Researchers have uncovered a new remarkable metal alloy that doesn’t crack at extreme temperatures due to kinking, or bending, of crystals in the alloy at the atomic level.
Unlike most materials, the new alloy displays impressive strength and toughness at extremely hot and cold temperatures, a combination that seemed nearly impossible to achieve until now.
Researchers say the alloy’s resilience to bending and fracture across an enormous range of conditions could open the door to a novel class of materials for next-generation engines that can operate at higher efficiencies.
Perfect crack-resistant metal alloy
The research was carried out by a 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.
The metal alloy is composed of niobium, tantalum, titanium, and hafnium.
In these terms, 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 team discovered the alloy’s surprising properties and then figured out how they arise from interactions in the atomic structure.
As per the team of researchers, the alloy is from a new class of metals known as refractory high or medium entropy alloys (RHEAs/RMEAs).
Majority of the metals used in commercial or industrial applications are made by mixing one main metal with small quantities of other elements.
However, RHEAs or RMEAs are made by mixing near-equal quantities of metallic elements with very high melting temperatures, which gives them unique properties.
The team has investigated these alloys for several years because of their potential for high-temperature applications.
‘Shocked’ at exceptionally high 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 PhD student in Ritchie’s lab.
He added that the ability to optimize further the materials we currently use at high temperatures has been exhausted. Therefore, there’s a need for “novel metallic materials. That’s what this alloy shows promise in.”
Co-corresponding author Punit Kumar, a postdoctoral researcher in the group, said that their earlier research on other RHEAs or RMEAs had shown they were very strong but possessed extremely low fracture toughness.
“Which is why we were shocked when this alloy displayed exceptionally high toughness,” Kumar said.
According to Cook, most RMEAs have a fracture toughness less than 10 MPavm. The best cryogenic steels, specially engineered to resist fracture, are about 20 times tougher than these materials.
However, this new alloy beat even the cryogenic steel, clocking in at over 25 times tougher than typical RMEAs at room temperature.
The scientists also evaluated strength and toughness at five temperatures total: – -320.8°Fahrenheit (196°C) (the temperature of liquid nitrogen), 77°Fahrenheit (25°C) (room temperature), 1472°Fahrenheit (800°C), 1742°Fahrenheit (950°C), and 2192°Fahrenheit (1200°C). The last temperature is about 1/5 the surface temperature of the sun.
Engines of the future?
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.
“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 alloy will undergo a lot of testing and research before anything like a jet plane turbine or SpaceX rocket nozzle is made from it.
However, this study indicates that the metal has potential to build the engines of the future.
