Development of creep-resistant Al-Si cast alloys strengthened with nanoscale dispersoids

This doctoral thesis aimed at creating thermally stable precipitates (dispersoids) via the use of slow-diffusing elements, Ni, Mo and Mn to develop creep resistant Al-Si cast alloys to be used for Diesel engine applications above 250°C and in the 20-30MPa stress range. CALPHAD-based thermodynamic calculations (FactSage, ThermoCalc) were carried out to determine the phase selections and solidification paths in the alloy systems. The precipitates and their influence on creep deformation were studied via conventional and high-resolution transmission electron microscopy.Ni up to 0.6wt.% increased the strength of the base alloy, Al-7Si-0.5Cu-0.3Mg at 300°C, but lowered the creep resistance because of the severe cracking of the brittle T-Al9FeNi intermetallics. Mn addition imparted ductility to T-Al9FeNi, preventing cracking during creep and led to improved creep resistance. No dispersoid phase was observed in this alloy system. The second stage of the study investigated the effect of Mo addition on dispersoid formation. Mo formed nanoscale coherent dispersoids through a high temperature solution-treatment. The Mo-containing alloy exhibited significant improvement in mechanical properties at 300°C compared to the base alloy. A 90-95% decrease in the minimum creep rate was observed at 300°C and 30MPa, and the creep time-to-fracture increased by two orders of magnitude. The tensile yield strength and elongation at 300°C increased by 25% and 34%, respectively. These effects were attributed to the formation of the dispersoids with approximate stoichiometric formula of Al22(Fe6-8Mo)Si4 as determined by quantitative EDS. Their crystal structure was BCC with lattice parameter, ao =12.54±0.03 A° and space group Im3. The orientation relationship most commonly observed between the Al matrix and dispersoids was 〈001〉d//〈001〉m and {350}d//{002}m. The precipitation of the dispersoids in large amounts and their coherency was accounted for by the structural correlation that was found between the BCC dispersoid phase and the FCC Al matrix. The creep behaviour of the Mo-containing alloy was studied in details. On the basis of the TEM studies and steep changes in the stress exponent, three distinct creep deformation regions were identified as: I- diffusion creep (na ~1.5), II- dislocation creep (na ~20) and III- Orowan bowing (na >> 20).In the third phase of the study, Mn was added to the Mo-containing alloy to enhance the precipitation of the dispersoids. The combined additions of Mo and Mn increased the volume fraction of the dispersoids and lead to a more uniform distribution of the dispersoids. The beneficial effect of Mn was attributed to its solute characteristics in Al: Mn is a slow-diffuser, with relatively high solid solubility, and its solute partitioning behavior during solidification is opposite to Mo (kMn<1 vs. kMo>1). The minimum creep rate decreased with Mn content at 300°C, 30MPa. The creep time-to-fracture also increased from ~25 to ~180 hours as Mn increased from 0.0 to 0.5%. In this study, this alloy was determined to be the best candidate for use in elevated temperature engine applications.The final part of the thesis determined the effect of Mg level (0.3-0.7wt.%) on the strength and creep resistance of the base alloy at 300°C. Increasing the Mg to the solubility limit of 0.47wt.% increased the amount of Q and improved the strength and creep resistance at 300°C. The improvement was not as high as the combined addition of Mo and Mn to the base alloy.In conclusion, this thesis project was successful in developing a new alloy exhibiting significant improvements over existing alloys used in Diesel applications. The Al-Si-Mo-Mn system was introduced, which offers significant potential for development of elevated temperature cast alloys. This alloy system can be used for lightweighting in automotive industry and promote the wide use of Al-Si alloys in more severe operating conditions such as Diesel engine applications.Cette thèse de doctorat avait pour objectif principal la formation de fins précipités thermiquement stables (dispersoïdes) par l'addition d'éléments de diffusion lent, tels que: Ni, Mo et Mn, pour développer d'alliages Al-Si résistant au fluage pour l'utilisation dans de moteur Diésel à des températures au-dessus de 250ºC et de 20-30MPa. Des calculs thermodynamiques (FactSage et ThermoCalc) ont été réalisés afin d'effectuer la sélection de phases ainsi que les modes de solidification. Les précipités ainsi que leur influence sur la déformation par le fluage ont alors été étudiés par la méthode d'examen par la microscopie électronique conventionnelle et à haute résolution.Ni jusqu'à 0.6 % poids augmente la résistance des alliages à 300ºC, mais réduit la résistance au fluage à cause de la fissuration de la phase fragile du compose intermétallique T-Al9FeNi au début de la déformation lors des essais de fluage. Le Mn a permis d'améliorer la ductilité du T-Al9FeNi, éliminer sa fissuration lors du fluage et conduit à une résistance améliorée au fluage. Aucun dispersoïdes n'a été observé dans les alliages. L'étape 2 de cette étude a été orientée vèrs l'addition de Mo à cause de son potentiel pour la formation de dispersoïdes dans les alliages Al-Si. Le Mo a formé de dispersoïdes cohérents à l'échelle nanométrique au cours de traitements-thermiques effectués à haute température. L'alliage contenant du Mo a montré une amélioration significative des propriétés mécaniques à 300ºC. Une diminution de 90 à 95% du taux minimal de fluage a été observée à la température de 300ºC et à une contrainte de 30MPa et de plus, le temps de rupture au fluage a alors augmenté selon un ordre de magnitude de deux fois. La force de cédage en tension et l'allongement à 300ºC ont respectivement augmentés de 25% et 34%. Ces effets ont été attribués à la formation de dispersoïdes, Al22(Fe6-8Mo)Si4 avec structure cristalline CVC (ao =12.54±0.03 A°; groupe spatial Im3). La relation d'orientation observé le plus entre la matrice Al et le dispersoïdes était 〈001〉d//〈001〉m et {350}d//{002}m. La précipitation des dispersoïdes en grande quantité et leur cohérence a été expliqué par la corrélation structurale entre les dispersoïdes et la matrice Al. Trois différentes zones de fluage ont alors été identifiées pour l'alliage contenant du Mo telles que: I- fluage par diffusion (na ~1.5), II- fluage par le mouvement des dislocations (na ~20) and III- le mécanisme Orowan (na >> 20). Dans Phase 3, les additions combinées du Mo et du Mn ont augmenté la proportion en volume des dispersoïdes et ont conduit à une dispersion plus uniforme. Le taux minium de fluage a diminué avec la teneur en Mn à 300ºC, 30MPa. Le temps menant à la rupture lors du fluage a augmenté d'environ 25 à 180h en fonction de l'augmentation de la teneur en Mn de 0.0 à 0.5%. Ainsi, dans cette étude, cet alliage est le meilleur candidat pour ce qui a trait aux applications pour les moteurs à température élevée.La dernière partie de cette thèse est sur l'effet de la teneur en Mg (0.3-0.7% poids) sur la force et la résistance au fluage des alliages de base à 300 ºC. En augmentant la teneur en Mg jusqu'à sa limite de solubilité de 0.47% poids, celui-ci a augmenté la teneur du précipités Q, et a amélioré la résistance au fluage à 300ºC. L'amélioration n'a pas été aussi élevée que la combinaison des additions de Mo et de Mn dans l'alliage de base.Ce projet de thèse a été un succès pour ce qui a trait au développement d'un nouvel alliage qui procure des améliorations significatives relativement aux alliages conventionnels utilisé pour la fabrication des moteurs au Diésel. Le système Al-Si-Mo-Mn qui a été élaboré lequel offre un potentiel prometteur pour le développement des alliages pour l'application à température élevée. Ce système d'alliage peut ainsi faire la promotion d'un plus large éventail pour l'utilisation des alliages Al-Si dans des conditions exigeantes

Color comparison of Iranian traditional architecture into television studio sets

A national television channel should contain national contents in order to have acceptable representation of the nation through its historical and cultural backgrounds. Novel aesthetic approach to television besides modern intellect of architecture, as will be argued in this thesis, opens a way to compare television with other arts such as architecture. In this thesis, following to the argument above,Iranian national television and Iranian historical architecture have been focused as example. By concentration on Iranian historical architecture, one of the most influential Iranian architectures was specified and was suggested to Iranian national television people to consider it in their programs. Specifically in Iran, virtual media such as national television programmes should present contents that portray the representation of Iranian identity adapted from real media such as Iranian arts and architecture. These contents could appear in the topic or subject of the programmes or in design elements (e.g. forms, shapes, colours and textures) or symbols of spaces showed in the programmes. However, there are still lacks of studies on colour comparison between these two media as well as the convenient technique for studio sets designers and television producers to estimate such comparison. Therefore, the aim of this research is to conveniently estimate the colour similarity of Iranian national television studio sets with one of the most influential architecture in the Safavid period i.e. Ali-Ghapoo Palace. This study adopted a correlational research method by applying some statistical manipulations for comparison purposes. Sixty images of six studio sets from Iranian national television cooking shows and thirty images of Ali-Ghapoo Palace were collected. Numerical colour histograms of Red, Green and Blue were extracted from each image of both sets. Histograms of studio sets were then separately transformed to relative histograms by mathematical formula and this method was then applied to each chosen histogram of Ali-Ghapoo Palace. Subsequently, the resulting relative histograms of each studio sets were adapted mathematically to those of Ali-Ghapoo Palace; and finally, the resulting relative histograms of all studio sets were adapted together to those of Ali-Ghapoo Palace. The results reveal that, out of six studio sets in total, two studio sets have intertwined histograms with those of Ali-Ghapoo Palace suggesting that these studio sets have many analogous colours with Ali-Ghapoo Palace. Another one studio set has separated histograms with those of Ali-Ghapoo Palace indicating that this studio set contains many different colours in comparison to Ali-Ghapoo Palace. Finally, after adapting all six studio sets with Ali-Ghapoo Palace, the result shows that all six studio sets, despite several diversities, are generally matched with Ali-Ghapoo Palace in terms of colour comparison. These results will potentially guide Iranian studio set designers and producers in adapting appropriate sets of colours in the future television programs in order to support and strengthen Iranian traditional identity

Structural, optical, and physical properties of GeOշ-PbO-BiշO³ glass

The lead bismuth germanate glasses are of growing interest, due to their low cut-off optical phonon energy, high linear and nonlinear refractive index and excellent infrared transmission. Fabrication of metallic nanostructures embedded in glass matrix also attracts many researchers because of their enhancement ability of photoluminescence and optical nonlinearity. However, fabrication of metallic nanostructure other than sphere in glass matrix still is a challenge where met with a very limited success. To achieve the research objectives, the undoped and Er3+/Yb3+ doped germanate based glasses samples were prepared. This thesis reports the preparation of GeO2-PbO-Bi2O3 ternary glass system with and without Er3+/Yb3+ by melt cast-quenching method. All samples are studied by a wide range of characterization techniques which includes their structure, elastic and optical properties. An attempt has been made to add silver nanostructures into the germanate based glasses by thermal reduction of melt-quenched glass in order to study the formation of nanostructures in the vitreous network. In the germanate based glasses system the bismuth atom will be substituted by the lead atom to achieve almost mass independent properties due to their closeness of atomic masses. Structure and physical properties were studied by X-ray diffraction,Fourier transform infra-red (FTIR), density, ultrasonic velocities, UV-Visible absorption, photoluminescence and transmission electron microscopy (TEM). All of the samples were fully amorphous and their density, ultrasonic velocities and elastic moduli are relatively low for high lead content samples. FTIR peaks related to PbO covalent bond and bending Bi-O bond of BiO6 group which are formed when both Pb and Bi act the role of former in glass network, were observed in high lead content samples in contrast with low lead samples. The FTIR data also showed that germanium participated in glass structure with both of four-fold and six-fold coordination in all of samples. The UV-visible absorption of the glasses studied showed highest energy of band-gap for Pb-rich samples and also showed characteristic peaks of Er3+/Yb3+ ions. The photoluminescence spectrum obtained by excitation at wavelengths of highest absorption peaks, showed highest intensities for samples with highest lead content which related to lowest non-radiative relaxation in Pb-rich samples. The GeO2-PbO glass dopped with Er2O3, Yb2O3 and AgNO3 were also prepared by melt quenching method. Annealing of the glass were utilized for thermally reducing of Ag+ ions to metallic silver. The TEM results showed that the annealing process at 450˚C caused the formation of silver nanoparticles of about 3 nm mean diameter size. The samples which were annealed at 400˚C temperature showed the formation of silver nanoplates with mean length size of 60 nm embedded in glass matrix. The UV-Visible absorption also confirmed the existence of metallic silver nanostructure. The FTIR shows peaks at 470 cm-1 for 450˚C annealed samples, in contrast with 400˚C annealed samples, which suggest the existence of Pb-O chains in the germanate glass network. This is the main source of difference in formation of various nano structures due to different stabilizing medium and better physical isolation of glass matrix in 450˚C annealed samples

Phase formation in as-solidified and heat-treated Al–Si–Cu–Mg–Ni alloys: Thermodynamic assessment and experimental investigation for alloy design

Thermodynamic simulations based on the CALPHAD method have been carried out to assess the phase formation in Al–7Si–(0–1)Ni–0.5Cu–0.35Mg alloys (in wt.%) under equilibrium and non-equilibrium (Scheil cooling) conditions. Calculations showed that the T-Al9FeNi, γ-Al7Cu4Ni, δ-Al3CuNi and ε-Al3Ni phases are formed at different Ni levels. By analyzing the calculated isothermal sections of the phase diagrams it was revealed that the Ni:Cu and Ni:Fe ratios control precipitation in this alloy system. In order to verify the simulation results, microstructural investigations in as-cast, solution treated and aged conditions were carried out using electron probe microanalysis (EPMA), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Furthermore, cooling curve analysis (CCA) was also performed to determine the freezing range of the new alloys and porosity formation during solidification. Hardness measurements of the overaged samples showed that in this alloy system the δ-Al3CuNi phase has a greater influence on the overall strength of the alloys compared to the other Ni-bearing precipitates