There are a number of experiments showing that the ductility of aluminum alloys decreases during age-hardening heat treatment. Observing the grains of age-hardened aluminum alloys at the micron scale, one can notice that there are precipitate-free zones (PFZs) along the grain boundaries. PFZ has yield stress three times lower than the grain interior (bulk) due to absence of alloying elements. As a result, PFZ is suspected to be the reason for ductility reduction of alloys. On the other hand, a number of experiments performed on specimens with micron-scale dimensions have shown that the plastic deformation of crystalline materials is size-dependent. These micron-scale dimensions which can influence the mechanical behavior, such as yield stress or hardening, are not taken into account in the conventional plasticity theory, therefore another theory has been developed. That theory is Strain Gradient Plasticity (SGP). The specific SGP theory used here is a so called ‘higherorder theory’ in the sense that higher order stresses as well as additional boundary conditions are included in the theory. SGP theory also includes length scale parameters in order to be dimensionally consistent. On a recent study conducted by Fourmeau et al. (Fourmeau, 2015), transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of PFZ in the AA7075-T651 aluminum alloy. It is observed that the width of PFZ is about 20 to 40 nm. In the present thesis, the properties for PFZ and bulk material provided by that study are used for a micromechanical finite element model of a microstructure including the bulk, PFZ and the grain boundary (GB). A uniaxial loading condition is applied to the representative volume element (RVE) and SGP theory is hired in order to capture the plastic strain fields as well as the stress triaxiality in PFZ and bulk region. Moreover a damage criterion is employed and studied for models with PFZ and without PFZ to understand the role of PFZ in reduction of the ductility of aluminum alloys. It is found that the damage parameter is much higher in the presence of PFZ. Finally, the void growth is studied by adding voids at critical locations to the model.

The defect structure, atomic structure, and energy of the interphase boundaries between an fcc matrix and a lath-shaped bcc precipitate in Ni-45 wt% Cr were investigated. The interfacial structure on the side facet of the precipitate consists of regular structural ledges and misfit dislocations. No regular defect structure can be found on the habit plane, or broad face, of the lath except for atomic-scale structural ledges. High resolution electron microscopy (HREM) observations show the (12¯1)_f habit plane is coherent and is a good matching interface. Based upon conventional transmission electron microscopy (TEM) observations, the orientation of the habit plane results from advancing growth ledges on the conjugate plane of the Kurdjumov-Sachs orientation relationship. Using embedded atom method (EAM) simulations, the interfacial energy of the (12¯1)_f habit plane is calculated and the simulated interphase structure is compared with the HREM observations. The simulated interface represents a major portion of the observed interface. The calculated interfacial energy of the (12¯1)_f habit plane is 210 mJ/m², lower than typical grain boundary energies indicating this habit plane is a low-energy interphase boundary. A non-Bain lattice correspondence is identified and employed to predict the (12¯1)_f habit plane successfully, although a Bain correspondence is more successful at predicting the elongation direction for the precipitate. Geometric matching is proposed to be responsible for determining the orientation of the precipitate habit plane and the growth direction. Lattice correspondence-based approaches such as the invariant line model and the phenomenological theory of martensitic crystallography can mimic aspects of geometric matching, but they do not accurately reflect the transformation mechanism during precipitation of bcc laths from an fcc parent. / Ph. D.

We investigated the structure and defects associated with interphase boundaries between a γ (fcc) matrix and plate-shaped precipitates of the δ (orthorhombic) phase in Inconel 718. Based upon transmission electron microscopy (TEM) observations, the average habit plane was confirmed to be (111)_γ which is consistent with previous reports. A parallel array of misfit dislocations with Burgers vector b=1/6[112̅]_γ, (designated M1) are always observed lying along the [11̅0] direction. Another array of misfit dislocations appears in some regions of the interface with Burgers vector b=1/6[21̅1̅]_γ (designated M2). These dislocations also lie along the [11̅0] direction. Irregular ledges were identified on the interface and are believed to contribute to the thickening of δ plates. Dislocations in the matrix were also characterized. Most matrix dislocations have a 1/2[1̅01]_γ Burgers vector. The growth ledges in the habit plane of a single δ plate have a variety of effective Burgers vectors. A geometric matching approach based upon near-coincident sites was employed to explain the interfaces structure of interphase boundaries in Inconel 718, as well as fdc/bcc in Ni-45wt% Cr. In both cases, the conjugate plane is the plane with the highest areal density of near-coincident sites over a small region while the average habit plane is determined by the continuity of near-coincidence sites over a large area. The M1 interfacial dislocations in the γ/δ interface accommodate misfit in the habit plane whereas M2 dislocations do not and are probably a by-product of the dissociation of matrix dislocations. In the fcc/bcc system, the habit plane is not parallel to the conjugate plane and the partial dislocations associated with matrix stacking faults improve matching in the habit plane even though their Burgers vector lies out of this plane. / Ph. D.

Phytomining is a promising method for metal recovery, but rare studies have been devoted to metal recovery from hyperaccumulator biomass. The objective of this study was to propose efficient and sustainable methods for treating REE hyperaccumulators, aimed at enhancing REE recovery and obtaining value-added byproducts. Firstly, grass seeds fed with a solution containing Y, La, Ce, and Dy, were found to have the capacity to accumulate around 510 mg/kg (dry basis) of total rare earth elements (TREEs) in grass leaves. With the use of conventional hydrometallurgy, around 95% of Y, La, Ce, and Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destructed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, HHV, and energy densification, coupled with a decrease in hydrogen and oxygen contents of hydrochar. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbent via MHTC. In addition to utilizing conventional hydrometallurgy for REE recovery and employing MHTC to convert leaching residue into hydrochar, MHTC was also applied to directly recover REEs and produce hydrochar from the GL as a more efficient approach. The effects of acid type and acid concentration on REE extraction from GL using MHTC were investigated. The utilization of 0.2 mol/L H2SO4 led to the extraction of nearly 100% of REEs from the GL into the resulting biocrudes. Concurrently, the acid-mediated MHTC system also caused the degradation of amorphous hemicellulose and crystalline cellulose present in the GL, thereby enhancing the thermal stability of the resulting hydrochar. The physiochemical properties of the hydrochar were also influenced by acid type and acid concentration. Using 0.2 mol/L H2SO4 as the reaction medium, MHTC resulted in a yield of 28% hydrochar with enhanced high heating value and energy densification. These results suggest that MHTC in the presence of an appropriate concentration of H2SO4 is an effective way to extract REEs and produce hydrochar from the GL. A process that combines solvent extraction and struvite precipitation was developed for the treatment of biocrudes containing REEs and other elements. In the extraction step, 95.6% of REEs were extracted using 0.05 mol/L di(2-ethylhexyl)phosphoric acid (D2EHPA) with an aqueous to organic (A/O) ratio of 1:1 at pH 3.0. However, other impurity metals were co-extracted into the organic phase with the REEs. To solve this issue, a subsequent scrubbing step using deionized water was applied, with the removal of over 98% of these impurities, while incurring negligible loss of REEs. After the scrubbing step, over 97% of REEs were ultimately stripped out from the organic phase as REE oxalates using 0.01 mol/L oxalic acid. Furthermore, phosphorous (P) was found to be retained in the raffinate after the solvent extraction process. 94.4% of the P was recovered by forming struvite precipitate at pH 9.0 and a Mg/P molar ratio of 1.5. In general, high purity and value-added REE products and struvite precipitate were eventually achieved from biocrudes in environmentally friendly and economically viable ways. In summary, this study contributes a sustainable and efficient framework for REE hyperaccumulator treatment that integrates acid leaching, MHTC, solvent extraction, and struvite precipitation. This work supports a circular economy, minimizing waste and promoting resource reuse. / Doctor of Philosophy / Rare Earth Elements (REEs) are essential for technologies like smartphones and electric vehicles, but traditional mining is environmentally harmful and resource-intensive. Innovations are needed to reduce waste and enhance resource reuse. In this study, grass, a natural accumulator, was found to be able to extract REEs from contaminated soils. Nearly all REEs can be recovered efficiently using a mild sulfuric acid solution, and the residual biomass was also transformed into valuable byproducts such as energy-rich solid fuel and low-cost adsorbents. Furthermore, a more sustainable and efficient method, microwave-assisted hydrothermal carbonization, was also investigated to treat grass aiming at recovering REEs and achieving value-added products. High purity REE product and phosphorous-rich fertilizer were finally produced. This method reduces the environmental impact of REE mining, utilizes renewable resources, and cuts costs, thereby supporting economic sustainability. By turning environmental challenges into opportunities, this research highlights how innovative, greener methods can drive a more sustainable future in resource management.

[pt] Metais pesados contidos em efluentes industriais são fontes de grande potencial de degradação ambiental; visto que eles produzem alterações físicoquímicas na qualidade da água. Estas alterações têm um impacto direto na mortandade da flora e fauna afetando indiretamente a saúde dos seres humanos. Esta dissertação refere-se ao estudo dos aspectos cinéticos da remoção de cromo (III) por flotação de precipitados a partir de cloreto de cromo hexahidratado, utilizando dodecilsulfato de sódio (DSS) como coletor aniônico. Para este estudo, foram realizadas medidas de potencial zeta, bem como ensaios de flotação do cromo (III) para determinar os parâmetros adequados de operação: vazão de ar, concentração de coletor e concentração de espumante. Medições de potencial zeta das partículas de hidróxido de cromo na presença de eletrólito indiferente KCl, indicaram ponto isoelétrico (PIE) em pH em torno de 8,5. A presença do coletor aniônico (DSS) no sistema hidróxido de cromo-água indicou uma redução do potencial zeta, para os três níveis de concentrações investigados. Havendo um deslocamento do PIE na direção de valores de pH mais inferiores, atribuindo tal comportamento à adsorção do coletor sobre o Cr(OH)3 através de interação eletrostática. Com relação aos resultados dos ensaios de flotação de precipitados do cromo (III), obtiveram-se remoções de aproximadamente 96,2%, conseguindo-se chegar a valores inferiores ao estabelecido pela norma do CONAMA (20/86), ou seja, [Cr3+]£0,5 mg/L. O estudo da cinética da flotação mediante as análises integral e diferencial evidenciaram que o processo de remoção do precipitado de hidróxido de cromo segue um modelo de primeira ordem. / [en] Heavy metals contained in industrial effluents are sources of great potential environmental degradation, since their presence produce physicochemical alterations harming in the quality of the water. These alterations have a direct impact in the mortality of the flora and fauna affecting indirectly the human health. This dissertation refers to the study of the kinetic aspects of chromium (III)removal by precipitate flotation from chromium chloride (CrCl3.6H2O), using sodium dodecylsulfate (SDS) as anionic collector. For this study, measurements of zeta potential were accomplished as well as chromium (III) tests of flotation to determine the appropriate parameters operation: gas flow rate, concentration of collector and frother. Measurements of zeta potential of the particles of chromium (III) hydroxide in the presence of indifferent electrolyte KCl, indicated an isoelectric point (PIE)in pH around 8,5. The particles of chromium (III) hydroxide in the presence of anionic collector (SDS) showed a decrease in module of the zeta potential, for the three levels of concentrations investigated, as well as a displacement of the PIE in the direction of the more inferior pH values. This behavior can be attributed to the adsorption of the anionic collector on Cr(OH)3 due to the electrostatic attraction. Chromium (III) removals of 96,2% were obtained by precipitate flotation. Residual values of chromium reached are lower than the standards values established by the CONAMA (20/86), that mean [Cr3+]£0,5 mg/L. The kinetic flotation study showed that the removal of chromium (III)hydroxide precipitates followed a first order model.

The municipality of Västervik, with support from Envipro Miljöteknik AB, is carrying out a main study of the minefields at Gladhammar. Mining of iron, copper and cobalt under different periods from the 16th century until the end of the 19th century has led to discharges of metals to the lakes situated downstream. The aim of the main study is to investigate the possibilities to reduce the environmental load on the surrounding ground and water caused by heavy metals from the mine. The present report is a part of the main study. The aim of the work is to investigate the geochemistry of the groundwater. As a starting point, questions concerning affected areas, occurring metals and possible processes for propagation and limitation of the pollutants, have been posed. The work is based on data from a pre-study and of the main study. In total, there are 25 groundwater pipes in the area. A subset of these pipes has been chosen in order to delimit the task at hand. The number of analysed measurements for each groundwater pipe range from five to 15. The data material has been compiled and subsequently evaluated with respect to natural background, variation in time, correlation with precipitation, depth and other measured parameters. Geochemical modelling using the computer code PHREEQC has also been conducted. The waste of the minefields at Gladhammar is constituted of waste rock, slag and tailings. Arsenic, cobalt, copper, lead and zinc are the prevailing metals in the area. Iron, manganese and sulphur control the behaviour of heavy metals in water to a great extent and they have, as well as the prevailing metals, therefore been in focus for the study. The data compilation shows that pipes in the proximity of slag and waste rock have the highest content of cobalt and copper. The groundwater is affected, with respect to guideline-values, by, above all, cobalt and copper, but also to a certain extent by lead. The content of arsenic and zinc is classified as low to moderate. The groundwater is most affected in the areas of Holländarefältet and Torsfall. The data analysis shows that out flush of secondarily retained metals is a likely pollutant process. Primarily, this is valid for cobalt and copper. The pollution propagation is likely limited by precipitation of secondarily minerals as well as adsorption to iron, manganese and aluminium particles. The groundwater in the pipes close to waste rock and slag most certainly consists of surface water that runs off from the heaps on top of the hill. This is demonstrated by the high metal content of the surface run off. According to calculations on mixing, the groundwater at the shore is made up by a mixture of lake water and mine entrance water.

Les turbines aéronautiques et terrestres utilisent comme matériaux de structure les superalliages à base nickel. Ils sont confrontés en utilisation à des environnements agressifs à très hautes températures, conduisant à l'usure et la corrosion, et à des sollicitations mécaniques qui entraînent fatigue et fluage. Pour permettre l'utilisation de ces matériaux dans des conditions toujours plus sévères de fonctionnement et augmenter la durée de vie des pièces, divers traitements de nitruration ont été proposés pour durcir la surface tout en conservant ou en améliorant la tenue mécanique et la résistance chimique. Les modifications induites par la nitruration, leur stabilité et l'influence de la microstructure initiale sont encore mal comprises dans ces matériaux complexes. Au cours de ces travaux, nous avons étudié les effets d'une nitruration assistée plasma en fonction de la microstructure et de la composition chimique des alliages. Différents types de superalliages à base nickel ont été choisis, de microstructures variées, comprenant éventuellement des précipités de type Ni3(Al,Ti,Nb) et/ou Ni3 (Nb). Nous avons alors caractérisé les modifications induites par l'introduction de l'azote dans les matériaux suite à un traitement de nitruration à basse température (400°C) : expansion de la maille, génération de contraintes résiduelles, comportement des précipités, formation de nitrures, plasticité, anisotropies... Les résultats obtenus suggèrent des effets différents selon la composition des précipités. Ces modifications structurales et leurs évolutions ont ensuite été étudiées lors d'un recuit à plus haute température (650°C) afin d'étudier la stabilité des couches formées. / Nickel-based superalloys are commonly used in pressurized water heat exchangers or in the hottest sections of aeroengines or industrial gas turbines, where they are subjected to high temperature and severe mechanical solicitations (fatigue, creep). To allow use of those materials in more and more difficult operating conditions and to improve their duration, different nitriding treatments have been proposed to harden the surface while maintaining or improving their mechanical strength and chemical resistance. However, modifications induced by nitriding, resulting stability in time and influences of the initial microstructure are still poorly understood in these complex materials. In this work, we investigated the behavior of plasma nitriding on superalloys according to their initial microstructure and chemical composition. Thus, different types of Nickel-based superalloys were selected with various microstructures, possibly including precipitates like Ni3(Al, Ti, Nb) and/or Ni3(Nb). Then, we have characterized the modifications induced by nitrogen introduction in the materials after nitriding treatment at low temperature (400°C): lattice expansion, generation of residual stress, precipitates behavior, nitrides formation, plasticity, anisotropy... The results suggest different behaviors depending on the composition of precipitates. Finally, these structural modifications and their evolutions have been studied during an annealing at higher temperature (650°C) in order to study the stability of the nitrided layers.

Through a series of carefully designed experiments, characterization and some modeling tools, this work is aimed at studying the role of thermal profiles on different microstructural zones and associated properties like strength and corrosion through a variation of weld parameters, thermal boundary conditions and material temper. Two different alloys belonging to the Al-Cu and Al-Cu-Li system in different temper conditions- peak aged (T8) and annealed (O) were used. A 3D-thermal pseudo mechanical (TPM) model is developed for the FSW process using heat transfer module in COMSOL Multiphysics and is based on a heat source wherein the temperature dependent yield shear stress is used for the heat generation. The precipitation and coarsening model is based on the Kampmann and Wagner theoretical framework and accounts for the competition between the various nucleation sites for both metastable and equilibrium precipitates. The model predicts different precipitate mean radius and volume fraction for the various zones in the friction stir welded material. A model for the yield strength is developed which considers contributions from different strengthening mechanisms. The predictions of the each models have been verified against experimental data and literature. At constant advance per rotation, the peak temperature decreases with a decrease in traverse speed and increases with an increase in tool rotation. Weld properties were significantly affected by choice of thermal boundary conditions in terms of backing plate diffusivity. Weld conditions with a higher peak temperature and high strain rate results in more dissolution of precipitates and fragmentation of constituent particles resulting in a better corrosion behavior for the weld nugget. For a peak aged temper of 2XXX alloys, the weld nugget experiences dissolution of strengthening precipitates resulting in a lower strength and the Heat affected zone (HAZ) experiences coarsening of precipitates. For an annealed material, both the weld nugget and HAZ experiences dissolution of precipitates with an increase in strength in the weld nugget.

No description available.

The commercial polycrystalline superalloy RR1000 developed for turbine disc applications contains a large number of alloying elements. This complex alloy chemistry is required in order to produce appropriate microstructures and the required mechanical properties, such that the most important strengthener γʹ displays complex alloy chemistry. The broad aim of this project is to develop an approach to measuring the composition of γʹ precipitates at a broad range of length scales from nanometres to hundreds of nanometres, and subsequently develop a better understanding of the role of thermodynamics and diffusion kinetics on γʹ phase separation and precipitate growth. A solution of the absorption-corrected EDX spectroscopy to spherical particles was developed in our work, therefore enabling the quantitative analysis of precipitates' composition using an absorption-corrected Cliff-Lorimer approach. By performing this quantification, size-dependent precipitate compositional variations were obtained. Examination of this quantitative approach was compared to thermodynamic calculations of primary γ' precipitates possessing equilibrium compositions. Given the development of semi-quantitative compositional measurements for spherical γʹ precipitates and that cooling is one of the most common and critical regimes in physical metallurgy of Ni-based superalloys, this approach was then applied to study the local compositional variations that are induced in γ' precipitates when the alloy RR1000 undergoes different cooling rates. These measured compositions have been compared to detailed thermodynamic calculations and provide new experimental evidence of the importance of the dominant role of aluminium antisite diffusion in determining the low-temperature growth kinetics of fine-scale γ' precipitates. We have applied a similar analysis approach to study the compositional variations of γʹ cores within the class of secondary precipitates upon cyclic coarsening and reversal coarsening. It was shown that supersaturated Co in secondary γʹ exhibits an overall trend towards the equilibrium but Co content can significantly increase as γʹ coarsens. It was demonstrated that the limited elemental diffusivity in γ and γʹ compared to the observed coarsening rate in the coarsening regime results in the long-lasting Co supersaturation in γʹ and builds up elemental enhancements or depletions. These inhomogeneous elemental distributions produce compressive elastic constraints on large-scale secondary γʹ, therefore inducing morphological instability of these γʹ and causing the reversal coarsening. These results enable us to better understand the role that both thermodynamics and limited diffusion kinetics plays in controlling the complex microstructures of γ' precipitates.