Size-Tuned Plastic Flow Localization in Irradiated Materials at the Submicron Scale.

Three-dimensional discrete dislocation dynamics (3D-DDD) simulations reveal that, with reduction of sample size in the submicron regime, the mechanism of plastic flow localization in irradiated materials transitions from irradiation-controlled to an intrinsic dislocation source controlled. Furthermore, the spatial correlation of plastic deformation decreases due to weaker dislocation interactions and less frequent cross slip as the system size decreases, thus manifesting itself in thinner dislocation channels. A simple model of discrete dislocation source activation coupled with cross slip channel widening is developed to reproduce and physically explain this transition. In order to quantify the phenomenon of plastic flow localization, we introduce a "deformation localization index," with implications to the design of radiation-resistant materials.

Controlling Strain Bursts and Avalanches at the Nano- to Micrometer Scale.

We demonstrate, through three-dimensional discrete dislocation dynamics simulations, that the complex dynamical response of nano- and microcrystals to external constraints can be tuned. Under load rate control, strain bursts are shown to exhibit scale-free avalanche statistics, similar to critical phenomena in many physical systems. For the other extreme of displacement rate control, strain burst response transitions to quasiperiodic oscillations, similar to stick-slip earthquakes. External load mode control is shown to enable a qualitative transition in the complex collective dynamics of dislocations from self-organized criticality to quasiperiodic oscillations.

Inhibiting Adatom diffusion through surface alloying.

Ab initio and kinetic Monte Carlo calculations elucidate the electronic nature of surface Sn alloying on the stability and mobility of a Cu adatom on the Cu-Sn (111) alloy surface. Sn atoms segregate on the surface and introduce forbidden areas around them within which adatom adsorption is strictly prohibited. In addition they reduce dramatically both the binding and the mobility of Cu adatoms in neighboring adsorption sites outside the forbidden areas, in contrast to experimental suggestions. Thus, Sn atoms act as blocking sites inhibiting the Cu adatom diffusion. The underlying mechanisms are the structural deformation associated with the oversized Sn atoms and the enhancement of the adatom-surface interaction in the vicinity of Sn atoms.