Atomistic Mechanisms of Surface- and Interface-Mediated Creep in Small-sized Metals

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Irreversible, plastic deformation in polycrystalline metals normally arises from the movement of line defects, also known as dislocations. However, in materials composed of nanoscale agglomerations of crystals, i.e., nanograins, the significant amount of crystal interfaces and free surfaces could dramatically facilitate mass (atoms) transport, thereby leading to fundamentally new atomistic deformation mechanisms and distinctive mechanical properties compared to those of their large grain counterparts. Such surface or interface-mediated diffusive plasticity (creep) has been found to play a significant role in mechanical behaviour of nanomaterials even at room temperature. This project will investigate the atomistic mechanisms governing the interface and surface controlled diffusive plasticity in nanostructured metals through in-situ high-resolution microscopy. The understanding achieved through this research will have direct impact on the development of nanoscale metals and alloys with high strength and ductility, facilitating development of advanced nanomechanical devices with superior reliability. The results from this research will advance experimental mechanics at the nanoscale, and the knowledge gained will advance the national health, prosperity, and welfare by benefiting the materials and manufacturing industries. The project will also embark on an extensive plan of undergraduate and graduate curriculum development, training of underrepresented undergraduate students in advanced engineering sciences through summer internships, and outreach to elementary school students in collaboration with the local science museum.The objective of this research is to investigate the atomistic mechanisms governing grain boundary and surface diffusive plasticity in nanostructured metallic systems through in-situ observation under high-resolution transmission electron microscope (HRTEM). Specifically, the research will be divided into two parts: firstly, the interplay/competition between dislocation plasticity and diffusional creep will be atomically resolved, with an emphasis on the coupled diffusive-displacive processes at nanocrystal surfaces and the size dependent impact of surface diffusion on the strength and ductility of nanocrystals; secondly, atomic scale grain-boundary mass transport will be investigated in nano-size metals consisting of low-angle grain boundaries during uniaxial stressing, and a quantitative model will be developed to understand the contribution of such grain-boundary-mediated diffusive process to the overall plasticity. Understanding diffusional plastic deformation process of nanostructured metallic materials will have direct impact on the development of nanoscale metals and alloys with high strength and ductility to be used for advanced MEMS/NEMS with superior reliability for elevated temperature applications.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.