Experimental investigation of microstructure and properties in structural alloys through image analyses and multiresolution indentation

This work addresses the challenges in the investigation of structural alloy microstructures and their mechanical properties at multiple length scales. The investigations are performed on small volume ferrite-pearlite steel samples that were excised from in-service gas turbine components after prolonged exposure (up to 99,000 hours) to elevated temperatures, which promotes microstructural changes (spheroidization of pearlite and graphitization) as well as their yield strengths. Recent advances in spherical indentation protocols are combined for the first time to investigate the mechanical response of microscale ferrite-pearlite constituents and estimates of bulk properties on macroscale. It is shown that indentation yield strength captured with large indenter tips on an ensemble of ferrite-pearlite grains correlate strongly to the bulk yield strength evaluated with tensile measurements. Measurements on the individual ferrite and pearlite constituents follow a similar trend of decreasing yield strength as the bulk measurements. Second, to advance the reliability and accuracy of microstructure characterization, an image segmentation framework is developed that consists of five main steps designed to achieve systematic image segmentation on broad classes of microstructures utilizing widely available image processing tools. The flexibility and modularity of the framework was demonstrated on various types of microstructures images. The developed framework was used to segment the microstructures of ferrite-pearlite samples. The extracted microstructure statistics from the segmented images and multiresolution indentation yield strength measurements were used to evaluate established composite theory estimates and have demonstrated highly consistent estimates for these material systems.Ph.D

New Trends in 3D Printing

A quarter century period of the 3D printing technology development affords ground for speaking about new realities or the formation of a new technological system of digital manufacture and partnership. The up-to-date 3D printing is at the top of its own overrated expectations. So the development of scalable, high-speed methods of the material 3D printing aimed to increase the productivity and operating volume of the 3D printing machines requires new original decisions. It is necessary to study the 3D printing applicability for manufacturing of the materials with multilevel hierarchical functionality on nano-, micro- and meso-scales that can find applications for medical, aerospace and/or automotive industries. Some of the above-mentioned problems and new trends are considered in this book

Proceedings of the Scientific-Practical Conference "Research and Development - 2016"

talent management; sensor arrays; automatic speech recognition; dry separation technology; oil production; oil waste; laser technolog

Bi-directional evolutionary structural optimization (BESO) for topology optimization of material’s microstructure

It is known that composite materials with improved properties can be achieved through modifications to the topology of their microstructures. Structural topology optimization approaches can be utilized as a systematic way for finding the best spatial distribution of constituent phases within the microstructures of materials/composites. This study presents a new approach for designing material’s microstructures based on the Bi-directional Evolutionary Structural Optimization (BESO) methodology. It is assumed that the materials/composites are composed of repeating microstructures known as periodic base cells (PBC). The goal is to find the best spatial distribution of constituent phases within the PBC, in such a way that materials with desired or improved functional properties are achieved. To this end, the Homogenization theory is applied to establish a relationship between properties of materials microstructure and their macroscopic characteristics. As the first step of this study, the optimization problem is formulated to find microstructures for materials with maximum stiffness, in the form of bulk or shear modulus, or thermal conductivity. Cellular materials, which are composed of one solid phase and one void phase, are considered at this stage. By conducting finite element analysis of the PBC, and applying the Homogenization theory, elemental sensitivity numbers are derived. By gradual removing and adding elements in an iterative process, the optimal topology of the solid phase within the PBC is found. In the next stage of this study, the aim is to combine additional performance constraint to the above procedure. Maximization of bulk or shear modulus is selected as the objective of the material design, subject to the constraint on the isotropy of material and volume constraint. The methodology is extended into topology optimization of microstructures for composites of two or more non-zero constituent phases. For design of material with maximum stiffness or thermal conductivity, the constituent phases are divided into groups and sensitivity analysis is performed between different groups. The developed methodology is also applied in designing functionally graded material (FGM), in which the mechanical property of material gradually changes. It is assumed that the microstructure of the FGM is composed of a series of cellular base cells in the direction of gradation and self-repeated in other directions. Finally, an approach is proposed for the topological design of FGMs with two non-zero constituent phases and multi graded properties. The objective of optimization is defined to find the stiffest materials with prescribed gradation of thermal conductivity. Similar to the approach used for cellular FGMs, the connectivity of base cells is maintained by considering three base cells at each stage. The effectiveness and computational efficiency of the proposed approaches are numerically tested, through designing a range of 2D and 3D microstructures for materials. A series of new and interesting microstructures of materials are presented. The results clearly indicate the advantages of BESO utilization in terms of computational costs and convergence speed, quality of generated microstructures, and ease of implementation as a post processing algorithm

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Cellular Metals: Fabrication, Properties and Applications

Cellular solids and porous metals have become some of the most promising lightweight multifunctional materials due to their superior combination of advanced properties mainly derived from their base material and cellular structure. They are used in a wide range of commercial, biomedical, industrial, and military applications. In contrast to other cellular materials, cellular metals are non-flammable, recyclable, extremely tough, and chemically stable and are excellent energy absorbers. The manuscripts of this Special Issue provide a representative insight into the recent developments in this field, covering topics related to manufacturing, characterization, properties, specific challenges in transportation, and the description of structural features. For example, a presented strategy for the strengthening of Al-alloy foams is the addition of alloying elements (e.g., magnesium) into the metal bulk matrix to promote the formation of intermetallics (e.g., precipitation hardening). The incorporation of micro-sized and nano-sized reinforcement elements (e.g., carbon nanotubes and graphene oxide) into the metal bulk matrix to enhance the performance of the ductile metal is presented. New bioinspired cellular materials, such as nanocomposite foams, lattice materials, and hybrid foams and structures are also discussed (e.g., filled hollow structures, metal-polymer hybrid cellular structures)

Прва међународна конференција о електронској микроскопији наноструктура ELMINA 2018, 27-29 август 2018. Београд, Србија

ELMINA2018 International Conference organized by the Serbian Academy of Sciences and Arts and the Faculty of Technology and Metallurgy, University of Belgrade, as the first in a series of electron microscopy conferences: Electron Microscopy of Nanostructures. The scope of ELMINA2018 will be focused on electron microscopy, which provides structural, chemical and electronic information at atomic scale, applied to nanoscience and nanotechnology (physics, chemistry, materials science, earth and life sciences), as well as advances in experimental and theoretical approaches, essential for interpretation of experimental data and research guidance. It will highlight recent progress in instrumentation, imaging and data analysis, large data set handling, as well as time and environment dependent processes

Engineered poly(vinylidene fluoride) based composites containing inorganic inclusions as materials for energy-related applications: process-structure-properties correlations

In recent years the continuous and rapid development of the electronic industry together with the need for more efficient electric energy harvesting have notably increased the demand for: (i) high dielectric constant and breakdown strength materials for high energy density capacitors and (ii) piezoelectric flexible materials, with the ability to bend into diverse shapes, for powering low-power portable devices and self-powered electronic systems. Polymer-based composites and nanocomposites with inclusions of a ceramic active phase are very attractive for these applications because they combine materials with different characteristics, allowing the possibility to tune and optimize the dielectric and piezoelectric properties in the ensuing composite systems. In particular, many parameters can affect the material performance: (i) the nature of the polymer matrix and active component; (ii) the phases connectivity; (iii) the filler concentration, shape and dimensions; (iv) the filler/matrix interactions; (v) the preparation technique and processing. All this variability expands the possible applications of polymer-composites for energy-related purposes but also increases the difficulty in realistically predicting their ultimate properties. The design of polymer composites thus requires a rational selection of components, good interface engineering and proper processing optimization. To achieve this, a thorough comprehension of the process-structure-properties correlations is very important. This is the principal aim of this thesis work, which focus on the preparation of poly(vinylidene fluoride) homopolymer (PVDF) or poly(vinylidene fluoride-co-hexafluoropropylene) copolymer (PVDF-HFP) based composites with 0-3 connectivity containing different perovskite fillers, namely, BaTiO3 (BT), Pb(Zr,Ti)O3 (PZT) and Na0.5Bi0.5TiO3 - BaTiO3 (BNBT). The filler particles were used as prepared or properly surface modified and several techniques were employed for the composites preparation (i.e., solvent casting, melt blending, hot-pressing, compression moulding). Initially, a study of the neat polymer matrices was performed, by using, for the first time in literature, the compression moulding technique to tune the polymorphism of PVDF. A principal component analysis was performed on the infrared spectra of the moulded films to validate the equation usually employed for determining the electroactive phase amount (FEA) then multiple linear regression was applied to better understand how the processing parameters affect the FEA value. A double-step procedure was proven fundamental in inducing the formation of PVDF \u3b2 phase and improving the dielectric properties of the ensuing polymer films. After this preparatory investigation, the study of the process-structure-properties correlations was extended to PVDF-based composites, addressing three main issues: (i) the influence of processing on the ultimate properties of the prepared samples; (ii) the influence of particles dimensions and surface modification on the dielectric behaviour of the composite materials; (iii) the response of flexible piezoelectric composites. The preparation technique affects the microstructure at different levels, but it was found that not always a flawless particles dispersion necessary leads to the best final performance of the composite. Whereas, a proper moulding method, by affecting the polymorphism of the polymer matrix and the compactness of the film, can improve significantly the dielectric response. The presence of an inorganic shell around BT particles allows a modulation of the effective permittivity of the composites; if intrinsic factors (i.e., the permittivity of the components) prevail on extrinsic ones (i.e., interfacial polarization), the composites response can be predicted by FEM calculations. However, in these conditions, the reduction in the dielectric constant compensates for the increase of the breakdown strength promoted by the shell and, as a whole, the stored energy decreases. It is worth noting that the composites containing core-shell particles are characterized by low tunability, a condition which is important for application as dielectric capacitors. The functionalization of the ceramic particles with the tested coupling agents, despite decreasing to a certain extent the dielectric permittivity of the ensuing composites (due to the intrinsic low permittivity of the silane moieties), increases the maximum electric field, thus leading to an energy recovering capability comparable or slightly higher than that of the composite containing pristine BT particles. The dielectric response of the composites is affected by the particles dimensions even though the films containing pristine BT and those containing TiO2-coated particles exhibit a different trend of dielectric permittivity with filler size; this suggests a not negligible contribution of the interfaces, which varies with the method of particles synthesis. As concerns the piezoelectric composites, the piezoelectric coefficient (d33), in general, increases if the filler dimensions increase significantly. The higher response of the samples containing sintered and crushed PZT or BNBT particles (with respect to simply calcined powders) probably derived from the higher particles connectivity inside the agglomerates, which in turn leads to higher local stresses inside the material. As far as we know, the piezoelectric properties of composites made of fluorinated polymer matrices and BNBT filler had not been studied yet. The obtained d33 are in line with those of many flexible lead-free composites made with particles different from BNBT, suggesting the potentiality of these composites in the field of energy harvesting. As principal achievements, I obtained: (i) an alternative and smart method to tune the polymorphism of PVDF homopolymer and its copolymers, by exploiting a simple and easily-scalable processing technique; (ii) solvent-free fabrication of polymer-based composites with dielectric properties improved by the moulding process; (iii) a better comprehension about the role of the interfaces, useful to tune the final performance of the dielectric composites; (iv) flexible lead-free polymer-based composites with a good piezoelectric response for potential application as safe energy harvesting devices