Surface Energy Driven Liquid-Drop-Like Pseudoelastic Behaviors and In Situ Atomistic Mechanisms of Small-Sized Face-Centered-Cubic Metals.

The elastic strain of conventional metals is usually below ∼1%. As the metals' sizes decrease to approximate a few nanometers, their elastic strains can approach ∼8%, and they usually exhibit pseudoelastic strain that can be as large as ∼35%. Previous studies suggested that the pseudoelastic behaviors of nanocrystals were attributed to distinctive mechanisms, including the release of stored elastic energies, the temperature-enhanced surface diffusion, etc. However, the atomistic mechanisms remain elusive. In this study, through large numbers of in situ atomic-scale tensile-fracture experiments, we report liquid-drop-like pseudoelastic behaviors of face-centered-cubic fractured single-crystalline nanowires with diameters varying from 0.5 to 2.2 nm. The ultralarge liquid-drop-like pseudoelastic strain ranged from 31.4% to 81.0% after the nanowire fracture was observed. The in situ atomic-scale investigations revealed that the atomistic mechanisms resulted from surface energy driven plastic deformation including surface diffusion mixed with shear plastic deformation as well as the release of true elastic energy. As the nanowires' diameters decrease below a critical value, the surface pressure can approach the ideal strength of metals. This ultralarge surface pressure drives atoms to diffuse mixed with dislocation nucleation/propagation, which ultimately leads to the fractured nanowires exhibiting liquid-drop-like pseudoelastic phenomena.