Size dependence of dislocation activities and independence on theoretical elastic strain limit in Pt nanocrystals revealed by atomic-resolution in situ investigation

Because of the lower total number and density of defects in nanocrystals than those in their bulk counterparts, the elastic strain limits and the plastic deformation behaviors of the former can be very different from those of the latter. Furthermore, as the surface atomic ratio increases, a surface-dominant elastic and plastic deformation characteristic may appear in nanocrystal metals. The competition between nano-strengthening and surface effects thus determines the apparent mechanical behaviors of nanocrystal metals. In this study, we conducted a series of in situ atomic-resolution deformation experiments on high stacking fault energy platinum nanocrystals using an aberration-corrected high-resolution transmission electron microscope. From the direct in situ atomic-scale observations, we provided direct atomic-resolution plastic deformation mechanisms for the Pt nanocrystals of size ranging from 20 to ∼0.7 nm. As the nanocrystal size decreased, a crossover occurred from dislocation slip-to dislocation-free-mediated plastic deformation. For nanocrystals of size above ∼6 nm, the plastic deformation was dominated by full dislocation. However, for nanocrystals of diameters below ∼2 nm, it was uncovered that the plastic deformation was dominated by the dislocation-free plastic deformation. In the elastic regime, the Pt nanocrystals reached a low elastic strain plateau by 1.5% when the size was 20 to ∼9 nm. The elastic strain increased when the crystal size was below ∼9 nm, and the Pt nanocrystals remained on the theoretical elastic strain limit plateau by above ∼7.0% when the crystal size was below ∼2 nm