MICROSCALE TESTING AND CHARACTERIZATION TECHNIQUES FOR BENCHMARKING CRYSTAL PLASTICITY MODELS

The desire to improve the performance of engineering alloys and introduce new materials into service has led to the development of advanced, multi-scale material property models that can accurately predict the deformation response of polycrystalline microstructures. These microstructure-dependent, multi-scale models have the ability to provide insight into the connections between material processing, microstructure and properties in a way that has not been available before. However, these advanced modeling techniques require microstructural characterization and experimentally obtained benchmarks at salient length scales. Accordingly, microtensile tests of the polycrystalline Ni-base superalloy René 88DT have been carried out in order to guide and benchmark parallel crystal plasticity finite element method (CPFEM) modeling of this material at appropriate length scales. Microscale machining processes, including wire electrical discharge machining (EDM), focused ion beam (FIB) and femtosecond laser machining, have been developed and optimized for machining microtensile samples across multiple sizes. Loading in uniaxial tension provides the full stress-strain behavior from which quantitative mechanical benchmarks such as yield strength, strain hardening, and modulus can be extracted. The effect of sample size was studied to observe the underlying effects of microstructural variations. It was found that average sample strength decreased, and stochasticity of strength increased, as sample size decreased, owing to a finite sampling of grain orientations with a biased distribution towards higher Schmid factor values for grains in a randomly textured FCC material. In addition, local strain accumulation on the surface of tested oligocrystalline samples, with a computationally tractable number of grains, has been measured through the use of 2D digital image correlation (DIC). It was observed that strain concentrations formed in regions of the microstructure where there was a significant mismatch in Schmid factor and elastic modulus across grain and twin boundaries, a microstructural feature that leads to local stress concentrations. These observations help to guide model development in highlighting deformation mechanisms in the material, and the developed strain maps provide both quantitative and qualitative benchmarks that can be directly compared with modeling results. The scale of these experiments allows for 3D characterization, via serial sectioning and electron backscatter diffraction (EBSD), of tested samples through collection of critical microstructural data, including size, shape and orientation of grains and twins within the tested volume. Experimentally capturing explicit microstructures, at a scale that is also computationally tractable in crystal plasticity modeling, and their attendant mechanical behavior highlights stochastic nature of plasticity in small volumes and provides quantitative metrics for model development

Fiscal Year 1995

The mission of the Engineering Research, Development, and Technology Program at Lawrence Livermore National Laboratory (LLNL) is to develop the knowledge base, process technologies, specialized equipment, tools and facilities to support current and future LLNL programs. Engineering`s efforts are guided by a strategy that results in dual benefit: first, in support of Department of Energy missions, such as national security through nuclear deterrence; and second, in enhancing the nation`s economic competitiveness through their collaboration with US industry in pursuit of the most cost-effective engineering solutions to LLNL programs. To accomplish this mission, the Engineering Research, Development, and Technology Program has two important goals: (1) identify key technologies relevant to LLNL programs where they can establish unique competencies, and (2) conduct high-quality research and development to enhance their capabilities and establish themselves as the world leaders in these technologies. To focus Engineering`s efforts, technology thrust areas are identified and technical leaders are selected for each area. The thrust areas are comprised of integrated engineering activities, staffed by personnel from the nine electronics and mechanical engineering divisions, and from other LLNL organizations. This annual report, organized by thrust area, describes Engineering`s activities for fiscal year 1995. The report provides timely summaries of objectives methods, and key results from eight thrust areas: computational electronics and electromagnetics; computational mechanics; microtechnology; manufacturing technology; materials science and engineering; power conversion technologies; nondestructive evaluation; and information engineering

DEFORMATION MECHANISMS IN NANOCRYSTALLINE ALUMINUM THIN FILMS: AN EXPERIMENTAL INVESTIGATION

Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unprecedented physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, nanocrystalline materials have garnered considerable interest and a quest to uncover the new deformation mechanisms that give rise to this superior response has revealed that nanoscale behavior is quite different from that described by continuum plasticity. While the production of nanocrystalline materials with reasonable sizes for structural applications remains a challenge, thin metallic films used in next-generation MEMS and NEMS devices can be nanostructured by virtue of their limited dimensions. Ultimately, the reliability and lifetime prediction of these devices will hinge on the accurate modeling of their mechanical response. This dissertation describes efforts to elucidate the deformation mechanisms operating in nanocrystalline aluminum freestanding submicron thin films. Results obtained from these films demonstrate unique mechanical behavior, where discontinuous grain growth results in a fundamental change in the way in which the material deforms. In contrast to the low tensile ductility generally associated with nanocrystalline metals, these nanocrystalline films demonstrate extended tensile ductility. In situ X-ray diffraction and post-mortem transmission electron microscopy point to the importance of stress-assisted room temperature grain growth in transforming the underlying processes that govern the ii mechanical response of the films; nanoscale deformation mechanisms give way to microscale plasticity. The findings highlighted in this work emphasize that the microstructure and the attendant properties are dynamic; they evolve as the nanocrystalline material is being deformed. Experiments designed to address the role of impurities in stabilizing the microstructure against an applied stress are used to demonstrate that a critical concentration of impurities can effectively pin the grain boundaries from any motion. A detailed comparison of the characteristics of grain growth with traditional driving forces for grain boundary migration reveals the need for an alternative description. Measurements of surface topography evolution indicate that shear stresses directly couple to grain boundaries, induce motion, and result in grain growth that dramatically changes the mechanical behavior of these films. Finally, comparison with recently published theoretical formulations and molecular dynamics simulations is shown

MC 2019 Berlin Microscopy Conference - Abstracts

Das Dokument enthält die Kurzfassungen der Beiträge aller Teilnehmer an der Mikroskopiekonferenz "MC 2019", die vom 01. bis 05.09.2019, in Berlin stattfand

Silica and Silicon Based Nanostructures

Silica and silicon-based nanostructures are now well-understood materials for which the technologies are mature. The most obvious applications, such as electronic devices, have been widely explored over the last two decades. The aim of this Special Issue is to bring together the state of the art in the field and to enable the emergence of new ideas and concepts for silicon and silica-based nanostructures