This alloy is kinky | Laboratory manager

Key learning points:

• Exceptional Properties: A new metal alloy consisting of niobium, tantalum, titanium and hafnium exhibits remarkable strength and toughness over a wide temperature range, promising for improved engine efficiency.
• Research and Discovery: A collaborative research team discovered the alloy’s surprising atomic-level properties and mechanisms, challenging the traditional understanding of materials at high temperatures.
• Insights at the atomic level: Advanced microscopy revealed that the alloy’s exceptional toughness comes from the formation of kink bands in the crystal structure, which inhibits crack propagation by redistributing damage.

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 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,” says first author David Cook, a PhD student in Ritchie’s lab. “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.

“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. That’s why we were shocked when this alloy showed exceptionally high toughness,” said co-corresponding 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).

To understand what was happening inside the remarkable metal, co-researcher Andrew Minor and his team analyzed the strained samples alongside unbent and uncracked control samples, using four-dimensional scanning transmission electron microscopy (4D-STEM) and scanning transmission electron microscopy ( VOTE). 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,” said Cook.

The Nb₄₅Ta₂₅Ti₁₅Hf₁₅ alloy will need to undergo much more fundamental research and engineering testing before being made into something like a jet turbine or SpaceX rocket nozzle, Ritchie said, because mechanical engineers rightly need a deep understanding of how their materials perform before using them in the real world. However, this study indicates that the metal has the potential to build the engines of the future.

– This press release was originally published on the Berkley Lab website and has been edited for style and clarity

AI use disclaimer: Parts of this text, such as the subtitle and main conclusions, may have been generated using AI. All AI-generated content is fact-checked and edited for clarity.