SUMMARY
As functional nano building blocks, nanowires are of great technical importance because of their unique structures, properties, and potential applications in nanoscale electronic, photonic, biological or chemical devices. The application of nanowires requires a fundamental understanding of the structural characteristics and thermomechanical properties, which are critical to the fabrication, assembling, and functioning of nanowires. This research focuses on the characterization of the structure and mechanical behavior of metal nanowires using molecular dynamics simulations with embedded-atom method (EAM) potentials. We discovered a novel shape memory effect and pseudoelastic behavior in single-crystalline FCC metal (Cu, Ni, and Au) nanowires. Upon tensile loading and unloading, these wires can recover elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. This novel behavior exists only at the nanoscale and is associated with a reversible lattice reorientation driven by the high surface-stress-induced internal stresses. The lattice reorientation process is also temperature-dependent because thermal energy facilitates to overcome the energy barrier for the transformation. Therefore, nanowires show either pseudoelasticity or shape memory effect depending on whether the transformation is induced by stress or heating. Based on the results of molecular dynamics simulations, a micromechanical continuum model is developed to characterize the preceding shape memory behavior of metal nanowires. The model captures the major characteristics of the unique behavior due to lattice reorientation and accounts for the size and temperature effects. The model predictions show excellent agreement with the results of molecular dynamics simulations. Overall, the results of this research can lead to important applications of nanowires, including sensors, transducers, and actuators in nano-electromechanical systems.