9/23/2026 Michael O'Boyle
Donald W. Hamer Professor Pascal Bellon, graduate student Nicholas Saunders and Professor Emeritus Robert Averback used atomistic simulations to show that "nano-necklaces" — chains of bead-like precipitates that form along defect lines in irradiated alloys — arise through self-organization, driven by a competition between thermal diffusion and irradiation-induced advection. The work was published in Physical Review Letters.
Written by Michael O'Boyle
Illinois Grainger Engineering researchers found that irradiated metal alloys undergo self-organization, forming “necklaces” of bead-like structures along defect lines that stabilize the alloy against brittleness. The result could have important implications for materials engineering in high-radiation environments.
When metallic alloys are subjected to radiation, lines of nanoscale beads called “nano-necklaces” can form around material defects. Curiously, these structures can strengthen the alloy against brittleness. This result could influence materials engineering in high-radiation environments such as nuclear reactors, but a quantitative theory is needed for the principles to be applied.
Researchers at the University of Illinois Urbana-Champaign’s Grainger College of Engineering have recently published a computational model in Physical Review Letters. They employed atomistic simulations to demonstrate that nano-necklaces can arise from the combination of two effects: diffusion, in which random noise causes precipitation, and advection, which prevents the precipitates from growing. In doing so, the researchers demonstrate that the nanostructures are an instance of self-organization, a phenomenon central to fundamental research in nonequilibrium systems.
“In general, irradiation makes materials more brittle, but recent experiments from our colleagues have shown that, when these necklace structures form, the material’s brittleness is reduced,” said Pascal Bellon, Illinois Grainger Engineering materials science and engineering professor and project lead. “My group’s theoretical work establishes a map of the self-organized structures, from necklaces to tubes, as a function of chemistry and irradiation conditions, a discovery with relevance in both basic science and applied engineering.”
Nanostructures arising from irradiation were first identified in irradiated reactor pressure vessel steels. Recently, the experimental work of Illinois Grainger Engineering mechanical science and engineering professor Janelle Wharry showed that neutron radiation drives the impurities in ferritic steel to form beads along the boundaries of structural defects, increasing the steel’s ductility — a measure of a material’s resilience against brittleness — from what it would have been in the beads’ absence.
“From our conversations with professor Wharry and other colleagues, what was missing was an understanding of how exactly material properties change with the presence and morphology of bead patterns,” Bellon said. “What impact does changing the size and separation of the beads have? How can the impact on material properties be controlled and optimized in the presence of radiation? It is a very new concept, so there is no theory right now. That is the gap we wanted to fill.”
The experimental work indicated that irradiation causes impurities in the steel to precipitate and become mobile. Bellon’s research group performed lattice kinetic computer simulations based on this fact, revealing that a competition between thermal diffusion and irradiation-induced advection causes the precipitates to accumulate at the defect boundaries and form nano-necklaces. This allowed the researchers to study how the beads change with material properties. Most interestingly, the revelation that nano-necklaces spontaneously form under an external influence indicated that the system undergoes self-organization.
“Self-organization is a very broad subject in which structures form without any apparent ‘planning’ when an underlying system is driven out of equilibrium,” Bellon said. “The example I like to use is mixing oil and water to make salad dressing. On their own, the two fluids separate. When you shake the bottle, though, the oil precipitates and forms tiny structures: droplets that mix into the water. Something similar is happening in the irradiated steel: the neutron radiation is effectively ‘shaking’ the alloy to precipitate the impurities that accumulate into beads.”
Bellon is hopeful that this new result could influence material design and engineering in high-radiation environments such as nuclear reactors and outer space. Understanding how nanostructures can help stabilize materials with defects can improve reliability and longevity in irradiated structures. In addition, the occurrence of self-organization in this context opens a new scientific front in which nonequilibrium dynamics can be exploited in applied engineering design.
Important contributions were made by graduate student Nicholas Saunders, who performed the simulations and part of the analysis, and Illinois Grainger Engineering materials science and engineering Professor Emeritus Robert Averback, who was crucial to the conceptual analysis and organization of the results.
The study, “Self-organized defect-phases along dislocations in irradiated alloys,” is available online. DOI: doi.org/10.1103/wwnl-8ds1
Support was provided by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.
Illinois Grainger Engineering affiliations
Pascal Bellon is a professor of materials science and engineering in the Department of Materials Science and Engineering. He is affiliated with the Materials Research Laboratory. He holds the Donald W. Hamer Professor appointment.
Robert S. Averback is a Professor Emeritus of materials science and engineering in the Department of Materials Science and Engineering. He holds the Donald W. Hamer Professor appointment.