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Yes / Heat-treatment is a frequently used technique for modifying the physical and chemical properties of materials. In this study, the effect of heat-treatment on the mechanical properties, thermal stability and surface morphology of two types of electrodeposited coatings (pure-Ni and Ni/Al2O3) were investigated. The XRD analyses showed that the crystal structure of the as-deposited coating changes from slightly amorphous to crystalline as the heat-treatment temperature increases. The heat-treatment of both the pure-Ni and the Ni/Al2O3 coating caused an increase of the grain size within the coatings. However, the unreinforced Ni coating experienced a faster growth rate than the Ni/Al2O3 coating, which resulted in a larger average grain size. The temperature-driven changes to the microstructure of the coatings caused a reduction in the hardness and wear resistance of the coatings. The presence of nanoparticles within the Ni/Al2O3 coating can successfully extend the operational temperature range of the coating to 473 K by pinning grain boundaries.

No description available.

No description available.

The automotive industry, has been innovating in the field of materials development in order to meet the demand for lower emissions, improved passenger safety and performance. Despite various attempts of introducing other lightweight materials (Al, Mg or polymers) in car manufacturing, steel has remained as the material of choice till date due to its excellent adaptability to systematic upgradation and optimization in its design and processing. One of the outcomes is the development of second generation high Mn TWin Induced Plasticity (TWIP) steels with excellent strength-ductility balance suitable for automotive applications. Cost effective high performance TWIP steel design is mostly based on its alloy design and advanced up and down stream processing methods (thermomechanical controlled processing (TMCP)) which can help achieve suitable microstructure to meet the property requirements. It has been observed that grain boundary migration (GBM) in austenite during high temperature TMCP stage dictates grain growth to control the final microstructure. This research work initially investigates the grain growth in Fe-30%Mn steel within a temperature regime of 1000-1200°C. Compared to conventional low Mn steel, austenite boundary mobility in Fe-30%Mn was found to be 1-2 orders of magnitude smaller. Atom probe tomography results showed no Mn segregation at austenite high angle grain boundaries (γ-HAGB) which rules out the effect of Mn solute drag on growth kinetics in Fe-30%Mn steels. Grain boundary character distribution (GBCD) study showed that the sample consists of two different population of grain boundaries. 50% of the grain boundaries are random HAGBs with high mobility. Remaining 50% are special in nature which introduce low mobility boundary/boundary segments in the global boundary network. The special boundaries are mostly in the form of Σ3 CSL boundaries or its variants like Σ9, Σ 27. These boundary/ boundary segments were introduced by the formation of annealing twins and their interactions with the random HAGBs. An attempt to investigate the effect of Mn on growth kinetics at 1200°C showed that Mn slows down growth kinetics up to 15 wt% predominantly by the formation of annealing twins. A qualitative study of the microstructures showed that as Mn concentration is increased from 1% to 15%, the annealing twin density increases resulting in Σ3 frequency to be 30%. The increased twinning frequency is attributed to the effect of Mn on lowering the stacking fault energy (SFE). Annealing twins, belonging to Σ3 CSL family, intersect the HAGBs resulting into twin induced boundary segments which possess very low mobility. In the light of this idea, slow grain growth in high Mn steel was attributed to the population of low mobility boundaries. The proposed ‘twin inhibited grain growth’ model clearly points to the low mobility boundary/boundary segments to be the rate controlling factor during grain growth in high Mn steels. The effect of carbon on grain growth in Fe-30%Mn steel showed that the presence of carbon makes the growth kinetics faster by a factor of 4 and 6 at 1200°C and 1100°C respectively. Although, atom probe tomography results indicated that in presence of carbon, Mn segregation takes place at γ-HAGBs in Fe-30%Mn steel, solute drag does not appear to play a role as it was seen that with increase in Mn content beyond 1%, the solute effect of Mn in slowing down HAGB migration becomes weak. Also, abovementioned higher mobility values are obtained from the growth kinetics of Fe-30Mn-0.5C. This once again highlights the fact that effect of Mn in slowing down grain growth is due to the low mobility of twin/twin related boundaries or boundary segments. Controlling grain growth has been commonly proposed to be accomplished through small addition (<0.1%) of microalloying elements (Nb, V and Ti) which can slow down GBM at high temperature by solute drag and at low temperature by precipitate pinning (Zener drag). This research work has also experimentally quantified the solute drag of Nb in a series of Fe- 30%Mn steels. Grain boundary mobility was estimated for various temperatures and niobium contents. An attempt was made to calculate the grain boundary mobility in presence of niobium using Cahn’s solute drag model. This calculated mobility, when used in the proposed ‘twin inhibited grain growth’ model, the predicted growth kinetics which showed very good fit with the experimentally obtained growth kinetics in case of Fe-30Mn-0.03Nb and Fe-30Mn-0.05Nb steels at 1100°C. The effect of Nb solute drag, thus captured using Cahn’s model, was shown to be slowing down only the HAGB migration in the microstructure, whilst the special boundary mobility was not affected by solute Nb. Another attempt was made through grain boundary engineering (GBE) to control grain growth in Fe-30Mn-0.5C steel. Using different TMCP schemes, GBCD was modified to produce maximum frequency of special boundary. Preliminary studies on grain growth of single step-grain boundary engineered samples did show a significant lowering of grain size compared to a no-GBE sample after grain growth. However, the effect of iterative GBE didn’t show any significant effect in controlling grain growth in spite of the fact that it increased Σ3 frequency to 64%. This probably indicates that the effect of GBE on grain growth by the formation of annealing twins/special low mobility boundaries is a complicated process which might involve twin/special boundary morphology, annihilation kinetics and formation of grain clusters in the microstructure other than the formation of immobile special triple junctions through the intersection of twins/special boundaries with the random HAGBs. / Thesis / Doctor of Philosophy (PhD)

Cemented carbides are composite materials consisting of a hard carbide and a ductile binder. They are powdermetallurgically manufactured, where liquid-phase sintering is one of the main steps. The most common cemented carbide consists of WC and Co and it is widely used for cutting tools. Two of the most important factors controlling the mechanical properties are the WC grain size and the grain size distribution and thus it is of great interest to understand the grain growth behavior. In this thesis the grain growth during sintering at 1430 °C is studied both experimentally and through computer simulations. The grain growth behavior in cemented carbides cannot be explained from the classical LSW-theory. The WC grains have a faceted shape necessitating growth by 2-D nucleation of new atomic layers or surface defects. A new model based on 2-D nucleation, long-range diffusion and interface friction is formulated. Three powders having different average sizes are studied and both experiments and simulations show that a fine-grained powder may grow past a coarse-grained powder, indicating that abnormal grain growth has taken place in the fine-grained powder. Fine-grained powders with various fractions of large grains are also studied and it is seen that a faster growth is obtained with increasing fraction of large grains and that an initially slightly bimodal powder can approach the logaritmic normal distribution after long sintering times. The grain size measurements are performed on 2-D sections using image analysis on SEM images or EBSD analysis. Since the growth model is based on 3-D size distributions the 2-D size distributions have to be transformed to 3-D, and a new method, Inverse Saltykov, is proposed. The 2-D size distribution is first represented with kernel estimators and the 3-D size distribution is optimized in an iterative manner. In this way both negative values in the 3-D size distribution and modifications of the raw data are avoided. / QC 20110426

<p>Spark Plasma Sintering (SPS) is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain Si₃N₄-based ceramics with tailored microstructures, consisting of grains with either equiaxed or elongated morphology.</p><p> The presence of an extra liquid phase is necessary for forming tough interlocking microstructures in Yb/Y-stabilised α-sialon by HP. The liquid is introduced by a new method, namely by increasing the O/N ratio in the general formula RE_xSi_12-(3x+n)Al_3x+nO_nN_16-n while keeping the cation ratios of RE, Si and Al constant. </p><p>Monophasic α-sialon ceramics with tailored microstructures, consisting of either fine equiaxed or elongated grains, have been obtained by using SPS, whether or not such an extra liquid phase is involved. The three processes, namely densification, phase transformation and grain growth, which usually occur simultaneously during conventional HP consolidation of Si₃N₄-based ceramics, have been precisely followed and separately investigated in the SPS process.</p><p>The enhanced densification is attributed to the non-equilibrium nature of the liquid phase formed during heating. The dominating mechanism during densification is the enhanced grain boundary sliding accompanied by diffusion- and/or reaction-controlled processes. The rapid grain growth is ascribed to a <i>dynamic ripening</i> mechanism based on the formation of a liquid phase that is grossly out of equilibrium, which in turn generates an extra chemical driving force for mass transfer. Monophasic α-sialon ceramics with interlocking microstructures exhibit improved damage tolerance. Y/Yb- stabilised monophasic α-sialon ceramics containing approximately 3 vol% liquid with refined interlocking microstructures have excellent thermal-shock resistance, comparable to the best β-sialon ceramics with 20 vol% additional liquid phase prepared by HP. </p><p>The obtained sialon ceramics with fine-grained microstructure show formidably improved <i>superplasticity</i> in the presence of an electric field. The compressive strain rate reaches the order of 10^-2 s^-1 at temperatures above 1500oC, that is, two orders of magnitude higher than that has been realised so far by any other conventional approaches. The high deformation rate recorded in this work opens up possibilities for making ceramic components with complex shapes through super-plastic forming. </p>

<div> <div> <div> <p>Surfaces and interfaces in ionic solids play a pivotal role in defining the trans- port properties and microstructural evolution in many of the existing and emerging material applications, including energy-related systems such as fuel cells, recharge- able batteries, as well as advanced electronics such as those found in semiconducting, ferroelectric, and piezotronic applications. Here, a variational framework has been developed to understand the effects of ionic species and point defects on the structural, electrochemical and chemomechanical stability of grain boundaries in polycrystalline ceramics. The theory predicts the equilibrium and phase transition conditions of charged interfaces, and quantifies the properties induced by the broad region of electrochemical and chemomechanical influence in front of a grain boundary capable of spanning anywhere from a few angstroms to entire grains. As an example application, the microstructural mechanisms leading to the onset of the flash during electric field assisted sintering were predicted, where the experimentally observed cascading charge flow, resulting in the onset of a flash event was rationalized. Also, the model was applied to describe the effects of grain boundary drag by the interfacially accumulated ionic species and charged defects during grain growth under electrical, chemical, mechanical, or structural driving forces. Finally, abnormal grain growth in ionic solids with an emphasis on the structural and electrochemical character of the grain boundaries was demonstrated. Here, two moving grain boundary types, highly mobile and immobile interfaces are identified, resulting in three grain size populations. </p> </div> </div> </div>

Made available in DSpace on 2017-07-21T20:42:35Z (GMT). No. of bitstreams: 1 AnaMariaSouza.pdf: 2164792 bytes, checksum: 84eae737acfeba032c0e3f99c598ccde (MD5) Previous issue date: 2011-07-04 / Alumina ceramic is one of the most widely used in industry due to its properties. However, some properties, such as low fracture toughness, limit its range of structural applications. In order to improve this property has been studied several ways to control the microstructure of these ceramics, seeking a refined and homogeneous microstructure. The firing curve control to manipulate the microstructure during sintering is a way that has been studied and has advantages such as simplicity and economy. This work studied the two-step sintering of alumina ultrafine powders using steps at a low temperature sintering and the two-steps sintering proposed by Chen and Wang. With the help of statistical analysis was determined the variables that most influence in sintering processes studied. After sintering, the samples were characterized by apparent density and grain size. It was observed that the use of two-step sintering is effective in controlling the final microstructure of alumina, altering both the densification and grain growth. / A alumina é uma das cerâmicas mais utilizadas na indústria, devido a suas propriedades. Entretanto, algumas propriedades, tal como a baixa tenacidade à fratura, limita sua gama de aplicações estruturais. Com o intuito melhorar essa propriedade vem sendo estudadas várias maneiras de se controlar a microestrutura dessas cerâmicas, buscando uma microestrutura homogênea e refinada. O controle da curva de queima para manipular a microestrutura durante a sinterização é uma maneira que vem sendo estudada e apresenta vantagens como simplicidade e economia. Neste trabalho foi estudada a sinterização em duas etapas de pós ultrafinos de alumina utilizando tanto patamares a baixa temperatura quanto a sinterização em duas etapas proposta por Chen e Wang. Com o auxílio da análise estatística de Planejamento de Experimentos determinou-se as variáveis que exercem maior influência nesses processos de sinterização, sempre visando atingir microestruturas com altas densidades e mínino crescimento de grãos. Após sinterização, as amostras foram caracterizadas por medidas de densidade aparente e de tamanho de grão. Observou-se que a utilização da sinterização em duas-etapas é eficiente em controlar a microestrutura final da alumina, alterando tanto a densificação quanto o crescimento de grão.

Ceramic materials are natural or synthetic, inorganic, non-metallic materials incorporating ionic and covalent bonding. Most ceramics in use are polycrystalline materials where grains are connected by a network of solid-solid interfaces called grain boundaries. The structure of the grain boundaries and their arrangement play a key role in determining materials properties. Developing a fundamental understanding of the formation, structure, migration and methods of control grain boundaries have drawn the interest of scientists for over a century.<br> While grain boundaries were initially treated as isotropic, advances in materials science has expanded to include energetically anisotropic boundaries. The orientation and structure of a grain boundary, determined by this anisotropy, controls the mobility of a grain boundary. The mobility is the controlling factor during grain growth impacting the microstructural evolution of a material.<br> This thesis covers fundamental research to model how a materials’ equilibrium crystal shape can be used as a grain growth control mechanism. First an overview of ceramic processing and microstructural development is presented with a focus on the role of grain boundaries in determining the properties of a material. The role of anisotropy and related recent work is highlighted setting the foundation for the link between the equilibrium crystal shape and grain growth. A discussion on the selection of the NiO-MgO system for all experimental work is included.<br> A novel production and processing route for NiO-MgO was developed. Mechanical alloying and milling resulted in significant impurity contamination therefore a chemical production route was used. A modified amorphous citrate process was developed where metal salts containing Ni and Mg were mixed with a polyfunctional organic acid. Rapid dehydration and calcination at 500°C resulted in chemically homogeneous powders. The amorphous citrate production route produced powder with crystallites ranging from 244-393 nm and agglomerates ranging from 20-300 μm with plate-like morphology.<br> NiO-MgO powders produced via the amorphous citrate method were sintered using various techniques. Conventional sintering was unable to produce fully dense samples peaking with relative densities from 95-96%. The introduction of pressure through spark plasma sintering and hot pressing improved the relative sample density to 97-100%. It was discovered that exposure to the vacuum required for spark plasma sintering and hot pressing resulted in the reduction of NiO. Spark plasma sintering created oxygen depleted regions and hot pressing further reduced NiO to pure nickel metal which precipitated out at the grain boundaries.<br> Due to the poor sintering behavior of NiO-MgO grain growth experiments were carried out on the large agglomerates formed during the amorphous citrate process. Agglomerates with more than 50 grains with a thickness of at least 1 μm were selected. Grain growth was measured across five compositions with Ni:Mg ratios of 100:0, 75:25, 50:50, 25:75, 0:100. The average grain size and growth rate increased with increasing nickel content with a significant jump between 50% and 75%. Increasing nickel content was also observed to correspond with a higher number of grains exhibiting surface faceting.<br> The NiO-MgO equilibrium crystal shape as a function of composition was measured previously. To make the equilibrium crystal shape a more viable control for grain growth a quantitative microstructural characterization technique was developed to measure a materials equilibrium crystal shape. Topographic surface information (surface facets measured by atomic force microscopy, AFM) and grain crystallographic orientation (measured by electron back-scattered diffraction, EBSD) were combined to produce the crystallographic topography of a sample surface. Surface crystallographic topography was used to identify the faceting behavior of grains with a range of orientations. Using the combined data, facet stability maps (n diagrams) for NiO-MgO were developed.<br> Controlling grain growth via the equilibrium crystal shape offers the potential to produce microstructures with a high frequency of desirable grain boundaries (grain boundary engineering) and therefore properties. The combination of using AFM and EBSD to create crystallographic topographical surface data and n-diagrams has been demonstrated. N-diagrams for most materials do not exist, but the technique used here can be applied to a wide range of materials and will expand the ability to control microstructures of ceramic materials.<br><br>