In our daily lives, we often have to make trade-offs between good, fast, and cheap. Material scientists also face trade-offs in their work, but sometimes they discover a rare material that allows them to have it all.
Scientists in the U.S. have developed a new metal alloy that is extremely strong and tough, and can keep these properties at very low and high temperatures, which was thought to be nearly impossible to achieve.
The new alloy, made of niobium, tantalum, titanium, and hafnium, is perfect for high-performance aerospace and other advanced technological uses. engines Unprecedented Material Toughness
“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 at the Lawrence Berkeley National Laboratory.
“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.”
Alloys are made by melting and combining two or more metals together. Most alloys are made of a base metal with other metals added to the molten mixture. For example, ordinary steel is over 99% iron, with the remaining being carbon.
Unlike typical alloys, this new alloy belongs to a class called refractory high entropy alloys (RHEAs) or medium entropy alloys (RMEAs). These alloys are made up of nearly equal amounts of elements, each with very high melting points, which gives them unique properties. elements Refractory high entropy alloys (RHEAs) and medium entropy alloys (RMEAs) are known for their strength but usually have low fracture toughness, making them prone to breaking under stress. However, this particular alloy (Nb45Ta25Ti15Hf15) breaks these expectations by having a fracture toughness over 25 times greater than typical RMEAs at room temperature.
The researchers tested the alloy at various temperatures, from -196°C to 1200°C, and found that it maintained its strength and fracture resistance in all conditions. This level of durability is unmatched by any other conventional material.
A ‘Defect’ That Adds Toughness This alloy defies expectations in its behavior due to a microstructural mechanism involving a defect called a kink band, which was revealed through advanced imaging techniques including four-dimensional scanning transmission electron microscopy (4D-STEM) at Berkeley Lab’s Molecular Foundry..
Kink bands are sudden bends in the crystal lattice that occur under stress and typically weaken a material by making it easier for cracks to spread. However, in this alloy, kink bands actually work to distribute stress and hinder crack propagation, preventing fractures.
To understand why this alloy behaves differently, the researchers used advanced imaging techniques like four-dimensional scanning transmission electron microscopy (4D-STEM) at Berkeley Lab’s Molecular Foundry. These tests showed that the alloy’s toughness is due to a defect known as a kink band.
“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 in a press release.
This image demonstrates kink bands formed near a crack tip during crack propagation testing in the alloy at -196 degrees Celsius. Credit: Berkeley Lab.
Kink bands are sudden bends in the crystal lattice that occur under stress. Normally, they weaken a material by making it easier for cracks to propagate. However, in this alloy, kink bands act as a beneficial factor by distributing stress and hindering crack propagation, thereby preventing fractures.
“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.
Although the results look good, more research and testing are needed before the alloy can be used in things like jet turbine engines or spacecraft parts such as a rocket booster’s nozzle.
The results were published in the journal Science.
