Plastic deformation in a model copper wire under tensile loading is modeled using three dimensional atomistic simulations. The primary objective is to gain fundamental insight into the nano-scale deformation features. An initial defect is utilized in the model to trigger plastic deformation in a controlled manner. A parametric study is performed by varying the atomic interaction range used in the model. When the interaction distance is small, dislocation slip is observed to be the dominant deformation mechanism. A slight increase in the interaction range results in phase transition from the FCC structure to a BCC structure. Re-orientation of the BCC lattice also occurs at later stages of the deformation. The phase transition mechanism is further enhanced if the nanowire is attached to a flat substrate.

We are developing dynamic atomic force microscopy (AFM) techniques to determine nanoscale elastic properties. Atomic force acoustic microscopy (AFAM) makes use of the resonant frequencies of an AFM cantilever while its tip contacts the sample surface at a given static load. Our methods involve nanosized silicon probes with tip radius R ranging from approximately 10 nm to 150 nm. The resulting radius of contact between the tip and the sample is less than 20 nm. However, the contact stress can be greater than a few tens of GPa, exceeding the theoretical yield strength of silicon by a factor of two to four. Our AFAM experiments indicate that, contrary to expectation, tips can sometimes withstand such stresses without fracture. We subjected ten tips to the same sequence of AFAM experiments. Each tip was brought into contact with a fused quartz sample at different static loads. The load was systematically increased from about 0.4 μN to 6 μN. Changes in tip geometry were observed in images acquired in a scanning electron microscope (SEM) between the individual AFAM experiments. All of the tips with R < 10 nm broke during the first AFAM experiments at static loads less than 1.6 μN. Tips with R > 40 nm plastically deformed under such loads. However, a group of tips with R from 25 nm to 30 nm neither broke nor deformed during the tests. In order to reach higher contact stresses, two additional tips with similar values of R were used in identical experiments on nickel and sapphire samples. Although the estimated stresses exceeded 40 GPa, we did not observe any tip fracture events. Our qualitative observations agree with more systematic studies performed by other groups on various nanostructures. The results emphasize the necessity of understanding the mechanics of nanometer-scaled bodies and the impact of size effects on measurements of mechanical properties on such scales.

Nanoporous gold (NPG) thin films offer an opportunity to investigate the effects of nanoscale geometric confinement on the mechanical properties of metals. In the present study, NPG films supported by substrates were fabricated by dealloying Au-Ag films on Kapton and silicon. The microstructural evolution of NPG at various stages of dealloying was observed and analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cracking occurred at grain boundaries during dealloying and is believed to result from pre-existing tensile stress in the film. Stress evolution of NPG films on silicon substrates during dealloying was measured with the wafer curvature technique and revealed an overall shift toward compressive stress.

Strained InGaAs/GaAs bridges were released by a focused ion beam in order to observe the relaxation dynamics of the structure. Releasing the bridges resulted in the formation of chiral nanotubes with diameter of 920 nm and length 8.5 microns. The total time required for nanoscroll formation took > 20 minutes. From observing the scrolling action through time, it was found that the strain relief process differed from traditional wet etched nanoscrolls due to the simultaneous relief of strain from the released structures.

The morphological evolution of a Co cluster deposited on a Cu substrate is investigated by means of molecular-dynamics simulation. The many-body potential based on the second moment approximation of a tight-binding Hamiltonian (TB-SMA) is employed to calculate the interactions between Co and Cu atoms. The results show that the effect of the substrate anisotropy appears in the morphology of a deposited cluster. It is also revealed that the total period of the structural change of such a cluster can be divided into the three stages, viz., (I)change to an epitaxial structure, (II)overspread of diffusing Cu atoms on the cluster, and (III)solid dissolution of Co atoms into the Cu substrate.

We use the ReaxFF reactive force field to model extreme tensile deformation of a (10,10) armchair carbon nanotube. The ReaxFF force field has been developed based on DFT quantum mechanical calculations without any empirical parameters (Duin et al., 2001). We report an analysis of the stress-strain relationship for the elastic and plastic regime, including a description of the microscopic fracture mechanisms. We find Young's modulus to be around 1 TPa, close to experimental values. Our modeling yields a fracture tensile strain of approximately 30%, with a maximum tensile stress of approximately 300 GPa. Fracture of the CNT originates from formation of 5-7 Stone-Wales-like defects, leading to formation of micro-cracks.

It was demonstrated, on general thermodynamic grounds, that, in non-hydrostatically stressed elastic systems, phase and grain interfaces undergo morphological destabilization due to different mechanisms of “mass rearrangement”. Destabilization of free surfaces due to the combined action of mass rearrangement in the presence of electrostatic field has been well known since the end of the 19th century. Currently, morphological instabilities of thin solid films with electro-mechanical interactions have found various applications in physics and engineering. In this paper, we investigate the combined effects of the stress driven rearrangement instabilities and the destabilization due to the electro-mechanical interactions. The paper presents relevant results of theoretical studies for ferroelectric thin films. Theoretical analysis involves highly nonlinear equations allowing analytical methods only for the initial stage of unstable growth. At present, we are unable to explore analytically the most important, deeply nonlinear regimes of growth. To avoid this difficulty, we developed numerical tools facilitating the process of solving and interpreting the results by means of visualization of developing morphologies.