The early 2000s witnessed a paradigm shift in alloy design in which three or more elements were mixed in approximately equal proportions. These hybrid materials—known as multiprincipal element alloys (MPEAs) or high-entropy alloys—exhibit exciting properties that provide a higher level of performance than their single-element counterparts.
Advances in MPEA research led to the development of refractory MPEAs in 2010. Refractory alloys are made from a group of nine metal elements on the periodic table that are highly resistant to heat and wear. According to a new paper published in Science magazine, refractory MPEAs “are attractive candidates for use at extremely high temperatures associated with many technology applications.”
The interdisciplinary team who co-authored the paper included researchers from UC Santa Barbara (Santa Barbara, California, USA), the University of Kentucky (Lexington, Kentucky, USA), the U.S. Naval Research Library (Washington, DC, USA), and the U.S. Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio, USA).
“While compositionally complex alloys have long been of interest to us, progress in exploring the large compositional space has been slow,” says Tresa Pollock, materials professor at UC Santa Barbara and one of the co-authors of the paper. “With the MPE project, we brought together a team that used emerging computational, machine learning, and experimental tools, which have enabled us to uncover new behaviors and rapidly explore new compositional domains. The very high melting points of the refractory materials of interest have made them notoriously difficult to fabricate and study in the past, but our new approaches, combined with the possibility of 3D printing, completely change the landscape.”
In their Science paper, the researchers suggest a method for accurately predicting which refractory MPEAs might have valuable properties. One key property is an alloy’s ability to deform—meaning to be molded or bent—without cracking and while maintaining material integrity under excessive load and high heat.
“On the atomic level, a material deforms, or changes its shape, as a result of moving atoms,” says Fulin Wang, postdoctoral researcher at UC Santa Barbara and one of the paper’s co-authors. The line separating the regions where atoms have moved and where they have not is called a dislocation—and by understanding how and where atoms move, researchers can gain greater understanding of the deformation properties of a given material.
While researchers have developed theories as to why a particular alloy had desirable properties, Wang says that “there is a lack of experimental evidence to inform some critical elements of the theory. When I started working on this project, my immediate question was, what’s special about the MPEAs compared to traditional alloys? Since we are interested in mechanical properties, we focus on the dislocations.”
In their study, the researchers used electron microscopy to investigate the configurations of dislocations and determine why certain alloys have their desirable properties. Through atomistic simulations developed by Irene Beyerlein, they determined that the random field of different elements unlock multiple pathways for dislocation movements that are not available in conventional alloys.
“For conventional dislocations, the force to break atomic bonds at a dislocation is single valued because all the atoms are alike,” says Beyerlein, a materials professor at UC Santa Barbara. “For the MPE dislocation, this force cannot be deterministic. The structure of an MPE dislocation becomes redefined as it tries to move through randomly changing atomic environments.”
“With our atomistic calculations, we took the approach of expecting the unexpected and probed not only the usual modes but additional higher modes of slip, typically neglected in the literature to date,” she adds. “We also performed thousands of calculations, which exposed just how widely varying this critical dislocation force can be and how favorable alternative higher modes of slip are.”