(428f) Ultra-High Strength Due to Dislocation Depletion In Small-Volume Structures of Face-Centered Cubic Metals

Metallic small-volume structures, such as nanometer-scale-thick metallic films, are increasingly used in modern technologies, from microelectronics to various areas of nanofabrication. The mechanical behavior of small-volume structures of metals is very different from that of their bulk counterparts. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms that govern the response of strained small-volume structures in order to establish rigorous links between their structural evolution and their mechanical behavior.

In this presentation, we report an atomic-scale analysis of the mechanisms of dislocation starvation in small-volume structures of face-centered cubic (FCC) metals, focusing on free-standing ultra-thin copper films that are subjected to biaxial tensile strain. Our study is based on large-scale molecular-dynamics simulations of dynamic deformation experiments (i.e., at constant strain rate and temperature), using an embedded-atom-method parameterization to describe the interatomic interactions. The initial dislocation density of the thin films is on the order of 10¹⁶ m^-2. Our analysis of the films' mechanical response to the applied tensile strain reveals three stages of deformation. During the first stage, most of the dislocations in the material are unpinned; these dislocations glide under the application of biaxial strain in such a direction that they unzip the stacking faults they bound and the dislocation networks that they are a part of. With the stacking fault area reduced, there are fewer barriers to dislocation glide in the thin films. Consequently, the remaining dislocations glide faster and farther. During the second stage, gliding dislocations interact with the stacking faults. These interactions lead to dislocation dissociation and aid in dislocation annihilation. The obstacle stacking faults are unzipped almost always during these interactions. We have identified three classes of dislocation-stacking fault interactions, where the stacking faults act as barriers to dislocation glide and as sources for dislocation cross-slip. As the dislocation density decreases, the thin film's strength increases significantly and the mechanical behavior of the material is observed to be closer to that of a perfectly elastic solid. During the third stage of film deformation, continued application of strain leads to increase in the film's stress and, eventually, to nucleation of new dislocations in the thin film. Dislocation nucleation and depletion in the thin film continue in cycles until the failure of the film.