The traces in this image indicate electronic states in a complex alloy; smeared traces show reduced electrical and thermal conductivity. Image: Oak Ridge National Laboratory, US Department of Energy; G. Malcolm Stocks.
The traces in this image indicate electronic states in a complex alloy; smeared traces show reduced electrical and thermal conductivity. Image: Oak Ridge National Laboratory, US Department of Energy; G. Malcolm Stocks.

Designing alloys to withstand extreme environments is a fundamental challenge for materials scientists. Energy from radiation can create imperfections in alloys, so researchers in an Energy Frontier Research Center led by the Department of Energy's Oak Ridge National Laboratory are investigating ways to design structural materials that develop fewer, smaller flaws under irradiation. The key, they now report in Nature Communications, is exploiting the complexity present in alloys made from equal amounts of up to four different metallic elements.

"Chemical complexity gives us a way to modify paths for energy dissipation and defect evolution," said first author Yanwen Zhang, who directs an Energy Frontier Research Center called ‘Energy Dissipation to Defect Evolution’ (EDDE), funded by the US Department of Energy Office of Science. The growing center is nearly 15 months old and brings together more than two dozen researchers with experimental and modeling expertise. EDDE has partners at Oak Ridge, Los Alamos and Lawrence Livermore national laboratories and the universities of Michigan, Wisconsin-Madison and Tennessee-Knoxville.

Radiation can harm spacecraft, nuclear power plants and high-energy accelerators. Nuclear reactions produce energetic particles – ions and neutrons – that can damage materials as their energy disperses, causing the formation of flaws that evolve over time. Advanced structural materials that can withstand radiation are a critical national need for use in nuclear reactors, which currently provide one-fifth of US electricity. Next-generation reactors will be expected to serve over longer lifetimes and withstand higher irradiation levels.

In a reactor, thousands of atoms can be set in motion by one energetic particle that displaces them from sites in a crystal lattice. While most of the displaced atoms return to lattice sites as the energy is dissipated, some do not. In this way, irradiation can damage structural materials made of well-ordered atoms packed in a lattice, even obliterating their crystallinity.

Over the lifetime of a typical light water reactor, all atoms in the structural components can be displaced on average 20 times, and accumulated damage may threaten material performance. To prepare for new reactor concepts, scientists will have to design next-generation nuclear materials able to withstand their atoms being displaced more than 200 times.

Metallic alloys typically comprise multiple phases with one or two dominant elements modified by the addition of other minor elements, but a very different class of materials has recently generated a great deal of interest. In these special alloys, several different types of atom, in equal proportions, distribute randomly in a simple crystal lattice, with high entropy alloys comprising five or more elements being exemplars. Indeed, researchers at Berkeley and Oak Ridge labs have recently shown that some of these alloys, discovered about a decade ago, exhibit exceptional strength and ductility at cryogenic temperatures. In all these alloys, chemical disorder is intrinsic to their behavior.

The goal of the EDDE study was to determine how compositional complexity can lead to differences in heat and electricity conduction, and thus influence defect dynamics at early stages that can affect the robustness of a structural material at later stages. The results revealed how advanced alloys can achieve greatly enhanced irradiation performance through chemical diversity.

The study involved investigating a novel set of alloys containing nickel and equal amounts of from one to three other elements. These alloys included nickel-cobalt, nickel-chromium-cobalt and nickel-chromium-iron-cobalt. The chemical elements, distributed randomly in the crystal lattice, create unique site-to-site, microscopic distortions, but the lattice nonetheless retains its macroscopic crystalline structure.

Integrating theory and experiment, the scientists grew alloy crystals of unrivaled quality. They then calculated the changes to electronic structures and intrinsic transport properties induced by chemical disorder, and confirmed the computational results with experimental measurements of each crystal's electrical resistivity and thermal conductivity. By combining the results from ion irradiation, modeling of defect production, ion-beam analysis and microstructural characterization, they were able to show that defect production and damage accumulation were significantly reduced in these alloys. The findings suggest a link between slow energy dissipation and suppressed defect evolution.

"We observed suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary [alloys]," Zhang said.

A material's electronic band structure determines how well electrons can conduct electricity and heat. In a typical metal, energy dissipates quickly because electrons barely scatter – when an energetic particle hits the perfect atomic ordering of the crystal, the resulting energy wave is free of obstructions and can rapidly propagate, leaving little energy at the collision site. In the willy-nilly atomic arrangement of a multicomponent disordered alloy crystal, however, when an energetic particle hits a lattice atom, the energy encounters obstructions and stays local, and for a longer time.

The EDDE study showed that fewer and smaller defects were produced as the alloy complexity increased. It also showed dramatic improvement in properties related to resistance to radiation damage.

It turns out that just increasing the number of elements (and therefore the disorder, or entropy) in the recipe doesn't necessarily produce the best alloys for targeted functions. Determining what combinations work best depends on aspects such as local structural distortions and the chemical, electronic and magnetic properties of constituent atoms.

With dramatically lower electrical and thermal conductivity than traditional alloys, next-generation alloys based on recipes with high chemical disorder may slow energy dissipation and experience far fewer of the defects that weaken structural materials over time. Evidence that slow energy dissipation can remove some local defects even hints at the possibility of developing self-healing nuclear structural materials.

Further studies are now needed to understand how alloy complexity can tailor material properties. The knowledge gained may spur new design principles for alloys for advanced energy systems. "These insights into defect dynamics at the level of atoms and electrons provide an innovative path forward toward solving a long-standing challenge in structural materials," Zhang said.

This story is adapted from material from Oak Ridge National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.