The objective of this proposal is to investigate mechanical properties of nanocrystalline materials and thin films with strong coupling between grain boundary and grain interior deformation mechanisms. A combination of theoretical analysis and numerical techniques (integro-differential equation modeling, finite element based mesoscopic modeling, discrete dislocation dynamics and molecular dynamics methods) will be adopted in the study. A systematic investigation will be conducted on a number of fundamental issues including competing deformation in grain boundaries and grain-interiors in systems with grain sizes/film thicknesses on the order of 100 nm or smaller, effects of heterogeneous grain boundary diffusivity and sliding resistance in nanocrystalline materials and ultrathin films, interplay between grain boundary diffusion and grain boundary sliding to accommodate compatible deformation, and mechanical behaviors of thin films with surface and grain boundary diffusion.
This project will generate new knowledge and fundamental understanding that may provide insights and guidelines for a wide range of applications in modern technologies including thin film structures in microelectronics and optoelectronics, MEMS devices, thermal barrier coatings in gas turbines, tribological coatings and nanotechnology of materials engineering. The educational components of the project include training of a PhD student, mentoring of undergraduates, and assimilation of state-of-the-art research results into the existing graduate and undergraduate courses in engineering at Brown where there are currently strong student interests in interfacial mechanics of nanoscale materials and structures. Additional outreach activities will be mediated by Brown?s Materials Research Science and Engineering Center (MRSEC) which has established a set of excellent outreach educational programs.This award is co-funded by CMMI - Mechanics & Structure of Materials and DMR-Metals programs.
This cutting edge research in the fields of material science and solid mechanics has produced the following findings. Plastic strain recovery in nanocrystalline metals. In conventional metals, plastic strain induced by motion and multiplication of dislocations (a typical line defect in solids) is regarded as permanent. However, in nanocrystalline metals, large residual stresses induced by grain boundary and grain interior deformation mechanisms renders plastic strain partially recoverable. Figure 1 shows that a dislocation moves in reverse driven by internal residual stresses during deformation of nanocrystalline aluminum. In this research, the PI has quantified the fractional contributions by grain-boundary and grain-interior deformation mechanisms to the overall recovered strain. This is the first quantitative analysis of competing grain-boundary and grain-interior deformation mechanisms in nanocrystalline materials. Deformation and fracture of nanotwinned metals. Nanotwinned metal is a new class of hierarchical nanostructured materials, where a large population of coherent twin boundaries (which separate the crystals in a mirror symmetrical manner) spaced at a few to a few hundred nanometers apart are embedded in micron- or sub-micron-sized grains. In contrast to conventional coarse-grained metals, nanotwinned metals can possess ultra-high strength/hardness and remarkable fracture properties, while retaining good ductility, uncompromised electrical conductivity and electromigration resistivity. Large-scale atomistic simulations revealed that there exists a deformation-mechanism transition in nanotwinned metals as the twin-boundary spacing decreases. The major discovery in this research is that dislocation nucleation at the intersections between grain boundaries and twin boundaries governs the maximum strength of nanotwinned metals (Fig. 2). Based on the insights from this discovery, the PI hss developed a scaling law which describes the relationship between materials strength and the microscopic length scales. In addition, the PI has investigated the fracture behaviors of nanotwinned copper. Atomic structures in Fig. 3 show that during deformation, a large number of dislocations emit from the crack tip and impinge on twin planes, leading to transformation of initially clean twin plan into a dislocation wall. This behavior enhances ductility without sacrificing strength. These understandings will guide material researchers to create hierarchical materials which simultaneously have high-strength and high-ductility. Deformation mechanisms in nanotwinned metallic nanopillars. In this research, large-scale atomic simulations of nanotwinned copper nanopillars (where the simulated samples are comparable to those used in the experiments done by our collaborators at California Institute of Technology) revealed that when the twin boundaries are perpendicular to the loading direction, dislocations slip across twin boundaries, leading to formation of shear bands and breakage of nanoscale twins (see Fig. 4). However, when the twin boundaries are inclined, dislocations nucleates from free surface and slip on the twin planes, as illustrated in Fig. 4. These two distinct deformation mechanisms result in the observed differences in strength of nanotwinned nanopillars with orthogonal and slanted twin-boundaries. In addition, another discovery of this research is a unique characteristic of nanoscale plasticity in nanopillars: unlike their tightly-spaced counterparts, the 100 nm-diameter pillars with orthogonal twin boundaries spaced at 4.3 nm fail in a brittle fashion upon tension, attaining very high ultimate tensile strengths. Such brittle fracture has been related to the intrinsic brittleness of twin boundaries via large-scale atomistic simulations. The above findings not only significantly advance the scientific understanding of the deformation in nanostructured metals, but also provide insights and guidelines for designing new hierarchical nanostructured materials with optimize mechanical properties, and applications of nanostructured metals in next-generation interconnect and packaging components of micro-/nano- electronics and microelectromechanical systems. These findings can also motivate the materials community to pay special attention to the spatial organization of nanostructures, in conjunction with the unique nanoscale interfacial phenomena, for achieving the optimized properties of engineering materials. Moreover, the advance of high-speed and large-scale computational capabilities continues to enable USA to maintain scientific and technical leadership in the world. Funded by this grant, the PI has published 11 papers in high-profile journals including Nature, Nature Nanotechnology, PNAS, Nano Letters and Journal of the Mechanics and Physics of Solids. In addition, the PI had delivered more than 10 plenary, keynote and invited talks/seminars at scientific conferences, workshops, universities and national laboratories. Supported by this grant, the PI trained two PhD students (Tanmay Bhandakkar and Xiaoyan Li) who have graduated from Brown University and now worked as faculty members at two top universities in Asia (Bhandakkar in IIT Bomday, India) and Li in Tsinghua University, China). Moreover, the PI guided a Brown undergraduate student (Mr. James McLaughlin) in research on nanostructured materials. For outreach activities, the PI gave guest lectures at Brown summer school for local high school students, participated in outreach activities sponsored by the Materials Research Science and Engineering Center (MRSEC) at Brown University (the PI is as the current PI of Brown MRSEC) and served as a judge for poster competition in nanotechnology at International Mechanical Engineering Congresses (IMECE) in 2008-2011.
