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

在多晶金属中，不可逆的塑性变形通常是由线缺陷（也称为位错）的运动引起的。然而，在由纳米级晶体团块（即纳米颗粒）组成的材料中，大量的晶体界面和自由表面可以极大地促进质量（原子）的传输，从而导致与大晶粒相比较的全新原子变形机制和独特的力学性能。即使在室温下，这种表面或界面介导的扩散塑性（蠕变）也对纳米材料的力学行为起着重要作用。本项目将通过原位高分辨率显微镜研究纳米结构金属中控制界面和表面控制扩散塑性的原子机制。通过本研究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发展产生直接影响，促进具有卓越可靠性的先进纳米机械器件的发展。本研究成果将推动纳米尺度实验力学的发展，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。该项目还将开展广泛的本科和研究生课程开发计划，通过暑期实习培训代表性不足的高级工程科学本科生，并与当地科学博物馆合作向小学生推广。本研究的目的是通过高分辨率透射电镜（HRTEM）原位观察，探讨纳米结构金属体系的晶界和表面扩散塑性的原子机制。具体而言，研究将分为两部分：首先，原子地解决位错塑性和扩散蠕变之间的相互作用/竞争，重点研究纳米晶体表面的扩散-位移耦合过程以及表面扩散对纳米晶体强度和延性的尺寸依赖性影响；其次，研究由低角度晶界组成的纳米尺寸金属在单轴应力作用下的原子尺度晶界质量输运，并建立定量模型来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直接影响到具有高强度和延展性的纳米金属和合金的发展，这些金属和合金将用于先进的MEMS/NEMS，在高温应用中具有卓越的可靠性。该奖项反映了美国国家科学基金会的法定使命，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.