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Les alliages Al-Cu-Li sont particulièrement attractifs pour les applications aéronautiques du fait de leur faible densité, haute limite d'élasticité et bonne ténacité. Ils reçoivent une attention particulièrement importante actuellement, depuis le développement de la troisième génération qui contient des concentrations relativement élevées pour le cuivre et relativement basses pour le Li. Ces nouveaux alliages sont caractérisés par une dureté élevée, une bonne résistance à la fatigue et une bonne stabilité thermique. La phase principale de durcissement est la phase T1 – Al2CuLi qui se présente sous la forme de plaquettes d'environ 1 nm d'épaisseur et 50 nm de diamètre, situées sur les plans {111} de la matrice avec une structure hexagonale. La germination efficace de cette phase durcissante entre en compétition avec d'autres précipités des sous-systèmes constituant ces alliages (comme Al-Cu et Al-Li), et nécessite des conditions particulières, en particulier la présence de dislocations (introduites par pré-déformation) et d'éléments d'alliage mineurs (Mg, Ag, Zn). Bien qu'il soit connu depuis longtemps que l'addition de ces éléments favorise la cinétique de précipitation dans ces alliages et le durcissement associé, leurs mécanismes d'action sont encore très mal compris.Dans ce contexte, l'objectif de la thèse est d'évaluer systématiquement l'effet des additions mineures de Mg, Ag et Zn sur la germination, la cinétique de précipitation et le durcissement correspondant. La caractérisation détaillée de la microstructure est utilisée pour comprendre les mécanismes de modification de la microstructure par les éléments mineurs. Les mesures de la diffusion des rayons X à petits angles et la DSC fournissent respectivement la cinétique de précipitation et la séquence de formation des phases. La microscopie électronique en transmission, utilisée en mode conventionnel, en résolution atomique et en mode de cartographie chimique met en évidence la structure et la distribution spatiale des phases. La dureté donne accès au durcissement. Des matériaux à gradient de concentration ont été élaborés et caractérisés pour évaluer l'effet de la concentration des alliages sur la précipitation et le durcissement.Les résultats mettent en évidence que le Mg est l'élément le plus efficace pour accélérer la cinétique de précipitation et de durcissement. L'addition d'Ag et de Zn augmente également la cinétique de précipitation mais dans une moindre mesure. L'addition de Mg change la séquence de précipitation tout au long de la séquence de vieillissement. La différence principale liée à la présence de Mg pour les premiers stades de traitement thermique est observée par rapport à la précipitation sur les dislocations. Dans les alliages qui contiennent du Mg, les dislocations sont décorées par des phases précurseur contenant de Cu et Mg. Par contre dans les alliages sans Mg celles-ci sont associés à des zones GP qui évoluent ensuite en précipités θ'. Cette différence est attribuée à la germination favorable de T1 sur les phases précurseur de Cu/Mg dans les alliages contenant du Mg, et par la saturation des sites de germination hétérogène par θ' dans les alliages sans Mg.L'augmentation de dureté associée à l'addition d'Ag et Zn est attribuée à une fraction volumique plus élevé de la phase T1. Ag est ségrège à l'interface entre T1 et la matrice et Zn est incorporé dans la structure de T1. Ces résultats suggèrent que les additions de Zn et Ag stimulent la formation de T1.L'influence de la concentration en éléments d'addition mineurs a été caractérisée par une approche résolue en temps et en espace, sur les matériaux contenant un gradient en composition. Cela révèle que l'effet de l'addition de Mg sur la précipitation se produit à une valeur seuil de ~0.1% en poids, suggérant que cela est la concentration nécessaire pour germer des phases précurseur sur les dislocations dans les premiers stades de la précipitation. / Al-Cu-Li alloys are very attractive for aerospace applications alloys due to their low density, high modulus and high strength. They are experiencing a strong interest since the so-called 3rd generation alloys, with relatively high Cu and low Li content, have been developed with high toughness, fatigue resistance and thermal stability. The main precipitating phase in these alloys is the T1-phase which precipitates on {111}Al-planes with a hexagonal structure. It is known that obtaining a fine dispersion of T1, and hence a high strength requires the presence of dislocations as nucleation sites. In addition, commercial Al-Cu-Li alloys contain several minor alloying elements such as Mg, Ag and Zn, which help reaching the desired properties. Although the effect of these minor additions on precipitation of T1 has been characterized, it has not been understood yet.In this context the aim of this thesis is to systematically investigate the effect of minor additions of Mg, Ag and Zn on precipitation nucleation, precipitation kinetics and related strengthening, and to use a detailed characterization of the microstructure to understand the mechanisms by which the modifications induced by these minor additions take place. In-situ Small-Angle X-ray Scattering and Differential Scanning Calorimetry provide the precipitation kinetics and sequence, respectively. Transmission Electron Microscopy, both in conventional mode, atomically-resolved and in chemical mapping mode, reveals the structure and distribution of phases. Hardness gives access to the strengthening. Compositionally gradient materials are fabricated and characterized to evaluate the effect of alloy composition on precipitation and strengthening.The results reveal that Mg is most effective in order to enhance precipitation kinetics and hardening. Additional Ag and Zn further enhance precipitation kinetics but to a lower extent. The addition of Mg changes the precipitation sequence at all times of ageing. The main differences in early aging conditions are observed with respect to precipitation on dislocations. In Mg-containing alloys, dislocations are decorated by Cu-Mg precursor phases, whereas dislocations in Mg-free alloys are mainly associated to GP-zones which evolve subsequently into θ'-phase. In fully precipitated conditions the microstructure of Mg-containing alloys is dominated by the T1 phase, whereas that of Mg-free alloys is dominated by the θ'-phase. This difference is attributed to the favourable nucleation of T1 on Mg-Cu precursor phases in the Mg-containing alloys, and to the consumption of T1-heterogeneous nucleation sites by the θ'-phase in the Mg-free alloys.The increase of hardness associated to the addition of Ag and Zn is associated to a higher volume fraction of the T1-phase. Ag was found to segregate at the T1/matrix interface and Zn was incorporated into the T1-phase, so that it is assumed that their additions stimulate the formation of T1.The influence of the concentration of the minor solute additions has been characterised by combined space and time-resolved experiments on compositionally gradient materials. It reveals that the effect of an Mg addition on precipitation occurs at a threshold level of ~0.1wt%, suggesting that this concentration is that necessary to form the precursor phase at the dislocations during early ageing.

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The problem of inaccurate measurement techniques for quantifying isothermal solidification kinetics during transient liquid phase (TLP) bonding in binary and ternary systems; and resulting uncertainty in the accuracy of analytical and numerical models has been addressed by the development of a new technique using differential scanning calorimetry (DSC). This has enabled characterization of the process kinetics in binary and ternary solid/liquid diffusion couples resulting in advancement of the fundamental theoretical understanding of the mechanics of isothermal solidification. The progress of isothermal solidification was determined by measuring the fraction of liquid remaining after an isothermal hold period of varying length. A 'TLP half sample', or a solid/liquid diffusion couple was setup in the sample crucible of a DSC enabling measurement of the heat flow relative to a reference crucible containing a mass of base metal. A comparison of the endotherm from melting of an interlayer with the exotherm from solidification of the residual liquid gives the fraction of liquid remaining. The Ag-Cu and Ag-Au-Cu systems were employed in this study. Metallurgical techniques were used to compliment the DSC results. The effects of sample geometry on the DSC trace have been characterized. The initial interlayer composition, the heating rate, the reference crucible contents, and the base metal coating must be considered in development of the experimental parameters. Furthermore, the effects of heat conduction into the base metal, baseline shift across the initial melting endotherm, and the exclusion of primary solidification upon cooling combine to systematically reduce the measured fraction of liquid remaining. These effects have been quantified using a modified temperature program, and corrected using a universal factor. A comparison of the experimental results with the predictions of various analytical solutions for isothermal solidification reveals that the moving interface solution can accurately predict the interface kinetics given accurate diffusion data. The DSC method has been used to quantify the process kinetics of isothermal solidification in a ternary alloy system, with results compared to a finite difference model for interface motion. The DSC results show a linear relationship between the interface position and the square root of the isothermal hold time. While the numerical simulations do not agree well with the experimental interface kinetics due to a lack of accurate thermodynamic data, the model does help develop an understanding of the isothermal solidification mechanics. Compositional shift at the solid/liquid interface has been measured experimentally and compared with predictions. The results show that the direction of tie-line shift can be predicted using numerical techniques. Furthermore, tie-line shift has been observed in the DSC results. This study has shown that DSC is an accurate and valuable tool in the development of parameters for processes employing isothermal solidification, such as TLP bonding.

The problem of inaccurate measurement techniques for quantifying isothermal solidification kinetics during transient liquid phase (TLP) bonding in binary and ternary systems; and resulting uncertainty in the accuracy of analytical and numerical models has been addressed by the development of a new technique using differential scanning calorimetry (DSC). This has enabled characterization of the process kinetics in binary and ternary solid/liquid diffusion couples resulting in advancement of the fundamental theoretical understanding of the mechanics of isothermal solidification. The progress of isothermal solidification was determined by measuring the fraction of liquid remaining after an isothermal hold period of varying length. A 'TLP half sample', or a solid/liquid diffusion couple was setup in the sample crucible of a DSC enabling measurement of the heat flow relative to a reference crucible containing a mass of base metal. A comparison of the endotherm from melting of an interlayer with the exotherm from solidification of the residual liquid gives the fraction of liquid remaining. The Ag-Cu and Ag-Au-Cu systems were employed in this study. Metallurgical techniques were used to compliment the DSC results. The effects of sample geometry on the DSC trace have been characterized. The initial interlayer composition, the heating rate, the reference crucible contents, and the base metal coating must be considered in development of the experimental parameters. Furthermore, the effects of heat conduction into the base metal, baseline shift across the initial melting endotherm, and the exclusion of primary solidification upon cooling combine to systematically reduce the measured fraction of liquid remaining. These effects have been quantified using a modified temperature program, and corrected using a universal factor. A comparison of the experimental results with the predictions of various analytical solutions for isothermal solidification reveals that the moving interface solution can accurately predict the interface kinetics given accurate diffusion data. The DSC method has been used to quantify the process kinetics of isothermal solidification in a ternary alloy system, with results compared to a finite difference model for interface motion. The DSC results show a linear relationship between the interface position and the square root of the isothermal hold time. While the numerical simulations do not agree well with the experimental interface kinetics due to a lack of accurate thermodynamic data, the model does help develop an understanding of the isothermal solidification mechanics. Compositional shift at the solid/liquid interface has been measured experimentally and compared with predictions. The results show that the direction of tie-line shift can be predicted using numerical techniques. Furthermore, tie-line shift has been observed in the DSC results. This study has shown that DSC is an accurate and valuable tool in the development of parameters for processes employing isothermal solidification, such as TLP bonding.

The A15-based Nb3Sn and V3Ga superconducting compounds are an integral part of synchrotrons and magnetic fusion reactor technology, especially where a magnetic field higher than 10 T is required, which lies beyond the limit of conventional Nb-Ti superconductors (~8 T). These brittle intermetallic compounds are difficult to manufacture in the form of wires, required for the application purpose, using the traditional wire-drawing process. Hence, bronze technique is adopted to fabricate such filamentary wires. This is based on the solid-state diffusion where A3B compound (A=Nb or V, B=Sn or Ga) forms during the interaction of Cu(B) and A. The operation of pure superconducting wires gets restricted to the field of 12 T, however, the ever-increasing demands for an improved efficiency have promoted the development of these A15 wires with the addition of alloying elements such as Ti and Zr. Many important physical and mechanical properties of such wires depend on the growth behaviour of these compounds. Therefore, understanding the growth of such compounds necessitates an in-depth analysis on diffusion behaviour of various elements in both bronze-based solid solutions as well as A15-intermetallics. Estimation of diffusion parameters makes use of the most commonly used diffusion couple technique. There are mainly three methods available for the estimation of the interdiffusion coefficients, proposed by Matano-Boltzmann (MB), Den Broeder (dB), same as Sauer-Freise (SF) and Wagner. Among these three, MB treatment is known to be the least accurate method, especially when there is a deviation of molar volume in a system from the ideality. At the same time molar volume might affect the estimation process differently for dB and Wagner’s approach. MB method is still being used neglecting the actual molar volume variation. On the other hand, the implementation of dB or Wagner’s approach for the estimation remains to be random. For the first time, we have critically examined the role of molar volume on estimated diffusion parameters and indicated the more accurate approach. Similar analysis for the estimation of the intrinsic diffusion coefficient is conducted considering Heumann and van Loo’s methods. Furthermore, the discussion is extended to the estimations of various diffusion parameters considering the measured composition profile in the V-Ga system. A detailed diffusion study has been conducted on Cu(Ga) and Cu(Sn) solid solutions to examine the role of the vacancy wind effect on interdiffusion. The interdiffusion, intrinsic and impurity diffusion coefficients are determined to facilitate the discussion. It is found that Ga and Sn are the faster diffusing species in the respective systems. The trend of the interdiffusion coefficients is explained with the help of the driving force. Following that, the tracer diffusion coefficients of the species are calculated with and without consideration of the vacancy wind effect. We found that the role of the vacancy wind is negligible on the minor element in a dilute solid solution, which is the faster diffusing species in this system and controls the interdiffusion process. However, consideration of this effect is important to understand the diffusion rate of the major element, which is the slower diffusing species in this system. Major drawback of studying diffusion in multi-component systems is the lack of suitable techniques to estimate the diffusion parameters. In this study, a generalized treatment to determine the intrinsic diffusion coefficients in multi-component systems is developed utilizing the concept of pseudo-binary approach. This is explained with the help of experimentally developed diffusion profile in the Cu(Sn, Ga) solid solution. Based on an interdiffusion study using an incremental diffusion couple in the V-Ga binary system, we have shown that V diffuses via lattice, whereas Ga does so via grain boundaries for the growth of the V3Ga phase. We could estimate the contributions from two different mechanisms, which are, usually, difficult to delineate in an interdiffusion study. Available tracer diffusion studies and the atomic arrangement in the crystal structure have been considered for a discussion on the diffusion mechanisms. Diffusion–controlled growth rate of V3Ga at the Cu(Ga)/V changes dramatically because of a small change in Ga content in Cu(Ga). One atomic percent increase in Ga leads to more than double the product phase layer thickness and a significant decrease in activation energy. Kirkendall marker experiment indicates that V3Ga grows because of diffusion of Ga. Role of different factors influencing the diffusion rate of Ga and high growth rate of V3Ga are discussed. The growth of Nb3Sn by bronze technique on two different single crystals and deformed Nb is studied. The grain boundary diffusion-controlled growth rate is found to be different for each of these three specimens. The difference is explained on the basis of the grain size of Nb3Sn. Elemental additions such as Ti and Zr to either bronze or metal are found to improve the superconducting properties. We have examined their effects on the growth rates of A15-phase formed in Cu(B,x)/A and Cu(B)/(A,x), where x is Ti or Zr. In either cases Ti and Zr-additions result in an improved growth rate of the product phase and reduces activation energy with increase in alloying addition; however few precipitates are formed in the interdiffusion zone for Cu(B,x)/A. Wavelength dispersive spectrometry (WDS)-mapping reveals these to be x-rich. Scanning transmission electron microscopy (STEM)-analysis suggests having composition gradient inside a single precipitate. TEM-diffraction demonstrates these to be Ti(A) solid solution crystallizing as BCC-structure for Cu(B,Ti)/A. These are located on grain boundaries of A15-phase. Electron back-scattered diffraction (EBSD)-analysis demonstrates grain morphology of product phase and found the average grain size to exhibit a decreasing trend with increasing x content. Columnar grains, on Ti and Zr addition tend to form as equiaxed ones. Based on the morphology and grain size pattern, the role of grain boundary diffusion is speculated to have a dominant effect with increase in elemental additions. The texture evolution of the product phase is also investigated and found the product phase to grow as a strongly textured one with the elemental additions. A peculiar pattern is observed for the texture of the product phase and its adjacent A or A(x) grains.

Ce travail de thèse porte sur l’étude de l’influence des paramètres nano et microstructuraux pour l’optimisation des propriétés thermoélectriques des siliciures de magnésium Mg2(Si,Sn) de type -n. Ces matériaux thermoélectriques ont été choisis pour leurs compatibilités avec une utilisation en génération de puissance dans le domaine de l’automobile.Par une approche combinatoire utilisant les couples de diffusion, il est premièrement ré investigué le diagramme de phase pseudo-binaire Mg2Si-Mg2Sn dans le but d’interpréter les microstructures observées. Il est ensuite présenté les effets expérimentaux des paramètres de densification par frittage flash (SPS) sur les microstructures et les propriétés thermoélectriques résultantes. Finalement, il est explicité à l’aide de la modélisation l’influence des paramètres microstructuraux sur les propriétés thermoélectriques de l’alliage optimisé et nanostructuré Mg2Si0,3875Sn0,6Sb0,0125. / This study presents the influence of nano and microstructural parameters to optimize thermoelectric properties of n-type magnesium silicides Mg2(Si,Sn). These thermoelectric materials are chosen for their compatibilities with power generation in automotive.From a combinatorial approach using diffusion couples, it is first reinvestigated the pseudo-binary phase diagram Mg2Si-Mg2Sn to rationalize the observed microstructures. Then the experimental effects of sintering parameters (SPS) on resulting microstructures and thermoelectric properties are presented. Finally, the influence of microstructural parameters on the thermoelectric properties of optimized and nanostructured Mg2Si0,3875Sn0,6Sb0,0125 alloys are explained through modelling.