A micromechanical continuum model for the tensile behavior of shape memory metal nanowires

A novel shape memory behavior is discovered recently in single-crystalline FCC nanowires of Cu, Ni, and Au with lateral dimensions below 5 nm. Under proper thermomechanical conditions, these wires can recover elongations up to 50%. This phenomenon only exists at the nanoscale and is associated with reversible lattice reorientations within the FCC lattice structure driven by surface stresses. This paper presents a mechanism-based micromechanical continuum model for the tensile behavior. This model uses a decomposition of the lattice reorientation process into a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative propagation of a twin boundary. The reversible part is associated with strain energy functions with multiple local minima and quantifies the energy conversion process between the twinning phases. The irreversible part is due to the ruggedness of the strain energy landscape associated with dislocation nucleation, gliding, and annihilation characterizes the dissipation during the transformation. This model captures all major characteristics of the behavior, quantifies the size and temperature effects, and yields results which are in excellent agreement with data from molecular dynamics simulations.