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

多晶金属中的不可逆塑性变形通常由线缺陷（也称为位错）的运动引起。然而，在由纳米级晶体聚集体组成的材料中，即，纳米颗粒、大量的晶体界面和自由表面可以显著地促进质量（原子）传输，从而导致与它们的大颗粒对应物相比的全新的原子变形机制和独特的机械性能。这种表面或界面介导的扩散塑性（蠕变）已被发现在纳米材料的机械行为中发挥重要作用，即使在室温下。本计画将借由高解析度显微术，探讨奈米结构金属中界面与表面控制扩散塑性的原子机制。通过这项研究所获得的理解将对具有高强度和延展性的纳米级金属和合金的开发产生直接影响，促进具有上级可靠性的先进纳米机械设备的开发。这项研究的结果将推动纳米级的实验力学，所获得的知识将通过造福材料和制造业来促进国家的健康、繁荣和福利。该项目还将着手实施一项广泛的本科生和研究生课程开发计划，通过暑期实习培训代表性不足的本科生学习高级工程科学，本研究的目的是探讨纳米结构金属系统中晶界和表面扩散塑性的原子机制，高分辨透射电子显微镜（HRTEM）原位观察。具体而言，研究将分为两个部分：首先，位错塑性和扩散蠕变之间的相互作用/竞争将从原子上解决，重点是纳米晶表面的耦合扩散-位移过程以及表面扩散对纳米晶强度和塑性的尺寸依赖性影响;其次，将研究在单轴应力期间由小角度晶界组成的纳米尺寸金属中的原子尺度晶界质量输运，并将建立一个定量模型，以了解这种晶界介导的扩散过程对整体塑性的贡献。了解纳米结构金属材料的扩散塑性变形过程，将对开发具有高强度和延展性的纳米级金属和合金产生直接影响，这些金属和合金将用于高温应用的高级MEMS/NEMS，具有上级可靠性。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.