Even with existing limitations, digital twins are providing valuable decision support in many different application areas," said Willcox. "Ultimately we would like to see the technology used in every engineering system. At that point we can start thinking not just about how a digital twin might change the way we operate the system, but also how we design it in the first place."

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