Surface-Stress-Driven Pseudoelasticity and Shape Memory Effect at the Nanoscale
| Summary The pseudoelastic deformation of some shape memory alloys (SMAs) such as Au-Cd, Au-Cu-Zn, Cu-Zn-Al, and Cu-Al-Ni proceeds through the reversible movement of twin boundaries (Ren and Otsuka [1]). The behavior of these materials is commonly referred to as rubber-like due to its resemblance to the behavior of soft and pseudoelastic rubber (Otsuka and Wayman [2]). A similar behavior and a shape memory effect (SME) are discovered in single-crystalline Cu nanowires with lateral dimensions between 1.76 and 3.39 nm through molecular dynamics simulations. This behavior at the nanoscale is due to reversible crystallographic lattice reorientations through the movement of twin boundaries within the FCC crystalline structure (Fig. 2), allowing Cu nanowires to exhibit recoverable strains of up to more than 50% which are well beyond the recoverable strains of 5-8% of most SMAs. The reorientation is driven by high internal stresses resulting from the surface stress and high surface-to-volume ratios of the nanowires. This phenomenon only occurs in nanowires within the size range of 1.76-3.39 nm and is not observed in bulk Cu. Furthermore, it is temperature-dependent and hence gives rise to an SME. Specifically, the critical temperature for spontaneous reorientation upon unloading increases from 100 to 900 K as the wire size increases from 1.76 to 3.39 nm, making it possible to design nanoscale components of varying sizes for operation over a wide range of temperature. Such an objective is more difficult to achieve with conventional bulk SMAs since their transition temperatures (martensite start and finish temperatures, austenite start and finish temperatures) only vary with material structures and composition, not size. Moreover, the nanowires have very short response times which are on the order of nanoseconds due to their extremely small dimensions compared with bulk SMAs. These unique properties can lead to important applications at the nanoscale, including sensors, transducers, and actuators in nano-electromechanical systems (NEMS). Atomistic simulations have also yielded the same rubber-like behavior and SME in Au nanowires with similar levels of recoverable strains and different critical temperatures and critical sizes. Since other FCC metallic (e.g., Pt and Ag) nanowires have similar structures and properties as those of Cu and Au nanowires (Rodrigues and Ugarte [3]), it is tempting to speculate that a similar behavior may also be found in them, with comparable levels of recoverable strains as those for Cu and Au wires. Of course, their critical temperatures and critical sizes would depend on their surface stress levels and the differences between their {001} and {111} surface energies. If such an effect is proven true, these materials could provide a whole class of nano building-blocks with unique rubber-like behaviors and SMEs operative over a wide range of temperatures and sizes. |