A computational model is developed to study the effects of alumina layer formation on an ablative surface when exposed to high temperature particle laden gas flow. The solidification dynamics i.e., the solid and liquid alumina layer growth rate, and the heat transferred to the ablative surface are investigated. A one-dimensional model is developed taking into consideration the thermal loading, particle loading and the temperature dependence of the thermo-physical properties of alumina. A fully implicit finite volume method is used to solve the coupled set of non-linear heat conduction equations. The solidification interface is tracked using the Lagrangian interpolation technique. The particle mass flux was found to be the major factor affecting the solid layer growth rate. The gas heat flux also has a major effect on the solid growth rate and the heat transferred to the ablative surface, but only for lower particle mass fluxes. On other hand the particle temperature has a linear effect on the solidification dynamics and the heat transferred to the ablative surface for all particle mass fluxes. The heat transferred to the ablative surface is reduced by approximately 39% to 88%, depending on the mass fluxes, due to the formation of the alumina layer. / Graduation date: 2003

Tungsten carbide-based metal matrix composite coatings are deposited by PTAW (Plasma Transferred Arc Welding) on production critical components in oil sands mining. Homogeneous distribution of the reinforcement particles is desirable for optimal wear resistance in order to reduce unplanned maintenance shutdowns. The homogeneity of the coating can be improved by controlling the heat transfer, solidification rate of the process and the volume fraction of carbide. The degree of settling of the particles in the deposit was quantified using image analysis. The volume fraction of carbide was the most significant factor in obtaining a homogeneous coating. Lowering the current made a modest improvement in homogeneity. Changes made in other operational parameters did not effect significant changes in homogeneity. Infrared thermography was used to measure the temperature of the surface of the deposit during the welding process. The emissivity of the materials was required to acquire true temperature readings. The emissivity of the deposit was measured using laser reflectometry and was found to decrease from 0.8 to 0.2 as the temperature increased from 900C to 1200C. A correction algorithm was applied to calculate the actual temperature of the surface of the deposit. The corrected temperature did increase as the heat input of the weld increased. A one dimensional mathematical model of the settling profile and solidification of the coatings was developed. The model considers convective and radiative heat input from the plasma, the build-up of the deposit, solidification of the deposit and the settling of the WC particles within the deposit. The model had very good agreement with the experimental results of the homogeneity of the carbide as a function of depth. This fundamental model was able to accurately predict the particle homogeneity of an MMC deposited by an extremely complicated process. It was shown that the most important variable leading to a homogeneous coating is to operate at the packing saturation limit of the reinforcement. In the case of the MMC explored, a fully homogeneous coating was obtained with 50 vol% WC in a NiCrBSi matrix. / Materials Engineering

No description available.

Rapid solidification experiment on Fe-Cr-Mo-Mn-Si-C alloy was performed to investigate metastable phases formed during the solidification. A wide range of cooling rate was used to analyse the sample from melt spinning technique (~107 K/s) to water quenching method (~102 K/s). A single phase featureless structure was obtaind initially in the melt spinning experiment for 77Fe-8Cr-6Mn-5Si-4C alloy. Reduction of C and addition of Mo led to form a complete featureless structure for 2.85 mm rod for 72.8Fe-8Cr-5Mo-6Mn-5Si-3.2C. Subsequent investigation of influence of Mo, Cr and Mn on the single phase featureless structure concludes that 7.5 mm thick complete featureless phase could be formed at 63.8Fe-15Cr-7Mo-6Mn-5Si-3.2C alloy composition. In a separate attempt, powder samples of 40 μm dia. size complete featureless powders were produced for three slightly different compostions for the same alloy system. Characterisation of the featureless phases reveals that it could be a single phase metastable structure of ε phase or austenitic solid solution with high amount of alloying element dissolved in it. Subsequent heat treatment of this featureless phase of the rod and the powder at different temperatures formed bainitic ferrite with fine carbides dispersed in the austenitic matrix. Hardness values measured on featureless phase found to have influenced by the alloying element specially Mo, Cr and Mn. In an attempet to improve clean melting condition to extend the featureless phase and to form amorphous, an elliptic short arc lamp vaccum furnace was designed with 10 kW lamp power. Around 30 g of iron based alloy system was melted and cast as a 7 mm rod sample in a copper mould. Design details of new mirror and the lamp furnace are presented. In a separate study, influence of the melt temperature on Al-Y and Al-Si alloys were investigated by levitaion casting in a silver mould at around 2000 K/s cooling rate. Plate like structure of Al8Y3 primary phase was observed at low melt temperature with small percentage of peritectic transformation of Al8Y3 and liquid melt into Al9Y2. A pre-dentritic star like crystal of Al3Y was observed in a fine eutectic matrix at very high melt temperature. Amount and number of primary Si crystals formed in a unit area during the solidification increases as the melt temperature increases. / QC 20100805

Aluminum alloys are widely used in many manufacturing areas due to good castability, lightness and mechanical properties. The purpose of this research is to investigate copper’s influence on an Al-Si-Mg alloy (A356). Copper in the range of 0.6 – 1.6 wt. % has been used in an A356 aluminum based alloy. In this work a simulation of three different casting processes, sand-, die- and high pressure die-casting has been employed with the help of gradient solidification equipment. The microstructure of the samples has been studied by optical and scanning electron microscopy. Materials in both as-cast and heat treated states have been investigated through tensile test bars  to get the mechanical properties of the different conditions.   Questions that have been subjected to answer are what influence does copper have on the plastic deformation and on fracture behavior and whether there is a relationship between the content of copper and increased porosity or not; and in that case explore this relationship  between the amount of copper and the mechanical behaviour.   It has been analyzed that a peak of mechanical properties is obtained with a content about 1.6 wt. % copper. The increment of copper seems to have a remarkable impact on the mechanical properties and especially after the aging process showing a large raise on the ultimate tensile strength and yield strength. Relationship between the copper content and increased porosity could not be found.

La formation de la structure de grains dans les métaux pendant la solidification est déterminante pour les propriétés mécaniques et électroniques des pièces coulées. En plus de la texture donnée au matériau, la germination et la croissance des grains sont liées en particulier avec la formation des phases thermodynamiques et les inhomogénéités en composition d'éléments d'alliage. La structure de grains est rarement modélisée à l'échelle macroscopique, d'autant plus que l'approximation 2D est très souvent injustifiée. Dans ces travaux, la germination et la croissance de chaque grain individuel sont suivies avec un modèle macroscopique 3D CAFE. La microstructure interne des grains n'est pas explicitement résolue. Pour valider les approximations faites sur cette microstructure, une comparaison directe avec un modèle microscopique "champ de phase" a été réalisée. Celle-ci a permis de valider les hypothèses de construction du modèle CAFE, de mettre en avant le lien entre données calculées par les modèles microscopiques et paramètres d'entrée des modèles à plus grande échelle, et les domaines de validité de chaque modèle. Dans un deuxième temps, un couplage avec la ségrégation chimique et les bases de données thermodynamiques a été mise en place et appliquée sur un alliage binaire étain-plomb. Une expérience de macroségrégation par convection naturelle a été simulée. L'accord entre les courbes de température expérimentales et simulées atteint une précision de l'ordre de 1K, et la recalescence est correctement prédite. Les cartes de compositions sont comparables qualitativement, ainsi que la structure de grains. Les avantages du suivi de la structure ont été mis en évidence par rapport à une simulation par éléments finis classique. De plus, il a été montré que le calcul 3D était ici indispensable. Enfin, une implémentation parallèle optimisée du code a permis d'appliquer le modèle CAFE à un lingot de silicium polycristallin industriel de dimensions 0,192 x 0,192 x 2,08m, avec une taille de cellules de 250µm. Au total, 4,9 milliards de cellules sont représentées sur le domaine, et la germination et la croissance de 1,6 million de grains sont suivies.

Le traitement des sols aux liants hydrauliques (chaux, ciment...) est un procédé de plus en plus pratiqué par les entreprises du BTP en technique routière. Il améliore les propriétés géotechniques et mécaniques des sols dits " médiocres " in situ. Les guides techniques " Traitement des sols à la chaux et/ou aux liants hydrauliques " (SETRA-LCPC, 2000 et 2007) présentent les méthodologies d'études géotechniques en laboratoire et in situ afin d'atteindre les performances requises. Cependant, l'analyse du retour d'expérience montre un certain nombre de perturbations du traitement. La présente thèse s'intéresse à : - la caractérisation physico-mécanique et physico-chimique de 3 sols types de la région Basse-Normandie avant et après un traitement mixte ; - la détermination de seuils de perturbations de la solidification dues à la présence des anions Cl-, SO42-, NO3-, PO43-dans le sol. Les concentrations seuil sont recherchées tant du point de vue des performances mécaniques que de la stabilité structurelle. A l'échelle microscopique, les perturbations sont étudiées à l'aide des analyses physico-chimiques et microstructurales des sols dopés. Il apparaît que les 4 anions présentent un potentiel de perturbation de la solidification dont l'expression diffère selon le sol, le traitement considéré, la forme chimique de l'anion, sa concentration, les conditions de cure et le type de sollicitation. Si l'introduction dans le sol d'un anion unique ne remet pas en cause une réutilisation du sol traité en techniques routières, les simulations d'interactions des anions révèlent des cas d'inaptitude fortement préjudiciables, dont le contrôle doit être poursuivi.

Carbonation is a naturally-occurring process whereby Ca-containing cement phases lose their hydration water and are converted to carbonate minerals by reaction with atmospheric CO&#8322;. As these secondary minerals develop in the microstructure of hydrated cement, porosity, pore-size distribution and permeability are decreased. These are all considered desirable properties in a wasteform. The objective of this study was to examine the effect of carbonation and different pozzolans on the leach performance and mechanical strength of ordinary Portland cement (OPC) wasteforms. Two methods of accelerated cement carbonation were used: <ol> <li>A vacuum carbonation method, where wasteforms are placed in an evacuated, sealed cell and subjected to small additions of CO&#8322; over several days at near vacuum conditions; and <li>A one-step carbonation method, where CO&#8322; gas is added to the wasteform paste as it is being mixed. </ol> Thirteen elemental constituents of interest to the safety assessments of long-term management of Ontario Power Generation's radioactive waste (Cl, N, S, Se, 13C, Th, Pb, Co, Ni, Cu, Sr, Ba and Cs) were stabilised/solidified via cement mix water. Wasteforms were produced with only OPC, OPC and fly ash, or OPC and silica fume. Most wasteforms were carbonated using one of the carbonation methods. Some wasteforms were not carbonated and served as controls. Wasteforms were subjected to either standard leach tests or compressive strength tests. The extent of carbonation was found to be about 20% for vacuum carbonation method, substantially higher than that for one-step treatment (up to about 10%). For vacuum carbonated wasteforms, carbonation occurred at the outer selvages of the wasteforms, whereas one-step treatment resulted in homogenous carbonation. Generally, compared to uncarbonated OPC wasteforms, vacuum carbonation increased leaching of elements that are anionic in cementitious conditions (Cl, N, S, Se, 13C, Th), decreased leaching of large metal cations (Sr, Ba, Cs, Pb) and had negligible effect on the leaching of the elements that form hydroxyl complexes (Co, Ni, Cu). 13C was the only anionic element whose leachability was reduced by vacuum carbonation, as it may be precipitated in the form CO32- in the large quantity of secondary carbonate minerals produced during the vacuum carbonation process. One-step carbonation did not result in substantial reductions in leachability, compared to uncarbonated OPC wasteforms. However, it had an interesting inverse effect on large metal cation leachability from fly ash- and silica fume-containing wasteforms. A model is presented that proposes that porewater pH changes can have an effect on waste element leachability because 1) the C-S-H Ca/Si ratio is dependent on the equilibrating porewater pH and 2) the degree of ion sorption on C-S-H is dependent on the C-S-H Ca/Si ratio. This model should be tested experimentally as it has important implications on wasteform design. Because of this inverse behaviour, overall neither pozzolan outperformed the other with respect to leachability. Generally, for uncarbonated wasteforms, OPC retained the elements more effectively than OPC with pozzolans. For pozzolans, the leachability of these elements from OPC with fly ash was lower than that of OPC with silica fume. Leaching of Cs was anomalously low from uncarbonated OPC wasteforms, but follow-up experimentation did not corroborate this anomaly. Further testing of these wasteforms to determine how the mineralogical fate of Cs can differ between wasteforms is recommended. All wasteforms tested were of acceptable strength (<0. 689 MPa). Fly ash, and, to a greater degree, silica fume, improved wasteform strength when compared to OPC wasteforms. Carbonation treatments had little effect on wasteform strength. This study has provided much information about the leaching characteristics of a representative set of waste elements from several cement-based wasteform treatments. Although it has not indicated a wasteform design that is ideal for all elements studied, it does suggest that some treatments may be effective for certain groups of elements. Most notably, vacuum carbonation shows promise in improving the immobilisation of isotopes of large metal cations such as Sr, Ba, Cs and Pb as well as 14C (as suggested by 13C here) in cement-based wasteforms.

Carbonation is a naturally-occurring process whereby Ca-containing cement phases lose their hydration water and are converted to carbonate minerals by reaction with atmospheric CO&#8322;. As these secondary minerals develop in the microstructure of hydrated cement, porosity, pore-size distribution and permeability are decreased. These are all considered desirable properties in a wasteform. The objective of this study was to examine the effect of carbonation and different pozzolans on the leach performance and mechanical strength of ordinary Portland cement (OPC) wasteforms. Two methods of accelerated cement carbonation were used: <ol> <li>A vacuum carbonation method, where wasteforms are placed in an evacuated, sealed cell and subjected to small additions of CO&#8322; over several days at near vacuum conditions; and <li>A one-step carbonation method, where CO&#8322; gas is added to the wasteform paste as it is being mixed. </ol> Thirteen elemental constituents of interest to the safety assessments of long-term management of Ontario Power Generation's radioactive waste (Cl, N, S, Se, 13C, Th, Pb, Co, Ni, Cu, Sr, Ba and Cs) were stabilised/solidified via cement mix water. Wasteforms were produced with only OPC, OPC and fly ash, or OPC and silica fume. Most wasteforms were carbonated using one of the carbonation methods. Some wasteforms were not carbonated and served as controls. Wasteforms were subjected to either standard leach tests or compressive strength tests. The extent of carbonation was found to be about 20% for vacuum carbonation method, substantially higher than that for one-step treatment (up to about 10%). For vacuum carbonated wasteforms, carbonation occurred at the outer selvages of the wasteforms, whereas one-step treatment resulted in homogenous carbonation. Generally, compared to uncarbonated OPC wasteforms, vacuum carbonation increased leaching of elements that are anionic in cementitious conditions (Cl, N, S, Se, 13C, Th), decreased leaching of large metal cations (Sr, Ba, Cs, Pb) and had negligible effect on the leaching of the elements that form hydroxyl complexes (Co, Ni, Cu). 13C was the only anionic element whose leachability was reduced by vacuum carbonation, as it may be precipitated in the form CO32- in the large quantity of secondary carbonate minerals produced during the vacuum carbonation process. One-step carbonation did not result in substantial reductions in leachability, compared to uncarbonated OPC wasteforms. However, it had an interesting inverse effect on large metal cation leachability from fly ash- and silica fume-containing wasteforms. A model is presented that proposes that porewater pH changes can have an effect on waste element leachability because 1) the C-S-H Ca/Si ratio is dependent on the equilibrating porewater pH and 2) the degree of ion sorption on C-S-H is dependent on the C-S-H Ca/Si ratio. This model should be tested experimentally as it has important implications on wasteform design. Because of this inverse behaviour, overall neither pozzolan outperformed the other with respect to leachability. Generally, for uncarbonated wasteforms, OPC retained the elements more effectively than OPC with pozzolans. For pozzolans, the leachability of these elements from OPC with fly ash was lower than that of OPC with silica fume. Leaching of Cs was anomalously low from uncarbonated OPC wasteforms, but follow-up experimentation did not corroborate this anomaly. Further testing of these wasteforms to determine how the mineralogical fate of Cs can differ between wasteforms is recommended. All wasteforms tested were of acceptable strength (<0. 689 MPa). Fly ash, and, to a greater degree, silica fume, improved wasteform strength when compared to OPC wasteforms. Carbonation treatments had little effect on wasteform strength. This study has provided much information about the leaching characteristics of a representative set of waste elements from several cement-based wasteform treatments. Although it has not indicated a wasteform design that is ideal for all elements studied, it does suggest that some treatments may be effective for certain groups of elements. Most notably, vacuum carbonation shows promise in improving the immobilisation of isotopes of large metal cations such as Sr, Ba, Cs and Pb as well as 14C (as suggested by 13C here) in cement-based wasteforms.

The size and morphology of the dendrite growth patterns are simulated for laser powder deposition of Ti-Nb alloys under steady-state and transient growth conditions. A phase field model using an adaptive grid technique was employed to simulate the steady-state growth of dendrites on rather small domains, in which fixed local solidification conditions are present. For simulation of dendrite growth patterns at transient conditions, a cellular automaton model was used along with a virtual front tracking technique on larger domains, containing various initial orientations of the solid-liquid (SL) interface. To obtain the required input thermal data, i.e., the temporal distribution of temperature, a finite element analysis was performed along with a novel numerical approach for the real-time addition of new deposition material in each time step, thus building the deposition geometry momentarily. Using the output of the thermal model, the motion and morphology of the SL interface was determined through tracking the isotherm of the solidification temperature. First, in this study, the appropriate set of processing parameters was found through an optimization process using a new concept, laser supplied energy Es, which combines the effects of the energy and powder density in the process. With the developed analytical/experimental procedure, crack and pore-free coatings of Ti-Nb with continuous beads were produced by examining the effects of a few sets of processing parameters, including laser power, laser scan velocity, laser beam diameter and powder feed rate. The results of the thermal model for the optimized set of parameters matched with the thermocouple temperature measurements with only ~5% deviation. The thermal model was able to predict realistic profiles for the temporal development of deposition geometry, thus predicting meaningful morphologies of the SL interface. The model output was easily treated for extraction of local processing parameters, such as the temperature gradient and solidification velocity. These data are very useful when simulating the dendrite growth patterns at steady-state conditions in directional solidification of selected regions in the microstructure. In order to define transient growth conditions, the simulated distribution of temperature can be also directly fed into the microstructure model at each solution time step. Phase field simulations of steady-state growth of dendrites during directional solidification showed a remarkable agreement with the experimental observations for the local dendrite arm spacing across the microstructure. Also qualitatively agreeing with the experiment, the simulated dendrite spacing exhibited a minimum around the mid-height region of the microstructure, which is explained by the counter effect of the temperature gradient and solidification velocity along the height of the sample. On a large domain containing different initial orientations of the SL interface, cellular automaton simulations for transient growth patterns of dendrites could reproduce most qualitative features observed in the microstructure. The dendrite arm spacing gradually decreased from the top of the microstructure. The competition was won by the dendrites growing in areas with higher cooling rates, i.e., in the regions closer to the top of the microstructure. The secondary arms of the primary dendrites, which are initially inclined on the vertical axis, grew extensively only along the overall growth direction and eventually became primary arms in some cases.