Generating uniformly ordered interfaces in bulk nanostructured metals is a challenge in designing materials that are stable under extreme environment conditions, such as next-generation, highly energy-efficient systems. Irene J. Beyerlein, Amit Misra, and their colleagues at Los Alamos National Laboratory have shown that by imposing an extreme amount of plastic strain in a Cu–Nb nanolayered crystal system, low-energy, well-ordered bimetal interfaces evolve.
According to Beyerlein, the project lead in this work, the research team has been studying interfaces in bimetallic nanocomposites in order to understand this phenomenon at the microscopic and atomic-scale level. She said that under extreme conditions defects, voids, or damage in a material is expected. However in this case, “what was surprising is that the interface that emerged was ordered, similar to the interfaces found in epitaxially grown films,” she said. Most remarkably, experimental evidence showed that this preferred interface occurs ubiquitously throughout the volume (>cm3) of the nanocomposite. This interface was also stable with respect to further straining, high-temperature exposure, and irradiation, giving the nanomaterial extraordinary tolerances in other extreme environments.
As reported in the March 25 issue of PNAS (DOI: 10.1073/pnas.1319436111; p. 4386), the researchers fabricated Cu–Nb nanolayered materials in bulk form (>cm3) and imposed extreme strains. To identify the stability conditions that govern the emergence of a preferred interface at extreme strain, they combined theory, atomic-scale modeling, and experimental characterization. They used accumulative roll bonding (ARB), a severe plastic deformation (SPD) process often proposed for the production of ultrafine grain metals. The figure shows that ARB is characterized by rolling a stack of metal sheets; the stack is repeatedly rolled to a severe reduction ratio, sectioned into two halves, piled again and rolled. The researchers used an alternating stack of sheets of these two dissimilar, immiscible metals, Cu and Nb, to carry out the ARB process. Unlike conventional rolling, ARB strains the sample through a cycle of rolling, cutting, and restacking, and maintains the original sample dimensions.
In this work, the researchers imposed extreme strains, decreasing the height by 5–6 orders of magnitude, from 2 mm to 20 nm. This is equivalent to stretching a nickel coin to 2.2 km in length (or strains exceeding 12).
(a) A severe plastic deformation process—accumulative roll bonding—is used to synthesize (b) Cu–Nb nanolayered composites of controllable layer thickness. Credit: Los Alamos National Laboratory.
Beyerlein said that the ARB technique can make nanostructured composites in large quantities sufficient for structural applications. “This work paves a way to looking at different materials structures that go beyond the Cu–Nb systems,” she said.
Additionally, to understand this phenomenon, the team applied atomic-scale and crystal-plasticity simulation. The results revealed that the preferred interface is one of few interfaces under extreme straining that can remain plastically stable while forming interfaces corresponding to a minimum in formation energy.
This finding has the exciting potential of eliminating the aforementioned tradeoff and permitting the creation of materials with pristine interfaces in stable nanocomposites of unlimited quantities. Most significantly, it points to other interfaces that could also emerge in extreme straining and exhibit similar stability properties. This work introduces an innovative way toward manipulating interfaces through severe plastic deformation for target material properties.
