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

We have previously discovered a novel shape memory effect and pseudoelastic behavior in single-crystalline face-centered-cubic metal (Cu, Ni, and Au) nanowires. Under tensile loading and unloading, these wires can undergo recoverable elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. This phenomenon only exists at the nanoscale and is associated with a reversible lattice reorientation driven by the high surface-stress-induced internal stresses. We present here a micromechanical continuum model for the unique tensile behavior of these nanowires. Based on the first law of thermodynamics, this model decomposes the lattice reorientation process into two parts: a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative twin boundary propagation process. The reversible part is modeled within the framework of strain energy functions with multiple local minima. The irreversible, dissipative nature of the twin boundary propagation is due to the ruggedness of strain energy curves associated with dislocation nucleation, glide, and annihilation. The model captures the major characteristics of the unique behavior due to lattice reorientation and accounts for the size and temperature effects, yielding results that are in excellent agreement with the results of molecular dynamics simulations.