Study of Nanowires Using Molecular Dynamics Simulations

In this dissertation I present computational studies that focus on the unique characteristics of metallic nanowires. We generated virtual nanowires of nanocrystalline nickel (nc-Ni) and single crystalline silver (Ag) in order to investigate particular nanoscale effects. Three-dimensional atomistic molecular dynamics studies were performed for each sample using the super computer System X located at Virginia Tech. Thermal grain growth simulations were performed on 4 nm grain size nc-Ni by observing grain sizes over time for temperatures from 800K to 1450K and we discovered grain growth to be linearly time-dependant, contrary to coarse grained materials with square root dependence. Strain induced grain growth studies consisted of straining the nanostructures in tension at a strain rate of 3.3 x 10^8 s⁻¹. Grain boundary movement was recorded to quantify grain boundary velocities and grain growth. It was shown that during deformation, there is interplay between dislocation-mediated plasticity and grain boundary accommodation of plasticity through grain boundary sliding. To further understand the effect of stress on nanocrystalline materials we performed tensile tests at different strain rates, varying from 2.22 x 10⁷ s⁻¹ to 1.33 x 10⁹ s⁻¹ for a 5 nm grain size nc-Ni nanowire with a 5 nm radius. The activation volume was given as ~2b³, where b is the Burger's vector and is consistent with a grain boundary dominate deformation mechanism. We expanded our research to 10 nm grain size nc-Ni nanowires with radii from 5 nm to 18 nm. Each wire was deformed 15% in tension or compression at a strain rate of 3.3 x 10⁸ s⁻¹. Asymmetry was observed for all radii, in which larger radii produced higher flow stresses for compression and small radii yielded higher flow stresses in tension. A cross over in the tension-compression asymmetry is found to occur at a radius of ~9 nm. A change in the dominate deformation mechanism in combination with the ease of grain boundary sliding contributes to the phenomena of the asymmetry. In the final chapter we focus on the energetic stability of multi-twinned Ag nanorods at the nanoscale. We used a combination of molecular statics and dynamics to find the local minimum energies for the multi-twinned nanorods and the non-twinned "bulk" materials and concluded that the stability of multi-twinned nanorods is highly influenced by the size of the sample and the existence of the ends. Using an analytical model we found the excess energy of the nanorods with ends and determined the critical aspect ratio below which five-twinned nanorods are stable.