Les fontes à graphite sphéroïdal sont aujourd’hui très largement utilisées en raison de leurs bonnes propriétés mécaniques. La forme sphéroïdale du graphite est obtenue le plus souvent par l’ajout de magnésium ou de cérium lors de l’élaboration des fontes. Le graphite sphéroïdal peut être obtenu par graphitisation à l'état solide des fontes totalement ou partiellement solidifiées dans le système métastable. L’objectif de ce travail est d’étudier l’effet du traitement de graphitisation à l’état solide sur la croissance du graphite nodulaire d’une fonte à paroi mince qui présente une structure truitée à l'état brut de coulée. Cette fonte a été étudiée par microscopie optique, microscopies électronique à balayage et en transmission, spectroscopie Raman et spectroscopie de perte d'énergie des électrons. Des traitements thermiques assurant une graphitisation totale et partielle pour décomposer la cémentite formée à la solidification en graphite et en austénite ont été réalisés. Les nodules deviennent plus nombreux et leur taille augmente en fonction du temps de graphitisation. La microstructure après traitement thermique est composée de nodules de graphite et de ferrite. La spectroscopie Raman a été utilisée pour caractériser les nodules de graphite d’échantillons ayant été entièrement graphitisés à différentes températures dans le domaine austénitique. L’analyse par spectroscopie Raman ne montre aucune différence significative entre les spectres Raman enregistrés sur le graphite formé lors de la solidification et à l’état solide. Les caractérisations microstructurales par microscopie électronique en transmission montrent que le graphite à l’état brut de coulée présente une structure caractérisée par une zone interne où le graphite est désorienté. Une déformation mécanique due à la contraction lors de la solidification métastable induit la formation de cette zone. Cette zone disparaît par recristallisation après traitement de graphitisation totale pour former à la fin des secteurs coniques rayonnant à partir du germe et se développant vers la périphérie. Les résultats de ces travaux ont permis une meilleure compréhension de la structure de graphite nodulaire à l’état solide et montre aussi que le mécanisme de croissance du graphite nodulaire est le même lors de la solidification et de la transformation à l'état solide. / Spheroidal graphite iron castings are today widely used because of their good mechanical properties. The spheroidal shape of graphite is most often obtained by the addition of magnesium or cerium during the casting process. Spheroidal graphite can be formed at the solid-state by graphitization of cast irons which solidified partly or totally in the metastable system. The purpose of this work is to study the effect of solid-state graphitization treatment on the growth of nodular graphite of a thin wall casting which has a mottled structure at the as-cast state. This cast iron was studied using optical microscopy, scanning and transmission electron microscopy, Raman spectroscopy and electron energy loss spectroscopy. Heat treatments ensuring a total and partial graphitization to decompose the cementite formed at the solidification in graphite and austenite were realized. The nodules become more numerous and their size increases according to the time of graphitization. The microstructure after heat treatment is composed of graphite nodules and ferrite. Raman spectroscopy has been used to characterize graphite nodules in as-cast state and in samples having been fully graphitized at various temperatures in the austenite field. The results show no significant difference between Raman spectra recorded on these various samples, suggesting graphite grows with the same mechanism during either solidification or hightemperature (so-called first stage) graphitization. Transmission electron microscopy characterizations show that nodules in the as-cast material presents a multi-fold structure characterized by an inner zone where graphite is misoriented and an outer zone where it is well crystallized. In heat-treated samples, graphite nodules consist of well crystallized sectors radiating from the nucleus. These observations suggest that the misoriented zone appears because of mechanical deformation when the liquid contracts during its solidification. During heat-treatment, this zone disappears by recrystallization. The results of the present work lead to a better understanding of the nodular graphite structure in the solid state and also show that nodular graphite growth mechanism is the same during solidification and solid-state transformation.

Correlations between a materials microstructure and mechanical behavior are important for materials development. As materials characterization methods must consider instrument accessibility, sample dimensions and economical aspects, developing functional techniques in order to obtain better understanding of materials behavior in micro and nano scale is crucial. Procedures for assessing and interpreting the mechanical responses at small scales, combined with investigating the microstructure, are considered as significant steps to design and develop the effective frameworks for evaluating bulk properties. This research demonstrates how fundamental understanding of microstructures can assist interpreting of mechanical performance of bulk materials. Testing of materials at small scales is very important because the mechanical failure of any bulk material starts with the formation, extension, or local accumulation of initially small defects, leading finally to a catastrophic fracture by an expanding crack. Thus, any bulk material profits from an in-depth understanding of its deformation and mechanical phenomena at the nano- and micrometer length scale.<div><br></div><div>This thesis shows how the micro constituents’ interactions and grain boundaries reactions to dislocations in alloys and thin films contribute to understanding material flow behavior and differences in the mechanical properties of these materials in a wide range of material systems with variations in appropriate sizes which need to be probed. Among other things, this work shows that sources of variation can be specified and quantified as predictive tools for designing materials. Several examples are presented. First, the strength and strain hardening of martensite and ferrite in a dual phase steel with a grain size less than 5 μm were determined using an inverse technique. The yield strength of the ferrite and martensite phases are obtained as 370 MPa and 950 MPa respectively. The calculated hardening exponent of the alloy was exactly the same as experimental tensile test results (0.11). The constraint phenomena was effective in restricting deformation of this elastic-plastic alloy. Secondly, the differences in hardness and pop-in behavior were used to understand of the influences of different types of grain boundaries, high density dislocations, and twins in Al thin films before and after plasticity. The third example assesses the strength of several species of diatom frustules for the first time using a combination of indentation techniques. Lightweight materials with densities well below 1000 kg/m3 demonstrated strengths on the order of 100’s of MPa. Finally, conditions for laser grown oxides and laser shock peening on a commercial steel which lead to an optical marking without a change in strength around the marking have been identified.<br></div>

The thesis is focused on microstructural stability of heterogeneous weld joint of ODS steel and tungsten. The theoretical part summarizes the basic information about the structure and properties of ODS steels and describes the methods of joining these steels with tungsten. In the experimental part, materials MA956 and WL10 were welded by using electron beam. The individual samples differ by preheating temperature or use filler material. After annealing at 800 °C/1h and 1000 °C/5h, a change of the weld metal microstructure was evaluated by using scanning electron microscope. The chemical composition was measured by energy dispersive spectroscopy. It has been found that during annealing, massive precipitation of particles occures in the whole volume of the weld metal. These particles were identified as Laves phase.

Mechanical properties of thermal spray deposited coatings are highly influenced by their microstructural characteristics. The objective of this investigation is to evaluate the mechanical properties of thermally sprayed coatings consisted of aluminum and zinc based on the coating microstructure, using an image based computational scheme. Microstructural images of coating samples were subjected to image-based finite element analysis and the results were validated by experimental tests and analytical models. Comparison of the experimental data with FEA was used to explain the microstructural basis of the mechanical characteristics of Al-Zn coatings and the differences between two methods of thermal spray techniques. It was concluded that the cold spraying technique produces higher-quality coatings with less porosity and higher hardness compared to wire arc deposition. An isotropic behavior was observed in the cold sprayed coating. Finally, the electrochemical tests showed that the coating with a higher amount of zinc had better anti-corrosion properties.

<div>This research aims at investigating the direct in-situ synthesis of high-performance multi-component alloys such as high entropy alloys, bulk metallic glasses, and metal matrix composites using the directed energy deposition (DED) process, and modeling the entire solidification and microstructure evolution of these alloys via a novel three-dimensional cellular automata-phase field (3D CA-PF) model. These alloys are currently the focus of significant attention in the materials and engineering communities due to their superior material properties. In the 3D CA-PF model, the growth kinetics including the growth velocity and solute partition at the local solid/liquid interface is calculated by the multi-phase and multi-component PF component, and the 3D CA component uses the growth kinetics as inputs to calculate the dendrite morphology variation and composition redistribution for the entire domain, which could save the computational cost more than five orders of magnitude compared to the PF modeling that can only be applied to small domains due to its heavy computational requirements. Coupled with the temporal and spatial temperature history predicted by the experimentally validated DED model, this computation-efficient 3D CA-PF model can predict the microstructure evolution within the entire macro-scale depositions, which is known to be nonuniform due to the particular nature of additive manufacturing (AM) processes. </div><div>To achieve the final goal of direct in-situ synthesis of five-component CoCrFeCuNi high entropy alloys (HEA), and modeling of the solidification and microstructure evolution during the DED process, the proposed research is carried out in progressive stages with the increasing complexity of alloy systems. First, a simple binary material system of Ti-TiC composite was studied. The thermodynamically-consistent binary PF model is used to simulate the formation mechanism of detrimental resolidified dendritic TiCx. To capture the polycrystalline solidification, a grain index is introduced to link different crystallographic orientations for each grain. This PF model simulates the microstructure evolution of TiCx in different zones in the molten pool by combining the temperature history predicted by the DED model. The simulated results provide the solution of limiting the free carbon content in the melt, according to which, the formation of TiCx dendrites is successfully avoided by experimentally controlling the melting degree of premixed TiC particulates.</div><div>Second, the solidification, grain structure evolution, and phase transformation in the DED-built ternary Ti6Al4V alloy under the influences of thermal history are systematically simulated using the established simulation framework and a phase prediction model. The thermal history in a three-track deposition is simulated by the DED model. With such thermal information, the 3D CA model simulates the grain structure evolution on the macro-scale. The thermodynamically-consistent PF model predicts the local grain structure and concentration distributions of solutes Al and V on the micro-scale. The meso-scale CA-PF model captures the sub-grain microstructure evolution and concentration distributions of solutes within the entire molten pool. The dendritic morphology is captured within the large β grains. When the temperature drops below the β-transus temperature, the solid-state phase transformation of β→α/ is studied by the phase prediction model. Based on the predicted volume fractions of and α, the microhardness is also successfully assessed using rules of mixtures. </div><div>Third, the material system is expanded to a four-component ZrAlNiCu bulk metallic glass composite, whose raw composition is prepared by premixing the four pure elemental metals. The DED model is employed to obtain the temperature field and heating/cooling rates in single-track ZrAlNiCu bulk metallic glass composite, which provides insights for microstructure evolution. By delicate control of the material composition and utilization of the thermal history of the DED process, an amorphous-crystalline periodic structure is produced with in-situ formed crystalline particulates embedded in the amorphous matrix. This crack-free microstructure is successfully maintained within bulk parts, where a high fraction of the amorphous phase and crystalline phases are produced in the fusion zone and heat-affected zone, respectively. The large volume percentage of the amorphous phase contributed to the hardness, strength, and elastic modulus of the composite while the various soft crystalline phases improve the ductility by more than three times compared to monolithic metallic glasses. Nanoindentation tests are also performed to study the deformation behavior on the micron/sub-micron length scale. </div><div>Fourth, the material system is expanded to a five-component CoCrFeNiTi HEA alloy. Three CoCrFeNiTi HEA alloys with different compositions are designed and synthesized from premixed elemental powders via the DED process. Through a delicate design of composition and powder preparation, different microstructures are formed. H3-Co24.4Cr17.4Fe17.5Ni24.2Ti16.5 is mainly composed of a soft face-centered cubic (FCC)-γ phase while σ-FeCr, δ-NiTi2, and a small amount of Ni3Ti2 are precipitated and uniformed distributed in the FCC matrix for H1-Co22.2Cr16.1Fe19Ni21.8Ti20.9 and H2-Co25.9Cr15Fe17Ni20.8Ti21.3. With a large percent of the secondary phases, H1 exhibits a hardness value of about 853 HV0.5. These HEA alloys display a high oxidation resistance comparable to Inconel 625 superalloy. A detailed evaluation of the hardness, oxidation resistance, and wear resistance of these HEAs are conducted as compared with those of a reference HEA and two popular anti-wear steels.</div><div>Finally, a novel 3D Cellular Automata-Phase Field (CA-PF) model that can accurately predict the dendrite formation in a large domain, which combines a 3D CA model with a 1D PF component, is developed. In this integrated model, the PF component reformulated in a spherical coordinate is employed to accurately calculate the local growth kinetics including the growth velocity and solute partition at the solidification front while the 3D CA component uses the growth kinetics as inputs to update the dendritic morphology variation and composition redistribution throughout the entire domain. Taking advantage of the high efficiency of the CA model and the high fidelity of the PF model, the 3D CA-PF model saves the computational cost more than five orders of magnitude compared to the 3D PF models without losing much accuracy. By coupling the thermodynamic and kinetic calculations into the PF component, the CA-PF model is capable of handling the microstructure evolution of any complex multi-component alloys. Al-Cu binary alloys with 2 wt.% and 4 wt.% Cu are first used to validate the 3D CA-PF model against the Lipton-Glicksman-Kurz analytical model and a 3D PF model. Then, the 3D CA-PF model is applied to predicting the dendrite growth during large-scale solidification processes of directional solidification of Al-30wt.%Cu and laser welding of Al-Cu-Mg and Al-Si-Mg alloys. </div>

This contribution addresses recent advances in the experimental investigation of the phase stability, microstructural integrity and stress evolution of metal/metal (Cu/W) [1] and metal/ceramic (Ag/AlN, Ag60wt.%Cu40at.%/AlN, AgGe10at% /AlN) NML coatings during heating by advanced in-situ diffraction methods in combination with XPS, SEM and TEM analysis.

With growing world population and decreasing fossil fuel reserves we need to explore and utilize variety of renewable and clean energy sources to meet the imminent challenge of energy crisis. Solar energy is considered as the leading promising alternate energy source with the pertinent challenge of off sunshine period and uneven worldwide distribution of usable sun light. Although thermoelectricity is considered as a reasonable energy harvester from wasted heat, its mass scale usage is yet to be developed. By transforming window glasses into generators of thermoelectricity, this doctoral work explores engineering aspects of using the temperature gradient between the hot outdoor heated by the sun and the relatively cold indoor of a building for mass scale energy generation. In order to utilize the two counter temperature environments simultaneously, variety of techniques, including: a) insertion of basic metals like copper and nickel wire, b) sputtering of thermoelectric films on side walls of individual glass strips to form the thickness depth of the glass on subsequent curing of the strips, and c) embedding nano-manufactured thermoelectric pillars, have been implemented for innovative integration of thermoelectric materials into window glasses. The practical demonstration of thermoelectric windows has been validated using a finite element model to predict the behavior of thermoelectric window under variety of varying conditions. MEMS based characterization platform has been fabricated for thermoelectric characterization of thin films employing van der Pauw and four probe modules. Enhancement of thermoelectric properties of the nano- manufactured pillars due to nano-structuring, achieved through mechanical alloying of micro-sized thermoelectric powders, has been explored. Modulation of thermoelectric properties of the nano-structured thermoelectric pillars by addition of sulfur to nano-powder matrix has also been investigated in detail. Using the best possible p and n type thermoelectric materials, this novel energy generation technique promises 304 watts of thermoelectricity from a 9 m2 glass window utilizing temperature difference of 20 OC. In addition to be useful even during off sunshine hours of the day, these energy harvesting windows will be capable of power generation even in the absence of a cooling systems inside the building as long as a natural temperature gradient exists between the two counter environments. With an increasing trend of having the exterior of buildings and high rises entirely made up of glass, this work offers an innovative transformation of these building exteriors into mass scale energy harvesters capable of running average lighting loads inside the building hence providing a complimentary source of electricity to the main power grid.

The cast products Mg–Sn based alloys are promising candidates for automobile industries, since they provide a cheap yet thermally stable alternative to existing alloys. One drawback of the Mg–Sn based alloys is their insufficient hardness. The hardenability can be improved by engineering the microstructure through additions of Zn to the base alloy and selective aging conditions. Therefore, detailed knowledge about the microstructural characteristics and the role of Zn to promote precipitation hardening is essential for age hardenable Mg-based alloys. In this work, microstructural investigation of the Mg–1.4Sn–1.3Zn–0.1Mn (at.%) precipitation system was performed using TEM. The chemical composition of the precipitates was analyzed using EDS. APT was employed to obtain precise chemical information on the distribution of Zn in the microstructure. It was found from microstructural studies that different precipitates with varying sizes and phases were present; lath-shaped precipitates of the Mg2Sn phase have an incoherent interface with the matrix, unlike the lath-shaped MgZn2 precipitates. Furthermore, nano-sized precipitates dispersed in the microstructure with short-lath morphology can either be enriched with Sn or Zn. On the other hand, APT analysis revealed the strong repulsion between Sn and Zn atoms in a portion of the analysis volume. However, larger reconstruction volume required to identify the role of Zn is still limited to the optimization of specimen preparation.

Student Number : 9800022T - PhD thesis - School of Mechanical, Industrial and Aeronautical Engineering - Faculty of Engineering and the Built Environment / For many applications, it may be useful to be able to estimate creep properties of a material from simpler testing procedures such as tensile tests than the conventional creep testing procedures. Most alloys used for creep service conditions are in a hardened condition and thus tertiary creep, controlled by micro structural degradation, is dominant. The object of the study was to investigate a reasonably simple method for estimating the creep behavior of a low alloy 1% Cr, 0.25 % Mo steel from tensile yield data. The study involved performing of series of investigations, including age hardening, tensile and creep tests. Microstructural degradation was monitored from specimens held in a furnace for different times and temperatures, which were then tested in tension at room temperatures. Tensile tests were carried out at different temperatures and strain rates and the data used to determine material parameters for use in kinetic equations describing deformation. For comparison, creep curves were obtained from both creep tests and tensile tests results. Tests on furnace aged specimens were used to quantify softening due to material degradation and formulate a structure evolution and kinetic expressions used to determine creep curves. The modified equation by Dorn was used to determine the material parameters and to predict flow characteristics. Two sets of mechanisms were observed. At low temperature and high stress (above 550MPa) dislocation by glide mechanism was investigated. At higher temperatures and low stress (below 550MPa), some form of power law creep was observed. Glide mechanism was investigated and material parameters σ ) , n and activation volume v, were calculated. The calculated value of σ ) was assumed for both plastic deformation and the softening kinetics. A reasonably good estimate of the creep behavior of the low alloy steel used in this investigation in which tertiary creep dominates can be calculated from tensile yield stress values. Furthermore, the creep rate and recovery have similar stress dependences, with the stress and temperature dependence similar to that predicted by recovery theory. The value of activation energy observed for creep for this alloy is in line with the processes which could be related to self diffusion. In order to justify the significance of this study, four existing empirical models are discussed, highlighting their merits and demerits with respect to the models used in this study. These are θ-Projection, Damage Mechanics, Estrin-Mecking and the Internal Stress Methods. Generally, in this class of alloys, recovery process occurs under an effective stress (i.e. an applied stress less the internal stress). Thus the possibility of using tensile data obtained in this study in the internals stress model was explored. The model could replicate the one used in this study if the change in internal stress value o σ is assumed to be negligible. This could be assumed to be true for tensile data at high stresses and low temperature especially during secondary creep rate when the internal stress approximates to the applied stress and at short test durations.

Cellular metals made from aluminum, titanium, or other metals are becoming increasingly popular for use in structural components of automobiles, aircraft, and orthopaedic implants. Civil engineering applications remain largely absent, primarily due to poor understanding of the material and its structural properties. However, the material features a high stiffness to weight ratio, excellent energy dissipation, and low thermal conductivity, suggesting that it could become a highly valuable new material in structural engineering. Previous attempts to characterize the mechanical properties of steel foam have focused almost exclusively upon uniaxial compression tests, both in experimental research and in computational simulations. Further, computational simulations have rarely taken the randomness of the material’s microstructure into account and have instead simplified the material to a regular structure. Experimental tests have therefore been performed upon both hollow spheres and PCM steel foams to determine compressive, tensile, and shear properties. Computational simulations which accurately represent the randomness within the microstructure have been validated against these experimental results and then used to simulate other material scale tests. Simulated test matrices have determined macroscopic system sensitivity to various material and geometrical parameters.