(575d) Molecular-Dynamics Simulation Study of the Mechanical Behavior Under Biaxial Strain of Pre-Strained FCC Metallic Ultrathin Films

Nanometer-scale-thick metallic films are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. The mechanical behavior of metallic small-volume structures is very different from that of bulk metals; for example, recent experimental studies on nanopillars of face-centered cubic (fcc) metals have shown that the strength of such fcc small-volume structures increases with decreasing characteristic lengths and that the dislocation density always decreases during the application of the strain. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the physical mechanisms that govern the thin-film response to mechanical loading in order to establish links between the films' structural evolution and their mechanical behavior.

In this presentation, we report results of large-scale molecular-dynamics (MD) simulations for the dynamic deformation (i.e., at constant strain rate and temperature) under biaxial tensile strain of nanometer-scale-thick films of various fcc metals. The MD simulations employ an embedded-atom-method parameterization for the description of the interatomic interactions. The initial dislocation density of the thin films is on the order of 1017 m-2. Our results indicate that films of metals with moderate-to-high propensity for formation of stacking faults (such as Ni and Cu) exhibit an extended easy-glide regime followed by a sharp increase in the material stress, whereas those with low propensity for stacking-fault formation (such as Al) exhibit a monotonic increase in the stress during dynamic loading.

The mechanical response of the Ni and Cu films to applied biaxial tensile strain exhibits three stages of deformation. In the first stage, the response is nearly elastic, with a sharp increase in the stress; only a few dislocations are annihilated in this stage. During the second stage, gliding dislocations interact with stacking faults. These interactions lead to dislocation dissociation and aid in dislocation annihilation. Almost always, the obstacle stacking faults are unzipped during these interactions. We identify 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 strength increases and the material's mechanical behavior is found to resemble that of a perfectly elastic film. In the third stage, continued application of strain leads to increase in the thin-film stress, which leads eventually to nucleation of new dislocations in the thin film. Dislocation nucleation and depletion in the thin film continues in cycles until thin-film failure. In Al thin films, however, we find that the stress increases monotonically until thin-film failure and that the dislocation depletion rate is much lower than that in the Ni and Cu thin films. The analysis of the defect interaction mechanisms in Al thin films reveals that in metals with low propensity for stacking-fault formation dislocation annihilation is due to collinear interactions between dislocations. Furthermore, the plastic strain rate in Cu and Ni thin films is far greater than that in Al thin films, causing significant stress dissipation and an extended easy-glide regime in the Cu and Ni thin films.