Atomistic processes of surface-diffusion-induced abnormal softening in nanoscale metallic crystals
X Wang and SX Zheng and S Shinzato and ZW Fang and Y He and L Zhong and CM Wang and S Ogata and SX Mao, NATURE COMMUNICATIONS, 12, 5237 (2021).
As the sample size goes down to the nanoscale, the surface-related mechanism plays an important role in the deformation of nanoscale crystals. Here, the authors report breakdown of the traditional Hall- Petch-like relation in nanoscale Ag attributed to diffusion-involved nucleation behaviors. Ultrahigh surface-to-volume ratio in nanoscale materials, could dramatically facilitate mass transport, leading to surface-mediated diffusion similar to Coble-type creep in polycrystalline materials. Unfortunately, the Coble creep is just a conceptual model, and the associated physical mechanisms of mass transport have never been revealed at atomic scale. Akin to the ambiguities in Coble creep, atomic surface diffusion in nanoscale crystals remains largely unclear, especially when mediating yielding and plastic flow. Here, by using in situ nanomechanical testing under high- resolution transmission electron microscope, we find that the diffusion- assisted dislocation nucleation induces the transition from a normal to an inverse Hall-Petch-like relation of the strength-size dependence and the surface-creep leads to the abnormal softening in flow stress with the reduction in size of nanoscale silver, contrary to the classical "alternating dislocation starvation" behavior in nanoscale platinum. This work provides insights into the atomic-scale mechanisms of diffusion-mediated deformation in nanoscale materials, and impact on the design for ultrasmall-sized nanomechanical devices.
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