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An optimization problem is proposed for inferring physical properties of polycrystals given ultrasonic (elastic) wave velocity measurements, made across multiple sample orientations. The feasibility of the method is demonstrated by inferring both the effective grain elastic parameters and the grain c -axis orientation distribution function (ODF) of ice-core samples from Priestley glacier, Antarctica. The method relies on expanding the ODF in terms of a spherical harmonic series, which allows for a non-parametric estimation of the sample ODF. Moreover, any linear combination of the Voigt (strain) and Reuss (stress) homogenization scheme is allowed, although for glacier ice, the exact choice is found to matter little for bulk elastic behaviour, and thus for inferred physical properties, too. Finally, the accuracy of the inferred grain elastic parameters is discussed, including the well-posedness and shortcomings of the inverse problem, relevant for future adoptions in glaciology, geology and elsewhere.

For polycrystalline materials, key material properties, such as mechanical anisotropy or transformation behavior, and magnetic properties strongly depend on the crystallographic texture of the crystalline material. Assessment of texture is generally destructive and highly local. Only high energy X-ray diffraction at synchrotron sources and neutrons enable to study, non-destructively, the microstructure in the bulk of materials. Here, we report how progress in Laue three-dimensional neutron diffraction tomography enables to index the crystallographic orientation of several hundred grains and, thus, enables grain-resolved characterization of texture in the volume of centimeter-sized coarse-grained samples with statistical significance. To demonstrate the neutron technique for characterizing the crystallographic microstructure, we investigate a Fe-Ni-Mn austenitic alloy. A total number of 481 grains within a 1 cm(3) of material is indexed and the results in assessing the crystallographic texture are compared with electron backscatter diffraction measurements. The short exposure times and non-destructive nature of the Laue three-dimensional neutron diffraction render it a novel promising method for corresponding characterization. (C) 2022 The Author(s). Published by Elsevier Ltd.

The prediction of local field statistics from effective properties is an open problem in the field of micromechanics. Partial information on the local field statistics is accessible from homogenization assumptions. In particular, exact phase-wise second moments of stresses can be calculated analytically from the effective strain energy density. In recent years, full-field calculations have become efficient enough to sample large ensembles of microstructures in the plastic regime (e.g. Gehrig et. al [4]). In the present work, the maximum entropy method known from statistical thermodynamics is used to estimate first and second moments of local stresses from known eigenstrain distributions. The simple and refined formulations of the maximum entropy method proposed by Kreher and Pompe [9] are considered. While the simple method yields satisfactory results for a large amount of material classes (cf. Krause and Böhlke [7]), we prove that it does not respect the linearity of the eigenstrain problem. We further show that neither method corresponds to the exact second moments of stresses known from the effective strain energy density. By incorporating additional information, we find an improved maximum entropy method. As an example, we analyze stress fluctuations in polycristalline titanium.For the exact analytical solution and the maximum entropy methods, we use the singular approximation and the Hashin-Shtrikman bounds. For comparison, we numerically approximate full-field statistics using an FFT approach. In all methods, the stress fluctuations caused by the anisotropy of the single crystal strongly influence the elastic-plastic transition.

The effective mechanical properties of a polycrystal depend directly on the single-crystal properties of each grain and its crystallographic orientation with respect to the load axis. While the micromechanical approach has been used quite extensively to study the influence of grain shape and crystallographic texture on the resulting mechanical behavior of a polycrystal, the influence of the crystal plasticity parameters, which describe the constitutive behavior of the single crystal, requires to be investigated systemically because, typically, these parameters are fitted to describe a given material behavior. In the current research, this gap is filled by systemically studying the effect of changes in crystal plasticity parameters on the effective mechanical properties of polycrystals. The numerical model employed here consists of a representative volume element of 100 grains, and the material properties are described by using a non-local crystal plasticity model. A proper homogenization technique was used to homogenize the micromechanical results to an effective macroscopic material response. The equivalent stress versus equivalent plastic strain curve was obtained numerically by introducing the Voce-type hardening law, mimicking the material behavior in uniaxial tensile tests. The four parameters of the Voce-type hardening law were fitted to the macroscopic stress-strain curves, and the correlation between the crystal plasticity parameters and the Voce parameters has been studied, which is an efficient way to study the influence of microscopic material descriptions on the macroscopic behavior of polycrystals.

5 pags., 3 figs., 2 tabs., The present study addresses to the synthesis and determination of the dielectric function of CuZn(GeSi)Se solid solutions with x = 0.4 and 0.8 over the range 1–4.5 eV by spectroscopic ellipsometry analysis, with the aim to achieve a suitable band gap tuning. The dielectric function of the samples is determined using the Adachi model. From the analysis the lowest E transition and high energy E and E transitions are identified. It is found that the band gap varies nonlinearly on composition in the CuZn(GeSi)Se alloys and band gap values as large as 1.87 eV are obtained. These results are essential for the design of efficient tailored photovoltaic solar cells and show the high potential of the kesterite compounds for the development of low-cost sustainable future solar cells., This research was supported by the H2020 programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). The authors appreciate also the financial support from the Institutional Project ANCD 20.80009.5007.03.

Polycrystalline metal systems in the samples made of steel 40X with different morphology and parameters of the distribution of large-angle grains boundaries for energy have been examined. The effect of the structural-energy state of grains boundaries on the operational reliability of improved steel 40X has been established. Based on the hierarchical modeling of polycrystals structure, new approaches have been proposed and algorithms have been developed for defining relationships between the structure that is formed in the technological processing of materials and stages in the life cycle of parts. It has been revealed that it is advisable to use, as a digital prototype of the structure of polycrystalline alloys that describes their performance under conditions of contact loads, the matrix representation of a system model that would incorporate the quantitative characteristics of grains. By using the devised procedures, the ways to execute technological control over the energy state of grain boundaries in the structural components have been defined in order to improve durability of parts exposed under contact loads. An estimation-experimental method has been developed to assess the effect of quantitative characteristics of the structure on the parameters of strength of the grains boundaries and their ability to form intragrain damage under external loads. The energy level of grains boundaries and triple joints between the groups of small and large grains is higher than that between grains of the same size. The boundary surfaces with a high level of energy are places where damage occurs at technological processing and under external loads on structural materials. This points to the crucial role of large-angle boundaries placed between triple joints with a high energy gradient in the process of forming microstructurally short cracks and intragrain destruction of polycrystalline systems. The use of hierarchical modeling methods and computational materials science makes it possible to improve the operational reliability of articles by choosing the optimal parameters for internal boundary surfaces. The lower cost of the parts' life cycle is achieved by thermal treatment regimes, which alter the quantitative characteristics of the steel structure

Crack propagation in polycrystalline specimens is studied by means of a generalized finite element method with linear elastic isotropic grains and cohesive grain boundaries. The corresponding mode-I intergranular cracks are characterized using a grain boundary brittleness criterion that depends on cohesive law parameters and average grain boundary length. It is shown that load–displacement curves for specimens with the same microstructure and for various cohesive law parameters can be obtained from a master load–displacement curve by means of simple linear elastic fracture mechanics scaling relations. This property is a consequence of the independence of intergranular crack paths from cohesive law parameters. Perfect scaling is obtained for cases characterized by the same grain boundary brittleness number, irrespective of its value, whereas scaling is approximated for cases with different but relatively large values of the grain boundary brittleness number. The former case corresponds to grain boundary traction profiles that are identical apart from a scale factor; in the latter case, a large grain boundary brittleness number implies similar, apart from a scale factor, traction profiles. By exploiting this property, it is demonstrated that computationally expensive simulations can be avoided above a certain grain boundary brittleness threshold value.

The effects of grain size, source density, and misorientations on the dislocation configurational energy area density are investigated using two-dimensional discrete dislocation plasticity. Grain boundaries are modeled as impenetrable to dislocations. The considered grain size ranges from $$ 0.4\;\upmu{\text{m}}^{2} $$ to $$ 8.0\;\upmu{\text{m}}^{2} $$ . The configurational energy area density displays a strong size dependence, similar to the stress response. Two sets of materials are considered, with low and high source/obstacle density. The high-source-density specimens exhibit negative configurational energy, implying that the dislocation structure is more stable than for isolated dislocations . The contribution of misorientation to the configurational energy density is analyzed using specimens with a single orientation or a checkerboard arrangement. The configurational energy density is found not only to depend on the dislocation spacing but also to be related to the local stress states. Low source densities lead to higher (positive) configurational energy densities.

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.

Two codes have been developed: one allows columnar polycrystal geometries generated by software such as Neper or DREAM3D to be imported into the Complete Abaqus Environment for further geometric manipulation and meshing with arbitrary element types; the other code generates zero thickness, four noded interface elements at the boundaries between the crystals. Together these allow intergranular fracture simulations to be performed easily in Abaqus. This approach can automatically create different grain structures and allows the mechanical properties of metals and other crystalline materials to be simulated. This is important as it enables the sample to sample variability of fracture and mechanical properties that is observed in experiments to be reproduced. These scripts extend the simulation capabilities of the finite element solver Abaqus.

A.V. Stepanenko, a.v.stepanenko2@urfu.me. Ural Federal University named after the first President of Russia B.N. Yeltsin, Ekaterinburg, Russian Federation Степаненко Андрей Викторович, канд. физ.-мат. наук, доцент кафедры физики, Институт фундаментального образования, Уральский федеральный университет им. первого Президента России Б.Н. Ельцина, г. Екатеринбург; a.v.stepanenko2@urfu.me. Various methods of experimental study of the texture of polycrystalline metals and alloys are considered. The texture formed upon plastic deformation or upon recrystallization annealing leads to anisotropy of the physical and mechanical properties of metallic materials. For express calculations of the values of anisotropic physical properties of polycrystals, methods based on the use of integral characteristics of textures are preferred. An X-ray method for calculating orientation factors for textured metals with a hexagonal close-packed structure is discussed. Calculation of the orientational factor of texture Δi assumes averaging over the representative volume of the polycrystalline sample under study. The basis for calculating the orientational factors of a textured polycrystal is the reverse pole figure. In the standard method for calculating the reverse pole figure of hcp metals, 17 crystallographic orientations are used. Yttrium texture analysis can be performed using an extended set of pole densities on the reverse pole figure. A method for calculating the reverse pole figure for hcp metals is proposed, which takes into account 24 crystallographic orientations. The data of X-ray diffraction studies of polycrystalline yttrium after deformation by cold rolling with a degree of deformation ɛ = 45% are discussed. On the basis of the obtained X-ray data, the reverse pole figures of deformed yttrium were constructed using the standard method and using an extended set of pole densities. The orientation factor of the texture Δi of the deformed yttrium polycrystal was determined. A comparative calculation of the resistivity ρi of textured yttrium has been performed using two experimental procedures. The extended set of pole densities on the reverse pole figure is most convenient when studying samples in which a sharp deformation texture is formed. The proposed technique accurately describes the dynamics of texture formation, describes more precisely the features of the pole density distribution on the reverse pole figure, and allows one to determine the anisotropic physical properties of materials. В статье рассматриваются различные методы экспериментального исследования текстуры поликристаллических металлов и сплавов. Сформированная при пластической деформации или при рекристаллизационном отжиге текстура приводит к анизотропии физических и механических свойств металлических материалов. Для экспресс-расчетов величин анизотропных физических свойств поликристаллов предпочтительными являются методики, основанные на использовании интегральных характеристик текстур. Обсуждается рентгеновская методика расчета ориентационных факторов для текстурованных металлов с гексагональной плотноупакованной структурой. Расчет ориентационного фактора текстуры Δi предполагает усреднение по представительному объему исследуемого поликристаллического образца. Основой для расчетов ориентационных факторов текстурованного поликристалла является обратная полюсная фигура. Стандартная методика построения обратной полюсной фигуры ГПУметаллов требует учета 17 полюсных плотностей. Текстурный анализ иттрия возможно выполнить по расширенному набору полюсных плотностей на обратной полюсной фигуре. Предложена методика построения обратной полюсной фигуры для ГПУ-металлов на основе 24 полюсных плотностей. Обсуждаются данные рентгенографического исследования поликристаллического иттрия после деформации холодной прокаткой со степенью деформации ε = 45 %. На основе полученных рентгенографических данных построены обратные полюсные фигуры деформированного иттрия с использованием стандартного метода и с использованием расширенного набора полюсных плотностей. Определены ориентационного фактора текстуры Δi деформированного поликристалла иттрия. Выполнен сравнительный расчет удельного сопротивления ρi текстурированного иттрия по двум экспериментальным методикам. Расширенный набор полюсных плотностей на обратной полюсной фигуре наиболее удобен при исследовании образцов, в которых сформирована острая текстура деформации. Предлагаемая методика позволяет точнее описывать динамику текстурообразования, описывать неоднородность распределения полюсной плотности на обратной полюсной фигуре и определять анизотропные физические свойства материалов.

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 (

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.

Аннотация, Международный научно-исследовательский журнал, Выпуск 1 (103) 2021, Pages 139-144

The dependences of electrical conductivity, the Hall coefficient, the Seebeck coefficient, thermoelectric power factor and microhardness of Bi2Se3 polycrystals on the degree of deviation from stoichiometry 59.9–60.0 at. % Se and temperature (77–300 K) were obtained. The samples exhibited n-type conductivity in the studied ranges of compositions and temperatures. The boundaries of the Bi2Se3 homogeneity region were estimated. A non-monotonic behavior of the concentration dependences of the properties in the studied composition range, associated with a change in the phase composition and defect structure under the deviation from stoichiometry, was observed. The calculation of the power coefficient in the temperature dependence of electron mobility showed that in a stoichiometric Bi2Se3, electrons are predominantly scattered by acoustic phonons, and under the deviation from stoichiometry the contribution of impurity scattering increases. Based on the experimental data, the Fermi energy EF was calculated in the approximation of the relaxation time and within the framework of the single-band model with a quadratic dispersion law. The obtained values of EF showed that both in stoichiometric and non- stoichiometric Bi2Se3, the conduction is mainly due to electrons in the lower conduction subband.

It has long been known that the mechanical strength of finely grained solid state polycrystals could be enhanced when the grain size is reduced. Indeed, the equation linking the yield stress and the inverse square root of grain size was introduced in the 1950s by Hall and Petch. Since then this relationship has been widely used to engineer structural metals and alloys. To date, no similar behavior has been reported in materials other than atomic systems where the grain size usually lies in the nanometric range. The purpose of the present work is to study the influence of grain size on the mechanical strength enhancement of a soft colloidal 'alloy' made of micellar polycrystalline grains and silica nanoparticles. The nanoparticles act as nucleation sites and their concentration promotes the variation of the polycrystalline grain size. This system bears resemblance to solid state polycrystals; however the achieved grain length scale is situated in the micrometric domain. We show that the grain size evolves non-monotonically, first decreasing then increasing, when the nanoparticle concentration increases. Our main result is that the yield stress rigorously obeys the Hall-Petch law and follows a linear variation as a function of the inverse square root of the grain diameter. We believe that our experimental approach offers new possibilities to study the poorly understood mechanical aspects of polycrystalline and nanocrystalline structures, such as their plasticity, using non-destructive techniques.

The current Advanced Gas-cooled Reactors (AGR) in the UK are approaching their design life. The reactor operator, EDF Energy, is seeking to extend their life due to ecological and financial reasons. One challenge to plant life-extension is the accurate prediction of high-temperature deformation and failure of Type 316H stainless steel, used in components of the reactor boiler section. Under the operating conditions of these components (470-650°C; 10-300 MPa), creep and cyclic plasticity alter the material’s microstructure and failure response. Empirical models are used in industry to predict the deformation and damage response of Type 316H. Predictions can be in poor agreement with plant-recorded/experimental deformation response under complex loading histories. Furthermore, the models are deficient in capturing the sensitivity of the creep damage process to the microstructural state of the material. The overall aim of this research is to improve the understanding of high-temperature deformation and failure response of Type 316H under common plant conditions in order to make realistic life predictions. This is achieved through development of micromechanical models for deformation and damage, employed within polycrystalline modelling frameworks. An existing physically-based self-consistent model (SCM) for inelastic deformation, developed at Oxford University, captures accurately the global and grain-scale deformation response of Type 316H during short-term plasticity and creep. The model was found deficient in predicting cyclic deformation and stress relaxation. This study enhances the model by incorporating the fundamental physics of these processes. Predictions by the enhanced SCM provide insights into the evolution of deformation, microstructure and residual stress state of Type 316H under plant-relevant loading histories. The insights could inform assessment procedures in industry to more accurately account for the accumulation of inelastic strains and damage. Although the enhanced SCM predicts the global response of the material, localization effects, which are of importance to creep damage processes, are not captured. To address this, a crystal plasticity finite element (CPFE) model was developed. It employs the same micromechanical model as the SCM and predictions of global material response by the two frameworks are in agreement. The CPFE scheme captures stress and strain localization near grain boundaries and it was further extended to describe the grain interface response. A combined CPFE-interface element framework was developed and initially used to study the effects of grain-boundary sliding on creep deformation. Results suggest that the effects of grain-boundary sliding on the macroscopic deformation of Type 316H are limited. Features of the local stress and strain fields at grain boundaries, which could affect intergranular damage response, are captured by the CPFE-interface element scheme. A common creep damage mode in Type 316H under the operating range of interest is intergranular cavitation. Review of the literature confirmed that cavitation in Type 316H is controlled by cavity nucleation, which is not fully understood. In order to provide further insights into the physics of this process, existing strain-based empirical and stress-based (classical nucleation theory) nucleation models were modified in this study by considering experimentally-observed features of cavity nucleation in Type 316H. The models were employed locally within the developed CPFE-interface element framework. Modelling results suggest that the strain-based model as a function of local inelastic strain rate does not explain the physical nature of the nucleation process. By contrast, the modified classical nucleation theory captures both the macroscopic failure response and trends in distribution of cavities and failure in the microstructure. These findings outline key aspects of the nucleation process, which need to be examined experimentally. A number of missing features are identified in the mechanistic model, which need to be incorporated in future unified cavity nucleation theories.

6 pags., 4 figs., 2 tabs., The pseudo dielectric function of CuZnSn(SSe) [x = 0.35, 0.62, 0.81] bulk polycrystals is determined over the range 1.1–4.6 eV at room temperature from the analysis of spectroscopic ellipsometry data using the Adachi model. From the analysis, the lowest E transition and high energy E and E transitions are clearly identified, and used to follow the evolution of the pseudo dielectric function as a function of the composition. It is shown that the fundamental E and high energy E transitions can be tuned by increasing the sulfur content over a range of 0.3 eV. These results show the potential of the kesterite compounds for the design of efficient tailored photovoltaic solar cells., The research leading to the presented results was partially supported by the European Project INFINITE-CELL (Ref. H2020-MSCA-RISE-2017-777968, 2017e2021,www.infinitecell.eu) andthe Spanish MINECO Projects“WINCOST”(ENE2016-80788-C5-2-R) and“SENSIL”RTI2018-096498-B-I00 (MCIU/AEI/FEDER, UE).The authors from the Institute of Applied Physics appreciate the financial support from the Bilateral Project No. ANCD19.80013.16.02.01F/BL and from the Institutional Project No. CSSDT15.817.02.04A. Authors from IREC belong to the M-2E (Electronic Materials for Energy) Consolidated Research Group and the XaR-MAE Network of Excellence on Materials for Energy of the“Generalitat de Catalunya”. M. Guc thanks Generalitat de Catalunya for the TECNIO springþ fellowship (TECSPR18-1-0048).

A severe obstacle for the routine use of crystal plasticity models is the effort associated with determining their constitutive parameters. Obtaining these parameters usually requires time-consuming micromechanical tests that allow probing of individual grains. In this study, a novel, computationally efficient, and fully automated approach is introduced which allows the identification of constitutive parameters from macroscopic tests. The approach presented here uses the response surface methodology together with a genetic algorithm to determine an optimal set of parameters. It is especially suited for complex models with a large number of parameters. The proposed approach also helps to develop a quantitative and thorough understanding of the relative influence of the different constitutive parameters and their interactions. Such general insights into parameter relations in complex models can be used to improve constitutive laws and reduce redundancy in parameter sets. The merits of the methodology are demonstrated on the examples of a dislocation-density-based crystal plasticity model for bcc steel, a phenomenological crystal plasticity model for fcc copper, and a phenomenological crystal plasticity model incorporating twinning deformation for hcp magnesium. The approach proposed is, however, model-independent and can be also used to identify parameters of, for instance, fatigue, creep and damage models. The method has been implemented into the Dusseldorf Advanced Material Simulation Kit (DAMASK) and is available as free and open-source software. The capability of translating complex material response into a micromechanical digital twin is an essential precondition for the ongoing digitalization of material property prediction, quality control of semi-finished parts, material response in manufacturing and the long-term behavior of products and materials when in service.

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.

We present a thermal–mechanical–chemical-phase field model that captures the multi-physical coupling effects of precipitation creeping, crystal plasticity, anisotropic fracture, and crack healing in polycrystalline rock at various temperature and strain-rate regimes. This model is solved via a fast Fourier transfer solver with an operator-split algorithm to update displacement, temperature and phase field, and chemical concentration incrementally. In nuclear waste disposal in salt formation, brine inside the crystal salt may migrate along the grain boundary and cracks due to the gradient of interfacial energy and pressure. This migration has a significant implication on the permeability evolution, creep deformation, and crack healing within rock salt but is difficult to incorporate implicitly via effective medium theories compared with computational homogenization. As such, we introduce a thermodynamic framework and a corresponding computational implementation that explicitly captures the brine diffusion along the grain boundary and crack at the grain scale. Meanwhile, the anisotropic fracture and healing are captured via a high-order phase field that represents the regularized crack region in which a newly derived non-monotonic driving force is used to capture the fracture and healing due to the solution–precipitation. Numerical examples are presented to demonstrate the capacity of the thermodynamic framework to capture the multiphysics material behaviors of rock salt.

Одержано залежності мікротвердості H, коефіцієнта Зеєбека S та електропровідності σ від складу литих і гарячепресованих полікристалів напівпровідникових твердих розчинів заміщення PbSe1−xTex (x = 0–0,045) за кімнатної температури. Встановлено, що всі зразки мають дірковий тип провідності. На залежностях H(x), S(x) і σ(x) спостерігаються концентраційні аномалії властивостей поблизу x = 0,01 та 0,02 як для литих, так і для пресованих зразків. Наявність аномалій пов’язується з ефектами взаємодії атомів в домішковій підсистемі кристалу при переході від розведених до концентрованих твердих розчинів. Показано, що спосіб приготування зразків (литі або гарячепресовані) не впливає на наявність аномальних ділянок на залежностях властивостей від складу твердого розчину, але дещо змінює їх характер. При практичному застосуванні твердих розчинів PbSe1−xTex необхідно приймати до уваги немонотонний характер зміни властивостей. The room-temperature dependences of microhardness H, Seebeck coefficient S and electrical conductivity σ on the substitution of both the cast and hot pressed samples of PbSe1−xTex (x = 0–0.045) solid solution are obtained. As established, all of samples have a p-type of conductivity. Concentration anomalies in the dependences of H(x), S(x) and σ(x) in the vicinity of x = 0.01 and 0.02 for both the cast and pressed samples are detected. The existence of anomalies is connected with the interaction of atoms in the impurity subsys tem of crystal under transition from the diluted solid solution to concentrat ed one. As shown, a method of preparing (cast or hot pressed samples) has no impact on the existence of anomalous areas in composition dependences of solid solution properties, but slightly changes their character. The non monotonic character of the properties should be taken into account in practi cal application of PbSe1−xTex solid solutions Получены зависимости микротвёрдости H, коэффициента Зеебека S и электропроводности σ литых и горячепрессованных поликристаллов твёрдых растворов замещения PbSe1−xTex (x = 0–0,045) при комнатной температуре. Установлено, что все образцы имеют дырочный тип проводимости. На зависимостях H(x), S(x) и σ(x) наблюдаются концентрационные аномалии свойств вблизи x = 0,01 и 0,02 как для литых, так и для прессованных образцов. Наличие аномалий связывается с эффектами взаимодействия атомов в примесной подсистеме кристалла при переходе от разбавленных к концентрированным твёрдым растворам. Показано, что способ приготовления образцов (литые или горячепрессованные) не влияет на наличие аномальных участков на зависимостях свойств от состава твёрдого раствора, но несколько изменяет их характер. При практическом применении твёрдых растворов PbSe1−xTex необходимо учитывать немонотонный характер изменения свойств.

The excimer-laser annealed (ELA) polycrystalline silicon (p-Si or polysilicon) thin film, which influences more than 100-billion-dollar display market, is the backplane material of the modern advanced LCD and OLED products. The microstructure (i.e. ELA microstructure) and surface morphology of an ELA p-Si thin film are the two main factors determining the material properties, and they significantly affect the performance of the subsequently fabricated thin film transistors (TFTs). The microstructure is the result of a rather complex crystallization process during the ELA which is characterized as far-from-equilibrium, multiple-pulse-per-area and processing-parameter dependent. Studies of the ELA microstructure and the surface morphology closely related to the device performance as well as the microstructure evolution during the ELA process are long-termly demanded by both the scientific research and the industrial applications, but unfortunately have not been thoroughly performed in the past. The main device-performance-related characteristics of the ELA microstructure are generally considered to be the grain size and the presence of the dense grain boundaries. In the work of this thesis, an image-processing-based program (referred to as the GB extraction program) is developed to extract the grain boundary map (GB map) out of the transmission electron microscope (TEM) images of the ELA microstructure. The grain sizes are straightforwardly calculated from the GB map and statistically analyzed. More importantly, based on the GB maps, we propose and perform a rigorous scheme that we call the local-microstructure analysis (LMA) to quantitatively and systematically analyze the spatial distribution of the grain boundaries. The “local area” is mainly defined by the geometry and the location of a TFT. The successful extraction of the GB map and the subsequent LMA are permitted by our unique TEM skills to produce high-resolution TEM micrographs containing statistically significant number of grains for sensible quantitative analysis. The LMA unprecedentedly enables quantitative and rigorous analysis of spatial characteristics of the microstructure, especially the device geometry- and location-related characteristics. Additionally, we present and highlight the benefits of the LMA approach over the traditional statistical grain-size analysis of the ELA microstructure. From the grain-size analysis, we find that grain size across a statistically significant number of grains generally follows the same distribution as in the stochastic grain growth scenario at the beginning of the ELA process when the laser pulse (i.e. shot) number is small. As the shot number increases, the overall grain size monotonically increases while the distribution profile becomes broader. When the scan number reaches the ELA threshold (several tens of laser shots), the distribution profile substantially deviates from the stochastic profile and shows two sharp peaks in grain size around 300nm and 450nm, which is consistent with the previously proposed theory of energy coupling and nonuniform energy deposition during ELA. From the LMA, local nonuniformity of grain boundary density (GB density) at the device length scales and regions of high grain boundary periodicity are identified. More importantly, we find that the local nonuniformity is much more pronounced when p-Si film exhibits some level of spatial ordering, but less pronounced for a random grain arrangement. It is worth noting that the devices of different sizes and orientation have different sensitivity to the local nonuniformity of the ELA-generated p-Si thin film. In addition, based on the analysis results, the connection between the microstructure evolution and the partial melting and resolidification process of the Si film is discussed. Aside from the microstructure, the surface morphology of the ELA films, featuring pronounced surface protrusions, is characterized via an atomic force microscope (AFM). Attempts to planarize those surface protrusions detrimental to the subsequent device performance are conducted. In the attempts, the as-is (oxide-capped) ELA films and the BHF-treated ELA films are subjected to single shots of excimer irradiation. When the results are compared, an anisotropic melting phenomenon of the p-Si grains is identified, which appears to be strongly affected by the presence of the surface oxide capping layer. Conceptual models are developed and numerical simulations are employed to explain the observation of the anisotropic melting phenomenon and the effect of the surface oxide layer. Eventually, 41.8% reduction of root mean square (RMS) surface roughness is achieved for BHF-treated ELA films. The results gained in the systematic analysis of the ELA microstructure and the attempt of surface planarization further our understanding about (1) the device performance-related material microstructure of the ELA p-Si thin films, (2) the microstructure evolution occurring during multiple shots of the ELA process, and (3) the fundamental phase transformations in the far-from-equilibrium melt-mediated excimer-laser annealing processing of p-Si thin films. Such understanding could help engineers when designing the microelectronic devices and the ELA manufacturing process, as well as provide scientific researchers with insights on the melting and solidification of general polycrystalline materials, thus profoundly contributing to both the related scientific society and the technological community. The GB extraction program and the LMA scheme developed and demonstrated in the thesis, as another contribution to the related research filed, could also be generalized to the microstructural study of other polycrystalline materials where grain geometry and arrangement are of concern.

International audience; When ultrasonic waves are sent in polycrystalline media, scattering (single and possibly multiple) occurs at grain boundaries, where impedance mismatches are due to random crystalline orientations of the grains \textiti.e., to local changes of elastic constants. Scattering induces a progressive loss of coherence of the incident wave, with a characteristic length called the scattering mean-free path \ellₛ. This length depends not only on the grain size distribution, but also on the wave polarization (longitudinal or transverse). We present experimental results obtained by using an array of 128 transducers with central frequency 5.5~MHz. Experimental data consists in the inter-element impulse response matrix \mathrm\mathbfh(t), recorded using FMC (Full Matrix Capture): time signals are recorded by every element of the array when these are individually and sequentially excited by the same impulse. The samples studied here are two defect-free 9-cm-thick steel blocks manufactured by EDF R&D as part of a collaboration; the two samples show the same elastic properties but their grain size distributions differ. Large sets of matrices recorded on both samples allow for the estimation of coherent waves by spatial averaging. As the multielement array is used in contact with the medium, surface waves dominate the early arrivals. We measure two scattering mean-free paths through the decay of these surface waves, for frequencies ranging from 3.5 to 7.5~MHz. A particular interest is drawn to the estimation of experimental uncertainty, and to comparisons with homogenized models based on bulk P- and S-wave attenuations.

International audience; This work addresses the problem of the reconstruction of the local fields distribution occurring in heterogeneous linear elastic solids. The constitutive heterogeneities are crystals and cracks. Through comparisons with FFT computations, it is shown that self-consistent estimates together with an assumption of normal distribution at the phase scale provide an accurate description of the elastostatic field histograms in polycrystals without cracks. In the case of inter and transgranular cracks, full-field FFT simulations indicate that the field histograms present van Hove singularities. Their natures are determined analytically in the low-density regime, in the case of an homogeneous medium containing cracks.

We have developed a compact relativistic femtosecond electron diffractometer with a radio-frequency photocathode electron gun and an electron lens system. The electron gun generated 2.5-MeV-energy electron pulses with a duration of 55 &plusmn, 5 fs containing 6.3 &times, 104 electrons per pulse. Using these pulses, we successfully detected high-contrast electron diffraction images of single crystalline, polycrystalline, and amorphous materials. An excellent spatial resolution of diffraction images was obtained as 0.027 &plusmn, 0.001 &Aring, &minus, 1. In the time-resolved electron diffraction measurement, a laser-excited ultrafast electronically driven phase transition in single-crystalline silicon was observed with a temporal resolution of 100 fs. The results demonstrate the advantages of the compact relativistic femtosecond electron diffractometer, including access to high-order Bragg reflections, single shot imaging with the relativistic femtosecond electron pulse, and the feasibility of time-resolved electron diffraction to study ultrafast structural dynamics.

A coupled multiphysics phase field framework is proposed to model anodic dissolution induced by stress corrosion fracture growth at microstructual level. The effects of electrochemical-mechanical processes (including crystal anisotropy) are all taken into account. This new model is based upon: (i) an anisotropic phase transformation model based on a variational formulation to describe material dissolution along preferential directions; (ii) an efficient description of grain boundaries as a smeared cohesive zone; (iii) an explicit approximation to model the different electrochemical behaviors between grain boundary and grain interior. Both intergranular and transgranular stress corrosion cracking is simulated in an efficient manner. The abilities of the proposed model are illustrated through several numerical examples involving a full prediction of complex crack network growth induced by stress corrosion cracking within 2D polycrystaline models.

This study investigates the joint impact of preferred texture and latent hardening on the plastic anisotropy of face centered cubic (FCC) materials. The main result is that both aspects have significant impact on the anisotropy, but the two can either counteract each other or synergistically reinforce each other to maximize anisotropy. Preferred texture results in significant anisotropy in plastic yielding. However, the latent hardening significantly alters the texture-induced anisotropy. In addition, one latent hardening type can cancel out the anisotropy of another type. Consequently, if all dislocation-based latent hardening types are included at the same level as the self-hardening, the result might not reveal the complexity of plastic anisotropy. The present study of the synergistic influence of detailed latent hardening and texture presented helps provide new insights into the complex anisotropic behavior of FCC materials during multi-axial forming.

Accurate prediction of cladding mechanical behavior is a key aspect of modeling nuclear fuel behavior, especially for conditions of pellet-cladding interaction (PCI), reactivity-initiated accidents (RIA), and loss of coolant accidents (LOCA). Current approaches to fuel performance modeling rely on empirical constitutive models for cladding creep, growth and plastic deformation, which are limited to the materials and conditions for which the models were developed. To improve upon this approach, a microstructurally-based zirconium alloy mechanical deformation analysis capability is being developed within the United States Department of Energy Consortium for Advanced Simulation of Light Water Reactors (CASL). Specifically, the viscoplastic self-consistent (VPSC) polycrystal plasticity modeling approach, developed by Lebensohn and Tom [1], has been coupled with the BISON engineering scale fuel performance code to represent the mechanistic material processes controlling the deformation behavior of light water reactor (LWR) cladding. A critical component of VPSC is the representation of the crystallographic nature ( defect and dislocation movement) and orientation of the grains within the matrix material and the ability to account for the role of texture on deformation. A future goal is for VPSC to obtain information on reaction rate kinetics from atomistic calculations to inform the defect and dislocation behavior models described in VPSC. The multiscale modeling of cladding deformation mechanisms allowed by VPSC far exceed the functionality of typical semi-empirical constitutive models employed in nuclear fuel behavior codes to model irradiation growth and creep, thermal creep, or plasticity. This paper describes the implementation of an interface between VPSC and BISON and provides initial results utilizing the coupled functionality. (C) 2016 Elsevier Inc. All rights reserved.

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage. The garnet porphyroblasts in the Mogok pelitic gneisses, which formed during prograde metamorphism to the upper amphibolite-granulite facies (0.6–1.0 GPa/780–850 °C), are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite, plagioclase, and quartz or monomineralic biotite veins. The segment texture formed at about 0.3–0.4 GPa/610–650 °C or lower-grade conditions. The EBSD analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4° and show no evidence of deformation and/or rotation processes after segmentation. These data suggest that the Mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9–12 km.

Polycrystalline metal systems in the samples made of steel 40X with different morphology and parameters of the distribution of large-angle grains boundaries for energy have been examined. The effect of the structural-energy state of grains boundaries on the operational reliability of improved steel 40X has been established. Based on the hierarchical modeling of polycrystals structure, new approaches have been proposed and algorithms have been developed for defining relationships between the structure that is formed in the technological processing of materials and stages in the life cycle of parts. It has been revealed that it is advisable to use, as a digital prototype of the structure of polycrystalline alloys that describes their performance under conditions of contact loads, the matrix representation of a system model that would incorporate the quantitative characteristics of grains. By using the devised procedures, the ways to execute technological control over the energy state of grain boundaries in the structural components have been defined in order to improve durability of parts exposed under contact loads. An estimation-experimental method has been developed to assess the effect of quantitative characteristics of the structure on the parameters of strength of the grains boundaries and their ability to form intragrain damage under external loads. The energy level of grains boundaries and triple joints between the groups of small and large grains is higher than that between grains of the same size. The boundary surfaces with a high level of energy are places where damage occurs at technological processing and under external loads on structural materials. This points to the crucial role of large-angle boundaries placed between triple joints with a high energy gradient in the process of forming microstructurally short cracks and intragrain destruction of polycrystalline systems. The use of hierarchical modeling methods and computational materials science makes it possible to improve the operational reliability of articles by choosing the optimal parameters for internal boundary surfaces. The lower cost of the parts' life cycle is achieved by thermal treatment regimes, which alter the quantitative characteristics of the steel structure, Досліджено полікристалічні металеві системи зразків стали 40Х з різною морфологією і параметрами розподілу великокутових границь зерен за енергіями. Встановлено вплив структурно-енергетичного стану границь зерен на експлуатаційну надійність стали 40Х після покращення. На основі ієрархічного моделювання будови полікристалів запропоновані нові підходи і створено алгоритми для визначення взаємозв'язків між структурою, яка формується при технологічних обробках матеріалів і етапами життєвого циклу деталей. Виявлено, що в якості цифрового двійника структури полікристалічних сплавів, який описує їх поведінку в умовах контактних навантажень, доцільно використовувати матричне подання системної моделі з її наповненням кількісними характеристиками зерен. З використанням розроблених методик визначено шляхи технологічного управління енергетичним станом меж поділу зерен структурних складових для підвищення довговічності деталей, що працюють при контактних навантаженнях. Розроблено розрахунково-експериментальний метод для оцінки впливу кількісних характеристик структури на параметри міцності границь зерен і їх здатність до утворення межзеренних пошкоджень при зовнішніх навантаженнях. Рівень енергії границь зерен і потрійних стиків між групами зерен малого і великого розміру є вищим, ніж між зернами одного розміру. Поверхні розділу з високим рівнем енергії є місцями утворення пошкоджень при технологічних обробках і зовнішніх навантаженнях конструкційних матеріалів. Це вказує на вирішальну роль великокутових границь, розміщених між потрійними стиками з високим градієнтом енергії в процесах утворення мікроструктурно коротких тріщин і межзеренного руйнування полікристалічних систем. Використання методів ієрархічного моделювання і числового матеріалознавства дозволяє підвищувати експлуатаційну надійність виробів вибором оптимальних параметрів внутрішніх граничних поверхонь. Менша вартість життєвого циклу деталей досягається режимами термічної обробки, які змінюють кількісні характеристики структури сталей, Исследованы поликристаллические металлические системы образцов стали 40Х с различной морфологией и параметрами распределения большеугловых границ зерен по энергиям. Установлено влияние структурно-энергетического состояния границ зерен на эксплуатационную надежность стали 40Х после улучшения. На основе иерархического моделирования строения поликристаллов предложены новые подходы и созданы алгоритмы для определения взаимосвязей между структурой, которая формируется при технологических обработках материалов, и этапами жизненного цикла деталей. Выявлено, что в качестве цифрового двойника структуры поликристаллических сплавов, который описывает их поведение в условиях контактных нагрузок, целесообразно использовать матричное представление системной модели с ее наполнением количественными характеристиками зерен. С использованием разработанных методик определены пути технологического управления энергетическим состоянием границ раздела зерен структурных составляющих для повышения долговечности деталей, работающих при контактных нагрузках. Разработан расчетно-экспериментальный метод для оценки влияния количественных характеристик структуры на параметры прочности границ зерен и их способность к образованию межзеренных повреждений при внешних нагрузках. Уровень энергии границ зерен и тройных стыков между группами зерен малого и большого размера выше, чем между зернами одного размера. Поверхности раздела с высоким уровнем энергии являются местами образования повреждений при технологических обработках и внешних нагрузках конструкционных материалов. Это указывает на решающую роль большеугловых границ, размещенных между тройными стыками с высоким градиентом энергии в процессах образования микроструктурно коротких трещин и межзеренном разрушении поликристаллических систем. Использование методов иерархического моделирования и числового материаловедения позволяет повышать эксплуатационную надежность изделий выбором оптимальных параметров внутренних граничных поверхностей. Меньшая стоимость жизненного цикла деталей достигается режимами термической обработки, которые изменяют количественные характеристики структуры сталей

This is the accepted version of the following article: [Pérez, I, Muniz de Farias, M, Castro, M, et al. Modeling polycrystalline materials with elongated grains. Int J Numer Methods Eng. 2019; 118: 121– 131. https://doi.org/10.1002/nme.6004], which has been published in final form at https://onlinelibrary.wiley.com/doi/10.1002/nme.6004 A novel algorithm to reproduce the arrangement of grains in polycrystalline materials was recently published by the authors. In this original approach, a dense package of circles (or spheres) with the same distribution as the grains is generated to produce a set of Voronoi cells that are later modified to Laguerre cells representing the original structure. This algorithm was successfully applied to materials with somewhat equidimensional grains; however, it fails for long-shaped grains. In this paper, modifications are provided in order to overcome these drawbacks. This is accomplished by moving each vertex of the Voronoi cells in such a way that the vertex should be equidistant from the particles with respect to the Euclidean distance. The algorithm is applied to packages of ellipses and spherocylinders in 2D. An example for a package of spheres is also provided to illustrate the application for a simple 3D case. The adherence between the generated packages and the corresponding tessellations is verified by means of the Jaccard coefficient (J). Several packages are generated randomly and the distribution of J coefficients is investigated. The obtained values satisfy the theoretical restraints and the quality of the proposed algorithm is statistically validated.

This article presents a measurement technique and data analysis tool to perform 3D grain distribution mapping and indexing of oligocrystalline samples using neutrons: Laue three-dimensional neutron diffraction (Laue3DND). The approach builds on forward modelling used for correlation and multiple fitting of the measured diffraction spots relative to individual grains.This enables not only to identify individual grains, but also their position and orientation in the sample. The feasibility and performance of the Laue3DND approach are tested using multi-grain synthetic datasets from cubic ({\alpha}-Fe) and tetragonal(YBaCuFeO5) symmetries. Next, experimental results from two data sets measured at the FALCON instrument of Helmholtz-Zentrum Berlin are presented: A cylindrical alpha iron ({\alpha}-Fe) reference sample with 5 mm diameter and 5 mm height, as well as a 2 mm3 layered perovskite (YBaCuFeO5). Using Laue3DND, we were able to retrieve the position and orientation of 97 out of 100 grains from a synthetic{\alpha}-Fe data set, as well as 24 and 9 grains respectively from the{\alpha}-Fe and YBaCuFeO5 sample measured at FALCON. Results from the synthetic tests also indicate that Laue3DND is capable of indexing 10 out of 10 grains for both symmetries in two extreme scenarios: using only 6 Laue projections and using 360 projections with extremely noisy data. The precision achieved in terms of spatial and orientation resolution for the current version of the method is 430{\mu}m and 1 degree respectively. Based on these results obtained, we are confident to present a tool that expands the capabilities of standard Laue diffraction, providing the number, position, orientation and relative size of grains in oligocrystalline samples., Comment: 15 pages, 8 figures

The size of the structural elements in polycrystalline materials formed during thermomechanical processing is one of the factors determining the physical and mechanical properties of metals and alloys. The formation of the crystallographic texture and the evolution ofthe blockiness of the crystal structure are still little studied in rare–earth metals. The article discusses the X-ray diffraction method for studying the mosaic crystallographic structure ofgadolinium. Based on the analysis of the broadening of diffraction lines, the average size ofcoherent scattering regions was determined. The research results can serve as a basis for analyzing the physical and mechanical properties of metal samples and studying the dynamics ofstructural changes during thermomechanical processing.

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019; Aachen 1 Online-Ressource (147 Seiten) : Illustrationen, Diagramme (2019). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019, Most materials of technological importance are heterogeneous at a certain scale. Typical examples include polycrystalline aggregates, fiber-reinforced composites, high-strength ceramics, and porous media. The overall, macroscopic mechanical behavior of such materials is, to a large extent, determined by physical, topological and statistical details of the underlying - and possibly evolving - microstructure. Thus, the scientific and economic interest in developing continuum mechanical two-scale models that are capable of capturing microstructural features has been growing rapidly and will continue to grow in the long term. At the current stage of research, high fidelity two-scale simulations of complex engineering systems are still barely possible when dealing with inelastic micro-heterogeneous media. This cumulative thesis reports research progress on different fields of modeling multi-phase materials across the scales using fast Fourier transforms (FFT), phase-field approaches, and finite element (FE) methods. The overall goal is the development of efficient FE- and FFT-based two- scale methods for the modeling of bulk microstructural evolution and the elasto-viscoplastic constitutive behavior of polycrystalline media for isothermal deformations at small and finite strains. The content of this thesis is given through several research articles published by the author (and his coauthors), and presented in a condensed form, in what follows. This thesis starts with an article that is concerned with the development of a FE-, FFT- and phase-field-based two-scale numerical scheme for the modeling of stress-induced martensitic phase transformations in elastically-deforming polycrystals. Implicit FE methods are employed for the numerical solution of the macroscopic boundary value problem (BVP). The mechanical behavior of the macroscopic continuum is based on a model for each of its material points as a periodic unit cell whose average properties determine those of the corresponding material point. The numerical solution of the microscopic BVP is based on FFT, Green’s function, fixed-point methods, and semi-implicit time integration in Fourier space. Bulk microstructural evolution is modeled by means of non-conserved phase-fields. The next two works deal with two-scale FE-FFT-based small and finite strain crystal plasticity simulations of (textured) polycrystals. FFT and Newton-Krylov-based micromechanical solvers are employed to solve the microscopic BVP. For the geometrically linear case, a rather simple solution strategy for two-scale simulations of non-linear polycrystals is proposed which ensures a high-fidelity representation of micromechanical fields as well as moderate overall computing times. For the geometrically non-linear theory, a numerically robust algorithmic formulation for the calculation of the overall algorithmic tangent moduli of non-linear micro-inhomogeneous materials is presented. In contrast to existing numerical tangent computation algorithms, the proposed method yields the exact tangent using only a small number of stress evaluations. The last article is concerned with the development of a novel model order reduction technique for FFT solvers. The underlying concept is a sparse sampling technique which allows for the reconstruction of highly incomplete data, provided the data is sparse. This fact is used to reduce the set of frequencies significantly based on which the Lippmann-Schwinger is discretized and the BVP is solved using fixed-point based FFT solvers. The aforementioned reduced set of frequencies is identified a-priori by means of a fixed sampling pattern in Fourier space. Non- linear reconstruction algorithms are applied to the undersampled FFT solution to reconstruct highly-resolved micromechanical fields. Linear and non-linear composites (J2-plasticity) are studied as representative numerical examples., Published by Aachen

We consider low-energy configurations for the Heitmann–Radin sticky discs functional, in the limit of diverging number of discs. More precisely, we renormalize the Heitmann–Radin potential by subtracting the minimal energy per particle, i.e. the so-called kissing number. For configurations whose energy scales like the perimeter, we prove a compactness result which shows the emergence of polycrystalline structures: The empirical measure converges to a set of finite perimeter, while a microscopic variable, representing the orientation of the underlying lattice, converges to a locally constant function. Whenever the limit configuration is a single crystal, i.e. it has constant orientation, we show that the $$\varGamma $$ -limit is the anisotropic perimeter, corresponding to the Finsler metric determined by the orientation of the single crystal.

Ciclo de Conferencias del CENIM "Diálogos y Restos", Hardening of polycrystals is often described by phenomenological models such as the one by Kocks and Mecking (KM). With the Taylor equation, it describes strain hardening in single-phased materials. Grain boundary and precipitation hardening are then included by adding empirical corrections. Here, the theoretical bases of KM-will be explored. The Taylor equation is obtained from an evolution equation for shear stress as a function of dislocation density, which can be modified to include precipitates. Dynamic annihilation and the grain size effect will be analysed from a probabilistic analysis of the slip length used in KM, to provide a theoretical basis for combined hardening mechanisms in alloys.

Antimony sulfoiodide (SbSI) is a ferroelectric semiconductor with many interesting physical properties (optical, photoconductive, ferroelectric, piezoelectric, etc.). The electrical properties of textured polycrystalline SbSI obtained by the rapid cooling of a melted mass in liquid nitrogen are presented in this work using ac impedance spectroscopy over a wide temperature range (275–500 K) in the frequency range of 1 Hz to 100 kHz. Detailed studies of the impedance Z*(ω), conductivity σ*(ω), electric modulus M*(ω), and dielectric permittivity ε*(ω) of this material were performed using complex impedance spectroscopy for the first time. This study showed that the impedance and related parameters are strongly dependent on temperature. The internal domain structure and the presence of grain boundaries in textured polycrystalline SbSI explain the obtained results.

Pilgering is a complex incremental manufacturing process for seamless tubes. In this work, a miniature physical simulator for pilgering was designed and fabricated. This miniature simulator employs a grooved roll-die and a mandrel and can impose controlled reductions in both tube diameter and wall thickness. Pilgering deformation over a range of ratios of reductions in wall thickness and in tube diameter, known as the Q-factor, was imposed on hemi-cylindrical zirconium alloy specimens. The influence of the Q-factor on the microstructure and deformation texture of the deformed specimens was quantified. A polycrystal plasticity calculation based on the binary tree model was used to simulate texture evolution during the simulated pilgering process. The computer model quantitatively captured the variation with Q of the Kearns factors, as measured in the physically simulated specimen. The small differences noticed between the predicted and experimental final textures point to unaccounted transverse components of the flow field. These observations suggest that physical and/or computer simulations can form the basis of a rapid methodology for tool selection to realize prescribed post-pilgering textures. (C) 2016 Elsevier B.V. All rights reserved.

The design of phase boundaries has now become a consolidated strategy to improve the functional properties of piezoelectric oxides because of the unique properties that may be obtained in their vicinity. In particular, polymorphic phase boundaries (PPBs) have attracted significant interest in recent years because they represent a significant breakthrough in terms of enhanced piezoelectric activity of lead-free piezoelectric oxides. PPBs are temperature-driven phase transitions where both intrinsic and extrinsic contributions maximize, thereby enhancing the macroscopic properties of piezoelectric materials. This tutorial discusses potassium–sodium–niobate-based systems as model materials to reveal some of the most relevant advances in the design of PPBs through compositional modifications. We focus on how PPBs can be modulated by engineered doping and also discuss the direct relation between PPBs and the enhancement of piezoelectric activity. Finally, we briefly describe the main experimental techniques for detecting PPBs.

Based on rate equations, the kinetics of atom adsorption, desorption, and diffusion in polycrystalline materials is analyzed in order to understand the influence of grain boundaries and grain size. The boundary conditions of the proposed model correspond with the real situation in the electrolytes of solid oxide hydrogen fuel cells (SOFC). The role of the ratio of grain boundary and grain diffusion coefficients in perpendicular and parallel (to the surface) concentration profiles is investigated. In order to show the influence of absolute values of grain and grain boundary diffusion coefficients, we select four different cases in which one of the diffusion coefficients is kept constant while the others vary. The influence of grain size on diffusion processes is investigated using different geometrical models. The impact of kinetic processes taking place on the surface is analyzed by comparing results obtained assuming the first layer as a constant source and then involving in the model the processes of adsorption and desorption. It is shown that surface processes have a significant influence on the depth distribution of diffusing atoms and cannot be ignored. The analytical function of overall concentration dependence on grain and grain boundary volume ratio (Vg/Vgb) is found. The solution suggests that the concentration increases as a complementary error function while Vg/Vgb decreases.

We studied the influence of intergranular barrier fluctuations on the electrical response of 3D semiconductor polycrystals. We first computed with a numerical simulation model the dispersion in the intergranular barrier height on polycrystalline tin oxide due to the punctual character of the donors. Then, in order to quantify the effects of the barrier fluctuation in the overall conductivity of the semiconductor, we added the dispersion to the well known brick-layer model and determined the connection between impedance measurements and grain boundary resistivity. We found that, the brick-layer model gives lower values for the real intergrain resistivity. However, the error can be quantified indicating that the brick-layer model is not a bad approximation to determine electrical properties of intergrains of a polycrystal, specially for relatively large grains. Fil: Uriz, Alejandro José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Científicas y Tecnológicas en Electrónica. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones Científicas y Tecnológicas en Electrónica.; Argentina Fil: Buono, Camila. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina Fil: Aldao, Celso Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentina

This review paper shows that grain boundary sliding (GBS) is a general phenomenon occurring in all classes of inorganic materials: ceramics, metals and composite materials. The occurrence of relaxations attributed to GBS is also quite general and therefore the mechanical spectroscopy constitutes a sensitive and universal technique to study such phenomenon. GBS is widely observed in ceramics. It can be due to the presence of an amorphous layer between the grains as in zirconia or to dislocations, as in alumina. In each case, a high temperature GBS peak has been identified. In metals, GBS is observed in some deformed materials but the correlation of such phenomenon with internal friction peaks has been controversial. In 1941, C. Zener describes a geometrical model of GBS that could give rise to a relaxation mechanism. In 1947, Kê observed a large relaxation peak in polycrystalline aluminum. This peak being absent in single crystals, the relaxation was attributed to GBS. Today, the Zener model can still be used in most cases for relaxations occurring in the grain boundaries. Instead, according to the grain boundary type, the material or the temperature, either dislocations or the gliding of a disordered layer produce the grain boundary relaxations.

This thesis is part of the study of fuel elements of pressurized water reactors and, more specifically, focus on the understanding and modelling of the viscoplastic behavior of uranium dioxide (UO2) at polycrystalline scale. During the incidental operation of the reactor, the fuelundergoes a strong increase of temperature and thermal gradient between the center and the periphery of the pellet leading to viscoplastic strains due to dislocation movement mechanisms. First, a crystal plasticity model was developed in order to describe the viscoplastic anisotropy of the material considering the temperature and the loading rate. Finite element (FE) simulations on single crystals enabled to highlight that the three slip modes generally observed in UO2 are crucial to describe the anisotropic behavior of the material. Secondly, coefficients of the interaction matrix have been identified specifically for UO2 in order to improve the polycrystal modelling. Indeed, by calculating geometrically necessary dislocations (GNDs), which are responsible of the great increase of the stored dislocation density in polycrystals, the interactions between dislocations enable to simulate de grain size sensitivity and hardening of the fuel pellet. Finally, the model adapted for polycrystals, have been validated by comparing FE simulations with pellet compression tests and by comparing the simulated intra-granular behavior with EBSD measurements. Thanks to the latter comparison, it is possible to indirectly compare the strain heterogeneities in the grains., Dans le cadre de l'étude des éléments combustibles des réacteurs à eau pressurisée, cette thèse s'inscrit dans la compréhension et la modélisation du comportement viscoplastique du dioxyde d'uranium (UO2) à l'échelle du polycristal. Lors de fonctionnement de type incidentel du réacteur, le combustible subit une forte élévation de la température avec un gradient thermique entre le centre et la périphérie de la pastille engendrant des déformations viscoplastiques contrôlées par des mécanismes liés aux mouvements des dislocations. Dans un premier temps,un modèle de plasticité cristalline a été développé de manière à décrire l’anisotropie viscoplastique du matériau en fonction de la température et de la vitesse de sollicitation. Des simulations par éléments finis (EF) sur monocristaux ont permis d’identifier que les trois modes de glissement généralement observés dans l'UO2 ont une importance capitale pour décrire le comportement anisotrope du matériau. Dans un second temps, les coefficients de la matrice d'interactions entre dislocations ont été déterminés spécifiquement pour l’UO2 afin d’améliorer la modélisation des polycristaux. En effet, en calculant par EF les dislocations géométriquement nécessaires (GNDs), qui sont responsables d’une forte augmentation de la densité de dislocations stockées dans les polycristaux, les interactions entre dislocations permettent de simuler l’effet de taille de grain et l’écrouissage des pastilles. Finalement, le modèle, adapté pour les polycristaux, a été validé par comparaison avec les essais expérimentaux sur pastille et par comparaison du comportement intragranulaire simulé avec des mesures EBSD. Grâce à cette dernière comparaison, il est possible de remonter indirectement aux hétérogénéités de déformation dans les grains.

This work approaches the fields of homogenization and of materials design for the linear and nonlinear mechanical properties with prescribed properties-profile. The set of achievable properties is bounded by the zeroth-order bounds (which are material specific), the first-order bounds (containing volume fractions of the phases) and the second-order Hashin-Shtrikman bounds with eigenfields in terms of tensorial texture coefficients for arbitrarily anisotropic textured materials.

Стохастическая структура поликристаллических материалов является причиной высокой степени неоднородности кинематических и силовых полей в зернах материалов и появления больших флуктуаций этих полей. Неоднородность и флуктуации несущественны в одних случаях, но приобретают решающее значение в исследовании различных критических явлений, возникновение которых сильно зависит от типа микроструктуры материала. Флуктуации обусловлены в основном упругим взаимодействием зерен, которое имеет дальнодействующий характер. Поэтому необходим учет взаимодействий большого числа зерен, что трудно осуществимо традиционными методами (прямого компьютерного моделирования и другими). В настоящей статье флуктуации неоднородных мезодеформаций в зернах поликристаллических материалов оценены с помощью теоретико-полевого подхода к решению краевой задачи деформирования микронеоднородных материалов. Особое внимание уделено определению экстремальных величин флуктуаций, имеющих важное значение для некоторых критических явлений, таких как зарождение трещин при сверхвысокоцикловой усталости, когда амплитуда макронапряжений и средние напряжения в зернах намного меньше величин, входящих в любые макроскопические критерии повреждений или усталости. Максимальные мезодеформации в зернах могут превосходить макродеформации в несколько раз. Экстремальные флуктуации в каком-либо зерне генерируются в кластерах зерен специфических конфигураций. Примененный подход позволяет прогнозировать паттерны таких кластеров. Экстремальные значения флуктуаций во внутренних зернах поликристаллического тела значительно выше, чем в зернах на поверхности, что обусловливает различие в поведении поверхностных слоев и глубинных объемов материала. Количественные данные приведены для случая одноосного растяжения поликристаллического цинка., The stochastic structure of polycrystalline materials causes a high inhomogeneity of the kinematic and force fields in grains of materials and large fluctuations of these fields. The inhomogeneity and fluctuations are insignificant in some cases, but they become crucial in the study of various critical phenomena whose occurrence strongly depends on the type of material microstructure. Fluctuations mainly arise due to the elastic interaction of grains, which has a long-range effect. Therefore, it is necessary to account for the interaction of a large number of grains, which is difficult to do using conventional methods (direct computer modeling and others). In the present paper, inhomogeneous mesostrain fluctuations in grains of polycrystalline materials were estimated using a field-theoretical approach to a boundary value problem of microheterogeneous material deformation. Particular attention is paid to the calculation of extreme fluctuations that are important for some critical phenomena, such as, e.g., crack initiation under ultra high cycle fatigue when the macrostress amplitude and the mean stresses in grains are much lower than the quantities included in any macroscopic damage or fatigue criteria. The maximum mesostrains in grains can exceed several times the macrostrains. Extreme fluctuations in a grain are generated in grain clusters of specific configuration. The applied approach makes it possible to predict patterns of such clusters. Extreme fluctuations in the bulk grains of a polycrystalline body are much higher than in the surface grains, due to which the behavior of the material surface layers and bulk volumes is different. Quantitative data are given for the case of uniaxial tension of polycrystalline zinc.

The Antarctic Ice Sheet is a continental ice mass with circa 23 million gigatons of ice, which represent roughly 67 % of world's freshwater supply. This colossal mass of ice is by no means static, as the old ice slowly creeps under its own weight towards the ocean, while new ice is continually formed through the sintering of snow deposited on the ice sheet surface. A crucial role in this metamorphism is played by firn, which is the porous material in an intermediate state between the granular snow and the solid polycrystalline ice. Understanding the snow firn ice metamorphism is essential not only for a precise determination of the mechanical (creep) properties of polar ice, but also for comprehending the formation and decay of climate proxies widely used in ice-core studies. This work investigates the transition from firn to ice through the spatial and directional distributions of slip bands in bubbly ice. The analysis of high-resolution micrographs of ice sections extracted from the EPICA-DML Deep Ice Core allows us to identify a clear influence of strain-induced anisotropy (viz. c-axis preferred orientations) on the evolution of slip-band inclinations in deep bubbly ice. In contrast, we discover an unanticipated behaviour of slip bands in shallow bubbly ice, which prompts the introduction of the hypothesis of microstructural fading memory and the definition of a stabilization zone that may penetrate hundreds of metres into the bubbly ice. Within this stabilization zone, highly localized concentrations of strain energy and internal stresses once generated by force chains in the ancient firn are gradually redistributed by the newly formed bubbly-ice microstructure. We show that this hypothesis is compatible with the localized dynamic recrystallization episodes observed in polar firn (even at temperatures close to -45°C), and it may also explain the sluggish rotation of c-axes observed in the upper hundreds of metres of polar ice sheets. © 2018 Elsevier Ltd Financial support from the Ramón y Cajal grant RYC-2012-12167 of the Spanish Ministry of Economy, Industry and Competitiveness is kindly acknowledged. This work is a contribution to the European Project for Ice Coring in Antarctica (EPICA), a joint European Science Foundation/European Commission scientific programme, funded by the EU and by national contributions from Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Sweden, Switzerland and the United Kingdom. The main logistic support was provided by IPEV and PNRA (at Dome C) and AWI (at Dronning Maud Land). This is EPICA publication no. 310.