Melting of elemental solids can be identified and appreciated as a particularly simple example of discontinuous phase transitions involving condensed phases. Motivated, on the one hand, by the need to improve the microstructural quality of laser-crystallized columnar-grained polycrystalline Si films for manufacturing advanced AMOLED displays and, on the other hand, to investigate the fundamental details associated with phase transformations transpiring in condensed systems, this thesis examines the initiation and evolution of melting in polycrystalline thin films. Distilling the essence of the classical nucleation theory and extending its description to address more general cases of phase initiation and evolution, a general thermodynamic method based on capillarity effect is developed and applied to determine the shape of solid/liquid interfaces that are in mechanical equilibrium. We first explicitly identify and build our analysis based on how the shape of solid/liquid interfaces must comply with the contact angle conditions at the junctions and also the property of constant mean curvature. Bi-crystal and tri-crystal models are presented to capture the microstructural features such as junctions and vertices of interfaces in polycrystalline thin films. At each of the potential melt initiating sites, the parameter space of contact angles is divided into domains depending on the shape of the solid/liquid interface that can be established in mechanical equilibrium. Melting initiation mechanisms are subsequently determined based on the permissible shape for each domain. This analysis is further extended to the edges and corners of embedded cubic crystals (with nonidentical contact angles at different faces). Secondly, in order to facilitate the thermodynamic analysis of the melting initiation and interface propagation, we extend our curvature-evolution-centric method to identify and develop what we consider as the central function for discontinuous phase transitions. Specifically, starting with a local governing condition, identifies and builds on two curvatures: ρ^E (𝑉) and ρ* (𝑇). ρ^E (𝑉) captures the evolution of the mean curvature of the solid/liquid interface as a function of liquid volume for the case in which the mechanical equilibrium condition is satisfied, whereas ρ* (𝑇) incorporates the temperature effect on the difference between the volumetric free energy of solid and liquid phases using the corresponding equilibrium mean curvature. We define and identify the interface driving stress function ƒ(𝑉,𝑇)=∂𝐺/∂𝑉=σ(ρ^E (𝑉)-ρ* (𝑇)) of the phase transition as being an important fundamental quantity, which can be directly derived by taking the difference of the two curvature terms. In contrast to the conventional analysis that requires integration of volumetric and interfacial free energy terms over various geometric domains to derive the total free energy as a function of volume for a given temperature, this formation completely disentangles geometry from the thermodynamic aspects of the phase transition and allows them to be treated separately. In addition to providing essentially all relevant thermodynamic information about the phase initiation and evolution, the above method readily permits the use of powerful general-purpose numerical tools to calculate the potentially complex geometry of the solid/liquid and other interfaces and obtain ρ^E (𝑉) directly as the output. Plotting the ρ^E (𝑉) function together with the temperature-dependent iso-curvature line, ρ* (𝑇), unveils the critical thermodynamic information regarding the melting transition at the temperature, such as whether equilibrium points exist, the number of equilibrium points, their stability, and their corresponding volumes. The change of free energy as a function of liquid volume can be derived through integration of the interface driving stress function. The velocity of the solid/liquid interface is simply proportional to the interface driving stress function. The application of this method is demonstrated in both shape-preserving (which we term as isomorphic) and shape-changing (which we term as non-isomorphic) examples. The analysis and findings presented in this thesis are relevant and useful for understanding discontinuous phase transitions, in general, and can be particularly so for small, confined, and embedded systems that are increasingly being utilized in modern technologies.

This thesis examines the excimer-laser-induced melting of ELA-prepared silicon films using in situ transient reflectance and transmission analysis. The results clearly show that these polycrystalline films, which consist of columnar grains in contact with SiO₂, can melt in a largely and remarkably 2-D manner. Based on the presently and previously obtained experimental results, as well as considering the thermal, thermodynamic, and kinetic aspects of the melting-transition-relevant details, we suggest a model that consists of grain-boundary-initiated melting, followed by lateral melting proceeding into the transiently superheated interior of the grains. Additional experiments are performed which demonstrate how this 2-D melting behavior at least stems intrinsically from the presence in the material of melt-prone grain boundaries and superheating-permitting Si/SiO₂ interfaces. Next, the phase and temperature evolutions of the irradiated films are investigated using a numerical simulation program, which incorporates key material, thermodynamic, and kinetic parameters. We find that the center portion of the grains during (partial) melting (1) corresponds to, especially at the SiO₂-passivated surface, the hottest regions of the films during rapid heating, and (2) remains entirely solid throughout the thickness of the film, as the maximum temperature sustained in these unmelted solids remains well below the superheating limit of silicon at the Si/SiO₂ interface. Lastly, we discuss, and substantiate with results obtained from numerical simulations, the role that the manifested dimensionality of melting plays in dictating the efficiency with which the ELA crystallization technique can generate microstructurally uniform polycrystalline materials. The current discovery regarding the 2-D nature of melting should be recognized and appreciated as a critical process-enabling element for ELA, as the scenario permits microstructure evolution of the grains to take place in an effective manner.

This dissertation documents the development and demonstration of a new laser crystallization process called spot-beam annealing (SBA). The SBA method is a partial-melting-based laser-annealing method, which converts as-deposited amorphous Si films into high-mobility TFT-enabling polycrystalline films. SBA builds on the thermally additive utilization of multiple short-lived low-energy ultra-high-frequency pulses, achieved via substantially overlapped scanning of a small spot beam to incrementally and gradually heat and partially melt the beam-irradiated region. After a brief review of other laser crystallization technologies, the conceptual framework for the SBA process is introduced, and various possible implementation schemes and development paths are discussed. In the present work, the SBA method is implemented using a new class of ultra-high-frequency (>100 MHz), low-pulse energy (<1 𝜇J), short-pulse-duration (<1 ns) UV fiber lasers. The first half of the thesis (chapters 4 and 5) presents, the simulation- and calculation-based studies of the SBA process. A simple but relevant one-dimensional thermal analysis identifies the "dwell time" (associated with the overall intensity temporal profile defined by the collection of those pulses that irradiate a point in the film) as a key SBA parameter. Provided that a sufficient number of multiple shots are involved in irradiating the point in the film, this parameter dictates the overall thermal and transformation cycle of heating, primary melting, and solidification that enables the ultra-short-pulse-based SBA method to mimic the physical conditions encountered previously only using pulsed lasers with pulse duration in the range of tens to hundreds of nanoseconds; the precise range needed for optimally generating laser-annealed polycrystalline materials on glass and plastic substrates. Additionally, we also identify and examine an important differentiating feature of the SBA method, namely the highly transient temperature spikes that arise from the individual pulses incident onto a point on the film during overlapped scanning. By simultaneously considering the preliminary experimental results that are presented in this thesis (chapters 6 and 7), we suggest that these periodic temperature spikes, the specific degree of which depends on the temporal profile and energy density of individual pulses, can potentially play a key role in dictating certain important details of melting and solidification transitions encountered in SBA. In particular, we identify and elaborate on how the temperature fluctuations can affect how explosive crystallization of a-Si films is manifested in a different manner than has previously been observed. In addition, we point out how the fluctuations can control the degree to which the melting scenarios in SBA can deviate from the grain-boundary-melting-dominated 2-D transition scenario (as for instance encountered in pulsed-laser irradiation of columnar-grained polycrystalline films), where lateral melting is exclusively initiated at grain boundaries and propagates predominantly laterally into the superheated and defect-free interior of the grains. In the second half of the thesis, the experimental results that are obtained from a recently constructed research SBA system are presented, characterized, and evaluated. Specifically, the examination of single-scan and multiple-scan exposed Si films conducted using OM, AFM, and TEM material characterization techniques reveals that the method is capable of not only generating uniform polycrystalline Si films consisting of ordered grains with tight grain-size distribution around the beam wavelength, but it can furthermore be configured to produce polycrystalline films with an enhanced level of ordering as manifested in the films with a highly parallel ridge (HPR) pattern.

Dynamic loading of polycrystalline metallic materials can result in brittle or ductile fracture depending on the loading rates, geometry, and material type. At high strain rates, mechanical energy due to plastic deformation may lead to significant temperature rise and shear localization due to thermal softening. These shear bands reduce the stress-bearing capacity of the material and act as a precursor to ductile fracture (e.g. cracks that develop rapidly on top of a shear band). Reliable models are needed to predict the response of metals subject to dynamic loads. Understanding the heat transfer physics in thermo-mechanical problems when cracks are developed is of great importance. In particular, capturing the interplay between heat conduction and crack propagation is still an open research field. To accurately capture the heat transfer physics across crack surfaces, damage models degrading thermal-conductivity are necessary. In this thesis, a novel set of isotropic thermal-conductivity degradation functions is derived based on a micro-mechanics void extension model of Laplace's equation. The key idea is to employ an analytical homogenization process to find the effective thermal-conductivity of an equivalent sphere with an expanding spherical void. The closed-form solution is obtained by minimization of the flux differences at the outer surfaces of the two problems, which can be achieved using the analytical solution of Laplace's equations, so-called spherical-harmonics. Additionally, a new anisotropic approach is proposed in which thermal-conductivity, which depends on the phase-field gradient, is degraded solely across the crack. We show that this approach improves the near-field approximation of temperature and heat flux compared with isotropic degradation when taking the discontinuous crack solutions as reference. To demonstrate the viability of the proposed (isotropic and anisotropic) approaches, a unified model, which accounts for the simultaneous formation of shear bands and cracks, is used as a numerical tool. In this model, the phase-field method is used to model crack initiation and propagation and is coupled to a temperature-dependent visco-plastic model that captures shear bands. Benchmark problems are presented to show the necessity of the anisotropic thermal-conductivity approach using physics-based degradation functions in dynamic fracture problems. On the other hand, the computational burden in dynamic fracture problems with localized solution features is highly demanding. Iterative methods used for their analysis often require special treatment to be more efficient. Specifically, the nonlinear thermomechanical problems we study in this thesis lead to strain localizations, such as shear bands and/or cracks, and iterative solvers may have difficult time converging. To address this issue, we develop a novel updating domain decomposition preconditioner for parallel solution of dynamic fracture problems. The domain decomposition method is based on the Additive Schwarz Method (ASM). The key idea is to decompose the computational domain into two subdomains, a localized subdomain that includes all localized features of the solution and a healthy subdomain for the remaining part of the domain. In this way, one can apply different solvers in each subdomain, i.e. focus more effort in the localized subdomain. In this work, an LU solver is applied in both subdomains, however, while the localized subdomain is solved exactly at every nonlinear iteration, the healthy subdomain LU operator is reused and only selectively updated. Hence, significant CPU time savings associated with the setup of the preconditioner can be achieved. In particular, we propose a strategy for updating the preconditioner in the healthy subdomain. The strategy is based on an idealized performance-based optimization procedure that takes into account machine on-the-fly execution time. Three dynamic fracture problems corresponding to different failure modes are investigated. Excellent performance of the proposed updating preconditioner is reported in serial and parallel simulations.

<p> Quench-aged low carbon iron specimens, containing various distributions of carbide particles were fatigued in tension-compression under low amplitude strain control. </p> <p> Observations of the influence of second phase particles upon the development of dislocation substructure were made using transmission electron microscopy of thin foils. These observations were correlated with the cyclic mechanical response of the material, and with the response of material containing a fatigue saturation substructure of subsequent tensile overstrain. </p> <p> A rationale for the development of cyclic softening in the material is proposed, based on the requirements of continuity in plastically inhomogeneous materials. A source model for the observed instability of the cyclic substructure in tensile overstrain is described. </p> / Thesis / Master of Science (MS)

No description available.

La plupart des solides dans la vie quotidienne, tels que les métaux et les céramiques, sont des systèmes cristallins dans lesquels les atomes ou molécules sont arrangés sur une structure périodique. Les solides cristallins sont rarement composés d'un cristal unique, mais sont en général des systèmes polycristallins formés par un grand nombre de grains cristallins avec une même structure cristalline, mais différente orientation. On appelle joints de grain (JG) les réseaux 2D de défauts qui séparent deux grains d'orientation différente. Les polycristaux jouent un rôle important en science et technologie et une connaissance complète de leurs propriétés mécaniques est de grand intérêt. La plasticité des polycristaux est liée à leur microstructure, mais les mécanismes qui régissent leur comportement plastique sont encore mal compris, en partie du fait de limitations techniques pour les systèmes atomiques. D'autre part, les colloïdes, dont l'étude expérimentale est souvent plus aisée que celle des systèmes atomiques, du fait de temps et taille caractéristiques plus grands, sont souvent considérés comme des systèmes modèles pour les atomes. L'objectif de la thèse est l'étude des propriétés mécaniques d'un polycristal colloïdal formé par une suspension aqueuse d'un copolymère tribloc commercial (Pluronic F108), dopé par une faible quantité de nanoparticules (NPs). Le polymère présente une phase micellaire cristalline pour une gamme de température et de concentration. De manière similaire à ce qui est fait couramment pour les systèmes atomiques, on peut contrôler le taux de cristallisation en faisant varier la vitesse à laquelle l'échantillon est porté de la phase liquide à basse température (T ~ 0°C) à la phase micellaire cristalline à température ambiante. Une caractéristique importante est que la taille des grains peut être facilement contrôlée en faisant varier la vitesse d'augmentation de la température ou la concentration en NPs. Dans un premier temps, nous avons caractérisé la structure du polycristal par diffusion de neutrons (SANS) et de lumière. Les mesures par SANS ont permis de sonder la structure du polycristal à l'échelle du nanomètre, i.e. sur des échelles de longueur comparable à celle des micelles et des NPs. Nous avons constaté que la structure cristalline micellaire est conservée indépendamment de l'histoire thermique de l'échantillon et de la concentration en NPs. De plus, nous avons montré que la distribution des NPs dans l'échantillon est hétérogène: les grains sont pauvres en NPs alors que les JG sont enrichies en NPs. Par conséquent, les NPs ségrégent dans les JG et et jouent un rôle analogue aux impuretés dans les cristaux atomiques. En outre, en raison de leur ségrégation, les NPs sont structurées sur une échelle de longueur beaucoup plus grande que leur taille. Nous avons étudié la structure mésoscopique des NPs par diffusion statique de la lumière, grâce à un appareil de diffusion de la lumière (MALS) spécialement construit pour accéder à la plage correcte de vecteurs d'onde. D'autre part, afin d'étudier les propriétés mécaniques des polycristaux, des mesures de diffusion dynamique de la lumière ont été réalisée dans la configuration MALS sur un échantillon soumis à des déformations de cisaillement cycliques. Dans la configuration MALS, l'intensité diffusée étant dominée par les NPs dans les JG, la technique permet de sonder la dynamique du réseau de JG induite par le cisaillement. Expérimentalement, on calcule la corrélation de l'intensité diffusée mesurée après un nombre donné de cycles de déformation. Les données montrent systématiquement une décroissance de la corrélation après un nombre caractéristique de cycles, démontrant ainsi l'existence de plasticité dans les échantillons. À l'avenir, des échantillons avec des tailles de grain différentes seront étudiés. De telles expériences pourraient faire la lumière sur les liens entre plasticité d'un polycristal colloïdal et microstructure. / Most everyday life solids, such as metals and ceramics, are crystalline systems in which atoms or molecules are arranged in a regular periodic structure. Crystalline solids are rarely composed of one single crystal, but are usually polycrystalline systems made of a large number of crystalline regions (grains), which share a common crystal structure, but with different lattice orientations. The interfaces where crystallites meet are denoted as grain boundaries (GBs). Polycrystalline materials play an important role in science and technology and a complete knowledge of their mechanical properties, including their elasticity and plasticity, is of great interest. It is well known that the plasticity of polycrystals is related to their microstructure, but the mechanism governing the plastic behavior is still poorly understood, partly because of the limits of experimental techniques and simulations for atomic polycrystals. On the other hand, colloids are often regarded as model systems for atoms, since many of the forces governing the behavior of condensed matter govern also that of colloidal suspensions, whose experimental study is often easier than that of atomic systems because of the larger characteristic time- and length-scales. In particular, colloidal crystalline systems can be used to investigate mechanical properties of polycrystals. The aim of the PhD thesis is the investigation of the mechanical properties of a colloidal polycristal formed by an aqueous suspension of a commercial triblock copolymer called Pluronic F108, doped with a small amount of nanoparticles (NPs). The polymer presents a micellar crystalline phase for a given range of temperature and concentration. Similarly to what is commonly done for atomic systems, we can control the crystallization rate by varying the speed at which the sample is brought from the fluid, at low temperature (T ~ 0°C), to the crystal phase at room temperature. An important characteristic of our system is that the grain size can be easily tuned by changing the temperature rate or the nanoparticles concentration. We have first characterized the structure of the Pluronic polycrystal using neutron (SANS) and light scattering. The SANS measurements have permitted to investigate the (doped) Pluronic polycrystal at nanometer length scale, i.e. at the length scale of the micelles and nanoparticles. We have found that the micellar crystal structure is preserved independently of the thermal history of the sample and the amount of added nanoparticles. Moreover, we have shown that the NPs distribution into the sample is heterogeneous: grains are poor in NPs whereas GBs are enriched in NPs. Hence, NPs segregate into the GBs as impurities in atomic crystals. In addition, because of their segregation in the GBs, NPs form structures on a length scale much larger than their size, that we have investigated with static light scattering, thanks to a novel light scattering apparatus (MALS) specifically built to access the correct range of wave-vectors. On the other hand, in order to investigate the mechanical properties of the Pluronic crystal, dynamic light scattering measurements have been performed with the MALS setup on the Pluronic polycrystal submitted to cyclic shear deformations. Since, in the range of wave-vectors covered by the MALS apparatus, the scattered intensity is dominated by the NPs segregated in the GBs, the techniques allows the shear-induced dynamics of the GB network to be probed. Experimentally, one computes the correlation of the scattered intensity measured after a given number of shear deformation cycles. Data systematically show that the correlation decays after a characteristic number of cycles, demonstrating the existence of plasticity. In future, samples with different grain size will be investigated with this technique. Such experiments could shed light on how the plasticity of a colloidal polycrystal is related to its polycrystalline microstructure.

Melting and solidification constitute the fundamental pathways through which a thin-film material is processed in many beam-induced crystallization methods. In this thesis, we investigate and leverage a specific beam-induced, melt-mediated crystallization approach, referred to as Mixed-Phase Solidification (MPS), to examine and scrutinize how a polycrystalline Si film undergoes the process of melting and solidification. On the one hand, we develop a more general understanding as to how such transformations can transpire in polycrystalline films. On the other hand, by investigating how the microstructure evolution is affected by the thermodynamic properties of the system, we experimentally reveal, by examining the solidified microstructure, fundamental information about such properties (i.e., the anisotropy in interfacial free energy). Specifically, the thesis consists of two primary parts: (1) conducting a thorough and extensive investigation of the MPS process itself, which includes a detailed characterization and analysis of the microstructure evolution of the film as it undergoes MPS cycles, along with additional development and refinement of a previously proposed thermodynamic model to describe the MPS melting-and-solidification process; and (2) performing MPS-based experiments that were systematically designed to reveal more information on the anisotropic nature of Si-SiO₂ interfacial energy (i.e., σ_{Si-SiO₂}). MPS is a recently developed radiative-beam-based crystallization technique capable of generating Si films with a combination of several sought-after microstructural characteristics. It was conceived, developed, and characterized within our laser crystallization laboratory at Columbia University. A preliminary thermodynamic model was also previously proposed to describe the overall melting and solidification behavior of a polycrystalline Si film during an MPS cycle, wherein the grain-orientation-dependent solid-liquid interface velocity is identified as being the key parameter responsible for inducing the observed microstructure evolution. The present thesis builds on the abovementioned body of work on MPS. To this end, we note that the limited scope of previous investigations motivates us to perform more thorough characterization and analysis of the experimental results. Also, we endeavor to provide more involved explanations and expressions to account for the observed microstructure evolution in terms of the proposed thermodynamic model. To accomplish these tasks forms the motivation for the first portion of this thesis. In this section we further develop the thermodynamic model by refining the expression for the solid-liquid interface velocities. In addition, we develop an expression for the grain-boundary-location-displacement distance in an MPS cycle. This is a key fundamental quantity that effectively captures the essence of the microstructure evolution resulting from MPS processing. Experimentally, we conduct a thorough investigation of the MPS process by focusing on examining the details of the microstructure evolution of {100}-surface-oriented grains. Firstly, we examine and analyze the gradual evolution in the microstructure of polycrystalline Si films being exposed to multiple MPS cycles. A Johnson-Mehl-Avrami-Kolmogorov-type (JMAK-type) analysis is proposed and developed to describe the microstructure transformation. Secondly, we investigate the behavior of grains with surface orientations close to the <100> pole. Orientation-dependent (in terms of their extent of deviation from the <100> pole) microstructure evolution is revealed. This observation indicates that the microstructure of the film continues to evolve to form an even tighter distribution of grains around the <100> pole as the MPS process proceeds. During MPS melting-and-solidification cycles, a unique near-equilibrium environment is created and stabilized by radiative beam heating. Therefore, the microstructure of the resulting films is expected to be explicitly and dominantly affected by various thermodynamic properties of the system. Specifically, we identify the orientation-dependent value of the Si-SiO₂ interfacial energy as a key factor. This being the case, the MPS method actually provides us with an ideal platform to experimentally study the Si-SiO₂ interfacial energy. In the second part of this thesis, we perform MPS-based experiments to systematically investigate the orientation-dependent Si-SiO₂ interfacial energy. Two complementary approaches are designed and conducted, both of which are built on examining the texture evolution of different surface orientations resulting from MPS melting-and-solidification cycles. The first approach, “Large-Area Statistical Analysis”, statistically examines the overall microstructure evolution of non-{100}-surface-oriented grains. By interpreting the changes in the surface-orientation distribution of the grains in terms of the thermodynamic model, we identify the orientation-dependent hierarchical order of Si-SiO₂ interfacial energies. The second approach, “Same-Area Local Analysis”, keeps track of the same set of grains that undergo several MPS cycles. An equivalent set of information on the Si-SiO₂ interfacial energy is extracted. Both methods reveal, in a consistent manner, an essentially identical Si-SiO₂ interfacial energy hierarchical order for a selected group of orientations. Also, the “Same-Area Local Analysis” provides some additional information that cannot otherwise be obtained (such as information about the evolution of two adjacent grains of specific orientations). Using such information and based on the grain-boundary-location-displacement distance derived using the thermodynamic model, we further deduce and evaluate the magnitude of Δσ_{Si-SiO₂} for certain orientation pairs.

Herein we consider material tensors that pertain to thin sheets or thin films, which we model as two-dimensional objects. We assume that the thin sheet in question carries a crystallographic texture characterized by an orientation distribution function defined on the rotation group SO(3), which is almost transversely-isotropic about the sheet normal so that mechanical and physical properties of the thin sheet have weak planar-anisotropy. We present a procedure by which a special orthonormal basis can be determined in each tensor subspace invariant under the action of the orthogonal group O(2). We call members of such special bases irreducible basis tensors under O(2). For the class of thin sheets in question, we derive a representation formula in which each tensor in any given tensor subspace Z is written as the sum of a transversely-isotropic term and a linear combination of orthonormal irreducible basis tensors in Z, where the coefficients are given explicitly in terms of texture coefficients and undetermined material parameters. In addition to the general representation formula, we present also the specialized form for subspaces of tensor products of second-order symmetric tensors, a type commonly found in mechanics of materials.

Understanding the elastic-plastic response of polycrystalline materials is an extremely difficult task. A polycrystalline material consists of a large number of crystals having different orientations. On its own, each crystal would deform in a specific manner. However, when it is part of a polycrystalline aggregate, the crystal has to ensure compatibility with the aggregate, which causes the response of the crystal to change. Knowing the response of a crystal enables us to view the change in orientation of the crystal when subjected to external macroscopic forces. This ability is useful in predicting the evolution of texture in a material. In addition, by predicting the response of a crystal that is part of a polycrystalline aggregate, we are able to determine the free energy of each crystal. This is useful in studying phenomena like grain growth and diffusion of atoms across high energy grain boundaries. This dissertation starts out by presenting an overview of the elastic and plastic response of single crystals. An attempt is made to incorporate a hardening law which can describe the hardening of slip systems for all FCC materials. The most commonly used theories for relating the response of single crystals to that of polycrystalline aggregates are the Taylor model and the Sachs model. A new theory is presented which attempts to encompass the Taylor as well as the Sachs Model for polycrystalline materials. All of the above features are incorporated into the software program "Crystals".