Expanding the Materials Performance Envelope by Controlling Interfaces

The performance envelope of materials limits much of what technology can accomplish, for example in energy applications: from steam generators and batteries to high-voltage power lines and nuclear reactors, better materials translate into cleaner, safer, and cheaper energy. So how can we tailor matter to our needs, rather than being constrained by what’s available? One approach is to harness a ubiquitous, though hitherto underutilized, component found in most engineering materials: interfaces.

Although interfaces comprise less than 0.01% of the volume of polycrystalline solids with micron-sized grains, they play a decisive role in determining their mechanical, electrical, thermal, and diffusion properties^{1, 2}. Textbooks often portray interfaces schematically as two-dimensional and compositionally abrupt. This simplification is convenient and often necessary, but fundamentally false: at the atomic level, interface structure is inherently 3-D, often complex, and occasionally quite beautiful (Fig. 1).

Fig. 1. A 30° {111} twist grain boundary in Cu appears merely planar when viewed edge-on (a), but reveals a remarkably complex structure when viewed in-plane (b). Atoms are colored by their local pressure.

Interface structure governs interface properties. For example, Coble creep—a form of slow, low-temperature deformation—arises from the competition between interface diffusion and interface sliding. Both of these behaviors are highly sensitive to interface structure as well as the properties of interfacial point defects. Due to their susceptibility to corrosion and embrittlement, interfaces and interface-crack interactions often control material performance in aggressive chemical environments.

In nanocrystalline solids, atoms located at or near interfaces account for 15-30% of the material and may govern its behavior entirely, leading to dramatic effects such as elevated strength, superplasticity, and suppression of fatigue. For example, the competition between sliding of interfaces and their ability to trap, nucleate, and transmit crystal defects called dislocations determines whether, with decreasing grain size, the strength of a nanocrystalline material decreases³, saturates⁴, or continually increases⁵.

While the importance of the atomic-scale structure and composition of interfaces has been appreciated for decades, our ability to explore them has been limited. This situation is changing: due to improved characterization and modeling techniques, the full three-dimensional atomic structure interfaces may now be studied with unprecedented detail and precision and its effect on properties assessed⁶. Quantitative structure-property relations for specific interfaces may now be developed, similar to what has been done for single crystals before.

In addition to their intrinsic scientific interest, interface structure-property relations may have disruptive technical applications. Much recent research on new structural metals specifically aims to exploit the influence of interfaces through microstructure engineering⁷, e.g. to improve wear resistance of electrodeposited alloys⁸, heal radiation damage in magnetron sputtered nanocomposites⁹, or reduce susceptibility to hydrogen embrittlement¹⁰. Quantitative structure-property relations for interfaces could take the guesswork out of these efforts, advancing the transition of materials science from hit-or-miss materials development to first principles materials design.

In this feature, I describe three examples of research that aims to expand the materials performance envelope through atomic-scale design of interfaces. This work was performed in collaboration with the Center for Materials at Irradiation and Mechanical Extremes (CMIME), an energy frontier research center funded by the US Department of Energy at Los Alamos National Laboratory.

Healing radiation damage

The primary forms of radiation damage are individual and clustered vacancies and interstitials produced during collisions between energetic particles and target atoms. The subsequent diffusion and clustering of these defects leads to several forms of radiation-induced degradation¹¹. When a vacancy and interstitial approach each other sufficiently closely, however, they undergo mutual recombination, “healing” radiation damage. Enhancing this process is one strategy for improving the radiation resistance of crystalline materials.

Some interfaces are efficient sinks and recombination sites for radiation-induced point defects¹². We have shown that maximizing the density of interfaces in a material may simultaneously counteract several forms of radiation-induced degradation, but only if these interfaces have a sufficiently high sink efficiency¹³. Sink efficiency depends on interface structure and the properties of vacancies and interstitials trapped at interfaces, which are in turn functions of interface crystallography, processing history, and properties of the adjacent materials. Thus, interfaces may be tailored for radiation resistance by judiciously selecting these parameters to maximize sink efficiency¹⁴.

Stabilizing implanted helium

Alpha particles are helium (He) nuclei that are produced in several kinds of nuclear reactions. Prolonged exposure of a material to alpha particles therefore results in copious He implantation, which is a prime concern for future fusion and fast fission reactors. We have found, however, that some interfaces may be tailored to trap He in stable form, preventing He-induced degradation¹⁵. The structure of these interfaces consists of arrays of parallel line defects called dislocations¹². Intersections between these dislocations are the preferred trapping sites for He and determine how much of it may be stored in such an interface¹⁶.

Based on these insights, we may begin to envision new ways for how interfaces may be designed to manage He implanted into structural materials. For example, by controlling the distribution of misfit dislocation intersections in an interface, He bubbles or bubble nuclei may be templated with a desired spacing¹⁷. The spacing may be optimized to achieve desired behavior, e.g. to maximize or minimize He storage or to influence interface mechanical properties.

Making materials containing designer interfaces

The key to synthesizing interfaces with desired properties lies in controlling the morphology of materials not at the level of individual atoms, but at the level of individual crystalline grains. Grain sizes, in turn, span from single nanometers to several centimeters or larger. One example of how crystalline grain arrangement or “microstructure” affects properties may be seen in composites to two metals: copper (Cu) and niobium (Nb). According to thermodynamics, these two metals are immiscible. Thus, if they are forced together, they should separate, just as water and oil separate after being mixed. That is indeed what happens in most cases. If the Cu and Nb are arranged into alternating layers, however, they do not separate, even if the thickness of the layers approaches single nanometers. The cause of this apparent contradiction of thermodynamics is to be found in the stability of the layered microstructure against perturbations arising from high temperatures¹⁸, radiation⁹, or mechanical deformation¹⁹.

Interestingly, one way of synthesizing materials with controlled microstructures is, once again, to use interfaces. In a class of processing methods called “severe plastic deformation” (SPD), interfaces may act as “gatekeepers” that control the creation, annihilation, and transmission of dislocations. An improved understanding of interface-dislocation interactions may therefore enable better control of microstructures in materials synthesized using a variety of SPD methods^{20, 21}.

References

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