HIGHLIGHTS
• Radiation attacks the crystalline structure of a metal producing a cascading effect that eventually reduces the metal’s mechanical and physical properties.
• In a study of a thin film of platinum, researchers found that grain boundaries, where different crystal orientations come together, can be moved causing the deterioration in the properties of the metal. 
• The grain boundary movement is caused by shortrange effects and longer-range interaction. 

Nuclear power is becoming a more attractive power generation option as interest increases in finding ways to reduce carbon dioxide and other greenhouse gas emissions. But there are challenges in further utilizing nuclear power.

Fission has been the main source of nuclear power, but research continues to find an approach for making fusion self-sustainable. In a previous TLT article,¹ an update was provided on the progress made to determine how to have nuclear fusion produce more energy than is required to initiate the process. One of the requirements for self-sustaining nuclear fusion is to have the fuel consisting of the hydrogen isotopes, deuterium and tritium, ignite at incredibly high temperatures to generate a burning plasma state. This objective has been realized in recent experiments conducted in 2021.

In working with nuclear power, radiation is present and has been found to weaken the metal alloys used in the design of the reactors. Dr. Douglas Medlin, material scientist at Sandia National Laboratories in Livermore, Calif., says, “The damage that radiation inflicts on metal alloys is due to a number of competing processes that are exceptionally complicated on the atomic and microstructural scale.”

Rémi Dingreville, Center for Integrated Nanotechnologies scientist at Sandia National Laboratories in Albuquerque, N.M., adds, “Radiation can make high energy collisions on metals creating significant movements of metal atoms, which can produce defects in the crystal structure. This aging process leads to short-term and long-term effects that are still not completely understood. These collisions occur over tiny fractions of a second, but the effects may not start to be seen for some time if the metal is situated in a nuclear reactor and constantly exposed to radiation.”

Medlin claims that the impact of radiation on metal is similar to breaking in the game of pool where a cue stick is used to hit balls organized in the shape of a triangle. He says, “When the break takes place, balls move everywhere around the pool table. On the atomic level, the highly ordered crystalline state of a metal is disrupted by high energy ions that will push atoms out of the lattice creating defects that can negatively affect the metal’s physical properties.”

Dingreville terms the process as playing pool on “steroids.” He says, “The outcome of radiation attacking the crystalline structure of a metal is the generation of a cascading effect where displaced atoms will collide with other atoms in nearby crystalline lattices in a propagating effect that further reduces the metal’s physical and mechanical properties. The collisions between high energy radiation and atoms can be so intense that local melting is created due to a spike in temperature.”

This exceptionally dynamic process cannot be readily accessed by experimental means, which leads to the need for modeling. Dingreville and Medlin were part of a team that combined experimentation with modeling to further understand how radiation weakens metal at the microstructural scale.

Ion-irradiated platinum
The researchers studied irradiation in a thin film of platinum. Dingreville says, “Platinum is a model system for face-centered-cubic metals and is a noble metal meaning it will not oxidize when subjected to radiation. Medlin adds, “Platinum is a well understood metal, and with all of the other complications we face in studying radiation, this is one issue we did not have to worry about.”

The researchers irradiated platinum using gold (Au⁴⁺) that is produced using an in situ ion irradiation transmission electron microscope (see Figure 2). Dingreville says, “The gold irradiation acts as a heavy ion mass that emulates neutron radiation but can much more easily be detected. The experimental setup involves the use of an electron beam that is aimed at the sample concurrently with the heavy ion radiation. This enables us to image what is occurring to the platinum crystal in real time.”


Figure 2. The in situ ion irradiation transmission electron microscopes, shown in this figure, were used to irradiate platinum with gold (Au⁴⁺). Figure courtesy of Sandia National Laboratories.

The focal point for the researchers in this study is crystalline gain boundaries. Medlin explains, “Grain boundaries are locations where different crystal orientations come together. During irradiation, displaced atoms tend to move to grain boundaries making them sinks. Eventually, these atoms are then able to exit the metal. The net effect is that grain boundaries can be moved leading to a deterioration in metal properties such as strength.”

The researchers conducted the study using a thin film of platinum with a thickness of 18 nanometers. Medlin says, “The experimentation was a bit tricky because the researchers initially used a different transmission electron microscope to study the microstructure, then move the sample to the in situ ion irradiation microscope and then move it back to study the effects that the radiation had on the platinum sample.”

The modeling phase of the study gave the researchers an opportunity to study individual processes simplifying this complicated process. Dingreville says, “Modeling gives us a way to study individual issues such as local areas of displacement, long-range elastic interaction of defects and long-range coordination of boundary motion. We can then combine several of these phenomena to get a better idea about what happens to the metal’s microstructure.”

The researchers hypothesize that movement of the grain boundaries during irradiation occurs through the coupling of several mechanisms at multiple length scales. There are short-range effects, which include local step nucleation, and longer-range interactions that entail diffusion of ion-induced point defects to the boundary.

The researchers are looking at several different future objectives. Medlin says, “We have seen what happens at a simple grain boundary, but now we need to evaluate more general grain boundaries to determine if the same short and long-range effects occur. The big picture is to generalize a more complicated system.”

Dingreville says, “We are looking to eventually develop a more comprehensive model that can explain how the microstructure ages when irradiated.”

Additional information can be obtained from a recent article² or by contacting Troy Rummler, media relations specialist for Sandia National Laboratories, at trummle@sandia.gov.
 
REFERENCES
1. Canter, N. (2022), “Self-sustaining fusion,” TLT, 78 (5), pp. 20-21. Available here.
2. Barr, C., Chen, E., Nathaniel, J., Lu, P., Adams, D., Dingreville, R., Boyce, B. Hattar, K. and Medlin, D. (2022), “Irradiation-induced grain boundary facet motion: In situ observations and atomic scale mechanics,” Science Advances, 8 (23), DOI:10.1126/sciadv.abn0900.