A multidisciplinary research team led by scientists at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) (Richland, Washington, USA) and North Carolina State University (NCSU) (Raleigh, North Carolina, USA) combined atomic-scale experiments with theory to crate a tool to predict how high-entropy alloys (HEAs) will behave under high-temperature oxidative environments. The research, published in the journalNature Communications, offers a road map toward rapid design and testing cycles for oxidation-resistant complex metal alloys.
“We are working toward developing an atomic-scale model for material degradation of these complex alloys, which can then be applied to design next-generation alloys with superior resistance to extreme environments for a wide variety of applications such as the aerospace and nuclear power industries,” says Arun Devaraj, co-principal investigator of the study and a PNNL materials scientist specializing in understanding metal degradation in extreme environments. “The goal here is to find ways to rapidly identify medium- to high-entropy alloys with the desired properties and oxidation resistance for your chosen application.”
Scientists and engineers are working to design alloys that can resist extreme environments by experimenting with complex combinations of many metals mixed in equal proportions — such combinations are called multi-principal element alloys or medium- to HEAs. These alloys aim to achieve design goals such as strength, toughness, and resistance to corrosion. More specifically, researchers seek alloys resistant to oxidation, which occurs when metals react with oxygen in the atmosphere. These alloys are typically tested in a “cook-and-look” procedure where alloy materials are exposed to high-temperature oxidation environments to see how they respond.
For their recent experiments, the PNNL/NCSU research team studied the degradation of a HEA with equal amounts of the metals cobalt, chromium, iron, nickel, and manganese (CoCrFeNiMn), also called the Cantor alloy. The research team examined oxide formed on the Cantor alloy using a variety of advanced atomic-scale methods to understand how each element arranges itself in the alloy and the oxide.
They discovered that chromium and manganese tend to migrate quickly toward the surface and form stable chromium and manganese oxides. Subsequently, iron and cobalt diffuse through these oxides to form additional layers.
By adding a small amount of aluminum, they discovered that aluminum oxide can act as a barrier for other elements migrating to form the oxide, thereby reducing the overall oxidation of the aluminum-containing Cantor alloy and increasing its resistance to degradation at high temperatures.
“This work sheds light on the mechanisms of oxidation in complex alloys at the atomic scale,” says Bharat Gwalani, co-corresponding author of the study. Gwalani began the study while a scientist at PNNL and continued the research in his current role as an assistant professor of materials science and engineering at NCSU. He adds that “by understanding the fundamental mechanisms involved, this work gives us a deeper understanding of oxidation across all complex alloys.”
“Right now there are no universally applicable governing models to extrapolate how a given complex, multi-principal element alloy will oxidize and degrade over time in a high-temperature oxidation environment,” says Devaraj. “This is substantial step in that direction.”
The team’s careful analysis revealed some universal rules that can predict how the oxidation process will proceed in these complex alloys. Computational colleagues from NCSU developed a model called the Preferential Interactivity Parameter for early prediction of oxidation behavior in complex metal alloys.
Ultimately, the research team expects to expand this research to develop complex alloys with exceptional high-temperature properties, and to do so very quickly by rapid sampling and analysis. The ultimate goal is to choose a combination of elements that formation of an adherent oxide, according to Devaraj.
“You know oxide formation will happen, but you want to have a very stable oxide that will be protective, that would not change over time, and would withstand extreme heat inside a rocket engine or nuclear reactors,” Devaraj says.
A next step will be to introduce automated experimentation and integrative additive manufacturing methods, along with advanced artificial intelligence, to rapidly evaluate promising new alloys. That project is now getting underway at PNNL as part of the Adaptive Tunability for Synthesis and Control via the Autonomous Learning on Edge (AT SCALE) Initiative.
“That kind of discovery loop for materials discovery will be very relevant for further expanding our knowledge of these novel alloys,” says Devaraj, who also has a joint faculty appointment at the Colorado School of Mines.
Source: PNNL, www.pnnl.gov.