TECHNICAL: Meaningful results related to strength and plasticity in one-dimensional metallic nanowires can only be achieved if the influences of microstructure and sample size are fully understood. The two foci of these research activities are to (1) use parallel atomistic simulation to characterize the complex interactions between lattice dislocations and special grain boundaries in gold and nickel nanowires, and (2) measure the strength, ductility and strain-rate sensitivity of gold and nickel nanowires with controlled microstructures and grain size using applied electrochemical methods and atomic force microscopy. This would enable the PI to prove both theoretically and experimentally that special grain boundaries such as twin boundaries can dramatically improve these properties in nanowires. The intellectual merit lies in the presentation of a combined experimental/modeling research approach harnessing the power of both atomistic simulation and atomic force microscopy. This combination of methodologies will be very successful in bridging the gap between experiment and modeling in the mechanical characterization of individual nano-building block materials. Another potential outcome of such approach can be to rapidly help establish new standard methods and calibration procedures to test nanowires, as well-defined as nanoindentation technique for thin films, open up new research areas and lead to more findings. The research is also expected to show new ways to fabricate nanorods and nanowires with specific defects that make them stronger. NON-TECHNICAL: The educational component of this 5-year plan is strongly integrated to the multidisciplinary research program. The PI will make major educational and outreach contributions at the University of Vermont (UVM) to (1) spearhead the effort of revising the graduate engineering program with a focus on Multiscale Systems and Modeling; (2) structure international student exchanges with eight European engineering-specific programs at undergraduate level via the existing International Student Exchange Program (ISEP); and (3) develop a vigorous outreach program, via a novel scientific imaging which cross-cuts with the photographic arts, to improve the recruitment in engineering of students from rural high schools in Northern Vermont and the Abenaki Indian tribe in the Missisquoi Valley community. This CAREER award will also broadly advance the understanding in the field while promoting educational training of undergraduate and graduate students involved in the research project. In particular, this award is expected to enhance the education of students in the area of high performance and parallel computing, and high-resolution imaging. The graduate students will also have opportunities to broadly disseminate the results of the research at national conferences and meetings to enhance scientific understanding in the field. This CAREER award will stimulate the use of a multi-user facility, UVM's newly-established Vermont Advanced Computing Center, and involve the participation of scientists at a new US DOE's national research facility. The education program of this CAREER award will broadly revitalize the interest for materials science and engineering at UVM, help recruit engineering students from underrepresented groups, and improve the student's multicultural training.
The primary objectives of this research program were to fundamentally study the strength, plasticity and fracture behavior of low-dimensional metallic nanowires relevant to nanotechnology applications, and understand the important roles played by their size and microstructure at the nanoscale. A significant outcome is the discovery of new phenomena in the plastic deformation and failure of metallic nanowires under deformation as follows: 1. Large-scale molecular dynamics computer simulations were used to predict that the addition of nanoscale twins to metallic nanowires can either increase dramatically or decrease their strength in tension, depending on the nanowire diameter and number of twins per unit length. The simulations have also revealed for the first time that significant strain-hardening takes place in gold nanowires containing low-density twin boundaries. 2. We have discovered that a combination of twin boundary defects and zigzag surface facets can be utilized to approach the ideal theoretical strength of gold in nanowires containing angstrom-scale twins. The PI closely worked with scientists at the University of Pittsburgh to show direct experimental evidence for this unusual phenomenon, and proposed a new mechanism in such nanowires based on homogeneous dislocation nucleation. 3. We reported fundamental differences in the plastic behavior of nanotwinned metal nanowires in Au, Ag, Al, Cu, Pb and Ni, and showed a transition from abrupt yielding and strain-softening to significant strain-hardening as the stacking-fault energy of the metal decreases. This phenomenon was elucidated by considering the relative change between the stress required to nucleate new dislocations from free surfaces and that for a gliding dislocation to overcome the resistance of twin boundaries. 4. Another important finding was the strong size-dependence of hardness in metallic nanowires under nanoindentation, due to dislocation-surface interactions beneath the contacting tip. A mitigation strategy for such size effects in nanoindentation was found by nanostructuring Ni nanowires with a very small grain size to predominantly accommodate the plastic deformation by grain-boundary sliding instead of crystal slip. 5. An atomistic simulation method has been developed to study the origin of the size effect and microstructure evolution in nanoscale Cu pillars deformed uniformly, enabling studies of the key mechanisms of nanoscale plasticity observed in the past with different methods, such as in-situ nano-compression experiments or discrete dislocation dynamics simulations. The technique developed offers new opportunities to investigate more complex phenomena in extreme environments such as high temperature or radiation damage. 6. A new experimental method has been developed and validated to obtain more accurate measure of nanohardness in thin-films and nanowires from nanoindentation tests with diamond-tipped atomic force microscopy (AFM) sapphire cantilevers. The PI and his students have used this method to experimentally study the dependence of plasticity and hardness on diameter and stacking fault energy in bimetallic Ni-Au nanowires made by traditional template-assisted electrodeposition. This work was recently pursued on Ag and Cu nanowires synthesized directly by a new electrodeposition method. 7. The PI collaborated with scientists at Lawrence Livermore National Laboratory and Ames Laboratory to investigate the role of nanoscale twin boundaries in plasticity of bulk metals, because the ability of coherent twin boundaries in strengthening, maintaining the ductility and minimizing the electron scattering is well documented, but most of our understanding of the origin of these properties relies on perfect-interface assumptions. We have discovered with both experiments and simulations that as-grown twin boundaries in nanotwinned copper are inherently defective with kink-like steps and curvature, and that these imperfections consist of incoherent segments and partial dislocations. We have further showed that these defects play a crucial role in the deformation mechanisms and mechanical behavior of nanotwinned copper. Our findings therefore offer a view of the structure of coherent twin boundaries that is largely different from that in the literature, and underscore the significance of imperfections in nanotwin-strengthened materials. 8. The new scientific knowledge gained in this project was disseminated through the publication of 21 peer-reviewed articles in high-impact factor journals such as Nature Materials, Nano Letters and Nature Communications, 1 invited book chapter, 1 invited online summary, 2 invited conference proceeding papers, as well as 30 oral and poster presentations at conferences and seminars including 10 invited talks. 9. Major broader impacts associated with this project includes (1) the development of a new graduate-level curriculum in Multiscale Modeling at the University of Vermont, (2) several outreach activities to disseminate scientific knowledge in nanotechnology to the general public, (3) the training and mentoring of five undergraduate students and five graduate students who participated in the research, and (4) the organization of two international symposia on the properties of low-dimensional materials.
