Condensed-Matter Physics & Materials Science Seminar

The experimental observation of deformation mechanisms in sub-ten-nanometer-sized crystals is lacking during the past several decades. Here, through in-situ high resolution transmission electron microscopy (HRTEM) observations, we show that the discrete plastic deformation of sub-ten-nanometer Au nanocrystals is drastically different from that of the bulk counterpart. In sharp contrast to the scenario that plasticity is mediated by dislocation emission from Frank-Read sources and multiplication in bulk materials, partial dislocations emitted from free surfaces, which produce fresh surface steps after they annihilate at the free surfaces, dominate the deformation of Au nano-crystals. Stress-relief caused by the surface-mediated plastic deformation is directly visualized. Additionally, these experiments verify that the crystallographic orientation is not the only factor in determining the deformation mechanism of nanometer-sized Au. After failure, the Au nanocrystal shows a phase transformation from face-centered-cubic (FCC) to body-centered-tetragonal (BCT) structure. The transformation is facilitated by surface stresses, which are able to drive the <001> crystal along the Bain path from FCC to BCT. These findings provide direct experimental evidence for the vast amount of theoretical modeling work on deformation mechanisms of nanomaterials that have appeared in recent years. The central theoretical prediction that plastic deformation in nanometer-sized crystals is dominated by surfaces has been directly verified using in-situ HRTEM.