Aluminium alloys with Si as the major alloying element form a class of material providing the most significant part of all casting manufactured materials. These alloys have a wide range of applications in the automotive and aerospace industries due to an excellent combination of castability and mechanical properties, as well as good corrosion resistance and wear resistivity. Additions of minor alloying elements such as Cu and Mg improve the mechanical properties and make the alloy responsive to heat treatment. The aim of this work is studying the role of size and morphology of microstructural constituents (e.g SDAS, Si-particles and intermetalics) on mechanical properties of Al-Si based casting alloy at room temperatures up to 500 ºC. The cooling rate controls the secondary dendrite arm spacing (SDAS), size and distribution of secondary phases. As SDAS becomes smaller, porosity and second phase constituents are dispersed more finely and evenly. This refinement of the microstructure leads to substantial improvement in tensile properties (e.g. Rm and εF). Addition of about 280 ppm Sr to EN AC- 46000 alloy yields fully modified Si-particles (from coarse plates to fine fibres) regardless of the cooling conditions. Depression in eutectic growth temperature as a result of Sr addition was found to be strongly correlated to the level of modification irrespective of coarseness of microstructure. Modification treatment can improve elongation to failure to a great extent as long as the intermetallic compounds are refined in size. Above 300 ºC, tensile strength, Rp0.2 and Rm, of EN AC-46000 alloys are dramatically degraded while the ductility was increased. The fine microstructure (SDAS 10 μm) has superior Rm and ductility compared to the coarse microstructure (SDAS 25 μm) at all test temperature (from room to 500 ºC). Concentration of solutes (e.g. Cu and Mg) in the dendrites increases at 300 ºC and above where Rp0.2 monotonically decreased. The brittleness of the alloy below 300 ºC was related to accumulation of a high volume fraction damaged particles such as Cu- Fe-bearing phases and Si-particles. The initiation rate of damage in the coarse particles was significantly higher, which enhances the probability of failure and decreasing both Rm and εF compared to the fine microstructure. A physically-based model was adapted, improved and validated in order to predict the flow stress behaviour of EN AC- 46000 cast alloys at room temperature up to 400 ºC for various microstructures. The temperature dependant variables of the model were quite well correlated to the underlying physics of the material

<p>Al-Si cast alloys have been in the fore-front of commercial casting application for more than a century. Iron containing intermetallic phases that evolve during the solidification of these alloys play a major role in the resultant mechanical properties and performance of the cast products. Changes in alloy composition and casting parameters significantly alter the evolution of the Al-Si-Fe intermetallic phases. There was a lack of clear understanding of the complex relationships between the solidification parameters and nature intermetallic phases in these alloys. Current thermodynamic model predictions for the nature of these intermetallic phases in the Al corner of the Al-Si-Fe ternary system are strikingly different from the experimental results in this project. Trace levels of Sr (about 0.02wt%) are typically added to the Al-Si commercial alloys to effect a morphological modification of the eutectic phases to improve the properties and performance of the cast products.</p> <p>The nature of the Fe containing intermetallic phases have been characterized as a function of alloy composition (Si, Fe and Sr) and cooling rates during solidification. There was an anomalous evolution of the t₅-Al₈Fe₂Si phase which transformed into the t₆-Al₉Fe₂Si₂ phase during solidification at lower cooling rates and higher Fe concentration in the alloy, alike. Further, Sr addition to these alloys prevented the evolution of the t₅ phase and promoted the evolution of an unidentified Al₅Fe₂Si₃ phase which was noted as k in this dissertation; the k phase also transformed into the t₆ phase at lower cooling rates and higher Fe concentration in the alloy, alike.</p> / Doctor of Philosophy (PhD)

Les alliages aluminium-silicium (Al-Si) ont attiré une attention considérable en raison de leur importance pour les applications industrielles. Dans le présent travail, des alliages à haute pureté (Al-12.7 wt. % Si) avec et sans ajout de strontium (400 ppm), solidifiés lentement en creuset ou de façon dirigée (DS), ont été préparés et traités thermiquement. L'influence de l'ajout de strontium et des post-traitements thermiques sur les caractéristiques microstructuraux et cristallographiques des phases eutectiques a été étudiée de façon systématique. Les caractéristiques de croissance du silicium eutectique (Si) dans l'alliage non modifié ainsi que dans l'Al-12.7Si Sr-modifié ont été étudiés. Pour le cas du non-modifié, la formation répétée de variantes de macles mono-orientées permet une croissance rapide du silicium eutectique selon le mécanisme twin plane re-entrant (TPRE). Microscopiquement, les cristaux de silicium ont une forme de plaque allongée dans la direction <1 1 0> non conforme à la croissance selon <1 1 2> présumée par le modèle TPRE. L'élongation selon <1 1 0> est réalisée par des paires en zigzag <1 1 2> sur des plans de maclage parallèles, conduisant à une disparition alternative et à la création de macles rentrantes à 141°. Ce mécanisme de croissance permet aux cristaux de silicium de n'exposer que les plans {1 1 1} à faible consommation d'énergie à la consolidation. Pour les alliages modifiés au strontium, des changements importants de morphologie apparaissent dans le silicium eutectique, attribuable à la croissance de TPRE restreinte et au maclage induit par les impuretés (IIT). Ce dernier améliore la croissance latérale en formant de nouvelles macles avec des plans de macles parallèles, tandis que le second conduit à une croissance isotrope en formant des macles orientées différemment. Le traitement thermique provoque l'affinement des grains des deux phases eutectiques. L'affinement de l'α-Al se produit en même temps que la fragmentation et la sphéroïdisation du silicium et est principalement lié à la fracture des grains de silicium en raison de leur capacité limitée à accommoder la très grande dilatation thermique l'α-Al, ainsi qu'à la diffusion des atomes d'aluminium au cours du traitement thermique. La rupture du silicium génère une force de "capillarité" qui active la diffusion d'atomes d'aluminium dans la fissure. En raison du caractère de substitution de la diffusion de l'aluminium, la migration des lacunes vers l'intérieur de l'α-Al est induite lorsque l'aluminium se déplace dans les fissures, ainsi les vides de la fracture du silicium sont transférés à l'α-Al. De cette façon, les cristaux d'α-Al sont altérés et déformés. Les défauts cristallins produits, à leur tour, initient la restauration et même la recristallisation du α-Al, ce qui entraîne une diminution de taille de grain. La phase α-Al dans l'alliage de Al-12.7Si-0.04Sr solidifiée directionnellement, affiche une forte texture de fibre <1 0 0> parallèle à la direction de solidification. De très gros grains <1 0 0> α-Al sont principalement formés à la périphérie de l'échantillon cylindrique en raison des directions d'évacuation de chaleur favorables disponibles pour les trois directions [1 0 0]. Après traitement thermique, l'intensité de la texture de la phase α-Al diminue en raison de la restauration et de la recristallisation, mais le type de texture ne change pas. Pour la phase de silicium eutectique dans l'alliage de coulée, il y a deux fibres principales de texture, <1 0 0> et <1 1 0> parallèles à la direction de solidification, accompagnées de deux composantes faibles, <2 2 1> et <1 1 3> dans la même direction. Les fibres <1 0 0> et <1 1 0> correspondent à des grains de silicium situés sur la périphérie et dans le centre de l'échantillon. Les composantes <2 2 1> et <1 1 3> proviennent de plusieurs macles de grains orientés <1 1 0> et <1 0 0>. Les faibles intensités de ces deux composantes sont liées à leur fraction volumique mineure [...] / Al-Si alloys have attracted considerable attention due to their importance to industrial applications. In the present work, both crucible slowly solidified and slowly directionally solidified (DS) high-purity Al-12.7 wt. % Si alloys with and without 400 ppm Sr addition have been prepared and heat treated. The influence of Sr addition and post heat treatments on the microstructural and crystallographic features of the eutectic phases has been systematically studied. The growth characteristics of eutectic Si in the unmodified and the Sr-modified Al-12.7Si alloys were investigated. For the non-modification case, the formation of repeated single-orientation twin variants enables rapid growth of eutectic Si according to the twin plane re-entrant (TPRE) mechanism. Microscopically, Si crystals are plate-like elongated in one <1 1 0> direction that is not in accordance with the <1 1 2> growth assumed by the TPRE model. The <1 1 0> extension is realized by paired <1 1 2> zigzag growth on parallel twinning planes, leading to alternative disappearance and creation of 141° re-entrants. This growth manner ensures Si crystals to expose only their low-energy {1 1 1} planes to the melt. For the Sr-modification case, substantial changes appear in eutectic Si morphology, attributable to the restricted TPRE growth and the impurity induced twinning (IIT) growth. The first enhances lateral growth by forming new twins with parallel twinning planes, while the second leads to isotropic growth by forming differently oriented twins. Heat treatment brings about refinement of both eutectic phases. The refinement of the α-Al occurs concomitantly with the fragmentation and spheroidization of Si and is mainly related to the fracture of the Si crystals due to their limited capacity to accommodate the giant thermal expansion of the α-Al and the diffusion of Al atoms to the cracks during the heat treatment. The Si fracture generates “capillarity” force that activates the diffusion of Al atoms to the gap of the crack. Due to the substitutional feature of Al diffusion, the migration of vacancies toward the interior of the α-Al is induced when Al moves to the gaps, thus the voids of the Si fracture are transferred to the α-Al. In this way, the crystals of α-Al are distorted and defected. The produced crystal defects, in turn, initiate recovery and even recrystallization of the α-Al, resulting in grain refinement. The α-Al phase in the directionally solidified Al-12.7Si-0.04Sr alloy, displays a strong <1 0 0> fiber texture in the solidification direction. Giant <1 0 0> α-Al grains are mainly formed in the outer circle region of the cylindrical specimen due to the favorable heat evacuation directions available for the three <1 0 0> directions. After heat treatment, the texture intensity of the α-Al phase decreases due to the recovery and recrystallization, but the texture type does not change. For the eutectic Si phase in the as-cast alloy, there are two main fiber texture components, <1 0 0> and <1 1 0> in the DS direction, accompanied by two weak components, <2 2 1> and <1 1 3> in the same direction. The <1 0 0> and <1 1 0> components are from Si crystals located in the outer circle and center regions of the cylindrical specimen. The <2 2 1> and the <1 1 3> components are from multiple twins of the <1 1 0> and <1 0 0> oriented crystals. The weak intensities of these two components are related to their minor volume fraction. Once heat treated, the twinned parts with minor volume fractions enlarge at the expense of their twin related matrix, thus the <1 1 0> component is weakened and accompanied by the intensification of the components from the twins. The disappearance of the <1 1 3> component and the appearance of the <1 1 5> component are due to crystallographic rotation of Si crystals during their fragmentation