The Plasticity Of Metals At The Sub Micrometer Scale And Dislocation Dynamics In A Thin Film

Nanotechnology has played a significant role in the development of useful engineering devices and in the synthesis of new classes of materials. For the reliable design of devices and for structural applications of materials with micro- or nano-sized features, nanotechnology has always called for an understanding of the mechanical properties of materials at small length scales. Thus, it becomes important to develop new experimental techniques to allow reliable mechanical testing at small scales. At the same time, the development of computational techniques is necessary to interpret the experimentally observed phenomena. Currently, microcompression testing of micropillars, which are fabricated by focused-ion beam (FIB) milling, is one of the most popular experimental methods for measuring the mechanical properties at the micrometer scale. Also, dislocation dynamics codes have been extensively developed to study the local evolution of dislocation structures. Therefore, we conducted both experimental and theoretical studies that shed new light on the factors that control the strength and plasticity of crystalline materials at the sub-micrometer scale. In the experimental work, we produced gold nanopillars by focused-ion beam milling, and conducted microcompression tests to obtain the stress-strain curves. Firstly, the size effects on the strength of gold nanopillars were studied, and "Smaller is Stronger" was observed. Secondly, we tried to change the dislocation densities to control the strength of gold nanopillars by prestraining and annealing. The results showed that prestraining dramatically reduces the flow strength of nanopillars while annealing restores the strength to the pristine levels. Transmission electron microscopy (TEM) revealed that the high dislocation density (~1015 m-2) of prestrained nanopillars significantly decreased after heavy plastic deformation. In order to interpret this TEM observation, potential dislocation source structures were geometrically analyzed. We found that the insertion of jogged dislocations before relaxation or enabling cross-slip during plastic flow are prerequisites for the formation of potentially strong natural pinning points and single arm dislocation sources. At the sub-micron scale, these conditions are most likely absent, and we argue that mobile dislocation starvation would occur naturally in the course of plastic flow. Two more outstanding issues have also been studied in this dissertation. The first involves the effects of FIB milling on the mechanical properties. Since micropillars are made by FIB milling, the damage layer at the free surface is always formed and would be expected to affect the mechanical properties at a sub-micron scale. Thus, pristine gold microparticles were produced by a solid-state dewetting technique, and the effects of FIB milling on both pristine and prestrained microparticles were examined via microcompression testing. These experiments revealed that FIB milling significantly reduces the strength of pristine microparticles, but does not alter that of prestrained microparticles. Thus, we confirmed that if there are pre-existing mobile-dislocations present in the crystal, FIB milling does not affect the mechanical properties. The second issue is the scaling law commonly used to describe the strength of micropillars as a function of sample size. For the scaling law, the power-law approximation has been widely used without understanding fundamental physics in it. Thus, we tried to analyze the power-law approximation in a quantitative manner with the well-known single arm source model. Material parameters, such as the friction stress, the anisotropic shear modulus, the magnitude of Burgers vector and the dislocation density, were explored to understand their effects on the scaling behavior. Considering these effects allows one to rationalize the observed material-dependent power-law exponents quantitatively. In another part of the dissertation, a computational study of dislocation dynamics in a free-standing thin film is described. We improved the ParaDiS (Parallel Dislocation Simulator) code, which was originally developed at the Lawrence Livermore National Laboratory, to deal with the free surface of a free-standing thin film. The spectral method was implemented to calculate the image stress field in a thin film. The faster convergence in the image stress calculation were obtained by employing Yoffe's image stress, which removes the singularity of the traction at the intersecting point between a threading dislocation and free surface. Using this newly developed code, we studied the stability of dislocation junctions and jogs, which are the potential dislocation sources, in a free standing thin film of a face-centered-cubic metal and discussed the creation of a dislocation source in a thin film. In summary, we have performed both microcompression tests and dislocation dynamics simulations to understand the dislocation mechanisms at the sub-micron scale and the related mechanical properties of metals. We believe that these experimental and computational studies have contributed to the enhancement of our fundamental knowledge of the plasticity of metals at the sub-micron scale.

The modern understanding of metal plasticity and fracturing began about 100 years ago, with pioneering work; first, on crack-induced fracturing by Griffith and, second, with the invention of dislocation-enhanced crystal plasticity by Taylor, Orowan and Polanyi. The modern counterparts are fracture mechanics, as invented by Irwin, and dislocation mechanics, as initiated in pioneering work by Cottrell. No less important was the breakthrough development of optical characterization of sectioned polycrystalline metal microstructures started by Sorby in the late 19th century and leading eventually to modern optical, x-ray and electron microscopy methods for assessments of crystal fracture surfaces, via fractography, and particularly of x-ray and electron microscopy techniques applied to quantitative characterizations of internal dislocation behaviors. A major current effort is to match computational simulations of metal deformation/fracturing behaviors with experimental measurements made over extended ranges of microstructures and over varying external conditions of stress-state, temperature and loading rate. The relation of such simulations to the development of constitutive equations for a hoped-for predictive description of material deformation/fracturing behaviors is an active topic of research. The present collection of articles provides a broad sampling of research accomplishments on the two subjects.

The Advanced Study Institute on Synthesis, Functional Properties and Applications of Nanostructures, held at the Knossos Royal Village, Heraklion, Crete, Greece, July 26, 2002 - August 4, 2002, successfully reviewed the state-of-the-art of nanostructures and nanotechnology. It was concluded that Nanotechnology is widely agreed to be the research focus that will lead to the next generation of breakthroughs in science and engineering. There are three cornerstones to the expectation that Nanotechnology will yield revolutionary advances in understanding and application: • Breakthroughs in properties that arise from materials fabricated from the nanoscale. • Synergistic behavior that arise from the combination of disparate types of materials (soft vs. hard, organic vs. inorganic, chemical vs. biological vs. solid state) at the nanoscale. • Exploitation of natural (e.g. chemical and biological) assembly mechanisms that can accomplish structural control at the nanoscale. It is expected that this will lead to paradigms for assembling bio-inspired functional systems that accomplish desirable properties that are either unavailable or prohibitively expensive using top-down approaches.

A diverse set of materials science communities come together in this volume to review the extraordinary progress made in the development of computer simulation and modeling techniques for the prediction of film morphology, microstructure, composition, profile and structure. These techniques are rapidly moving out of the area of academic research and into technological and production design areas of thin-film-based industries. The book is loosely organized in ascending order of modeling-length scales - from atomic, up to the entire deposition reactor. Topics include: deposition and growth modeling; film morphology and topology; film microstructure; failure mechanisms; etching; process modeling and control and reactor-scale modeling.

In the past twenty years, new experimental approaches, improved models and progress in simulation techniques brought new insights into long-standing issues concerning dislocation-based plasticity in crystalline materials. During this period, three-dimensional dislocation dynamics simulations appeared and reached maturity. Their objectives are to unravel the relation between individual and collective dislocation processes at the mesoscale, to establish connections with atom-scale studies of dislocation core properties and to bridge, in combination with modelling, the gap between defect properties and phenomenological continuum models for plastic flow. Dislocation dynamics simulations are becoming accessible to a wide range of users. This book presents to students and researchers in materials science and mechanical engineering a comprehensive coverage of the physical body of knowledge on which they are based. It includes classical studies, which are too often ignored, recent experimental and theoretical advances, as well as a discussion of selected applications on various topics.

This thesis transports you to a wonderful and fascinating small-scale world and tells you the origin of several new phenomena. The investigative tool is the improved discrete dislocation-based multi-scale approaches, bridging the continuum modeling and atomistic simulation. Mechanism-based theoretical models are put forward to conveniently predict the mechanical responses and defect evolution. The findings presented in this thesis yield valuable new guidelines for microdevice design, reliability analysis and defect tuning.