The work was suggested by Prof, G. I, Finch and carried out under his supervision in his laboratory at the Imperial College of Science., London, It was undertaken to study the structure of the first thin layers of electrodeposited metals and their properties by means of electron diffraction with a view to obtaining some light on the mechanism of electrodeposition. The thesis is divided into the following parts: lo An electrochemical method for making transmission films for electron diffraction is described, Transmission-films of 14 metals were made, namely, Cu, Ag, Au, Fe, Co, Ni, As, Sb, Bi, Pt, Sn, Cr. W and Se, 2, The lattice dimensions of platinum electrodeposited on silver, gold and zinc on coppers nickel and chromium on gold, iron on nickel, copper and nickel on platinum and copper and iron on palladium were found to be normal, whilst platinum displaced by copper and silver formed alloys with the substrates respectively. 3. Deposits of iron, nickel, coppers brass, gold, tin and platinum show preferred orientations. 4, Double scattering of electrons occurred with composite films of copper and nickel on platinum respectively and copper on palladium. 5. The effect of substrates on the structure of the deposits was studied with copper, gold, iron, nickel, tin, platinum, silvers palladium, zinc, brass, bismuth, arsenic and chromium. The grain size of chromium films was found to have changed with the substrates. The orientation relation for the crystlline substrates studied show that the correspondingly dense planes of the deposit and the substrate are parallel. An amorphous arsenic substrate, however, exerted no effect on the orientation of a deposit, A zinc film was found to have dissolved in the polished layer of a copper surface. 6, Nickel, iron, copper and gold films were anodically oxidised and electron diffraction patterns for passive nickel, iron and gold were obtained.

No description available.

No description available.

No description available.

Nb-Si-Cr-Al-Mo base in situ composites with nominal compositions Nb-18Si-5Al-5Cr-5Mo (JG1), Nb-24Ti-18Si-5Al-5Cr-5Mo (JG2), Nb-24Ti-18Si-5Al-5Cr-2Mo (JG3), Nb-24Ti-18Si-5Al- 5Cr-2Mo-5Hf (JG4) and Nb-24Ti-18Si-5Al-5Cr-2Mo-5Hf-5Sn (JG6) were prepared by arc-melting followed by heat treatment. The effects of Mo, Ti, Hf and Sn additions on phase selection, phase transformations, microstructure and oxidation behaviour in the as cast and heat-treated in situ composites were investigated using XRD, SEM, and EPMA. All the alloys are classified as hyper-eutectic alloys since the betaNb5Si3 phase was formed in all the as cast microstructures. After heat treatment, the betaNb5Si3 partly transformed to the alphaNb5Si3 in the presence of Ti. The addition of Ti stabilised the betaNb5Si3 to lower temperatures and increased the solubilities of A1 and Cr in the Nb88. The lattice parameter of the Nb88 increased after heat treatment. This is attributed to the redistribution of solutes between the Nbss and the intermetallic phases and in particular the reduction in Si content. The NbaSi phase was formed in the as cast Hf-free alloys (JGl, JG2 and JG3). However, this phase was unstable at 1500°C and decomposed by a eutectoid transformation to Nb88 and alphaNb5Si3. The eutectoid decomposition was enhanced considerably by the Ti addition. The formation of Nb3Si was suppressed by the Hf addition. In the presence of Ti and Hf, there were Ti and Hf segregations in the Nb5Si3 phase. The Ti solubility in the Hf-rich Nb5Si3 was higher than in the Nb5Si3. The structure of the Nb5Si3 phase was mainly affected by the Hf addition, which prompted the formation of the alphaNb5Si3. The Hf-rich regions in the 5-3 silicide probably corresponded to the alphaNb5Si3, which was stable up to 1500° C in the presence of Hf. The alloying elements Hf and Sn preferentially substituted for Nb in the Nb5Si3 and Nbss, respectively. The addition of Sn had a significant effect on the Nbss, essentially leading to the formation of Sn-rich and Sn-poor parts in the solid solution in the as cast microstructure. In the presence of Sn, the Si solubility in the Nbss increased considerably whilst the Cr solubility decreased. The Sn addition promoted the formation of the Si-rich 014 Laves phase and stabilised this phase at 1300°C. The solubility range of Si in the C14 Laves phase formed in alloys of this study was ~ 6.6 to 10.5at.%. For the Ti containing alloys, titanium nitride formed near the surface of the specimen during heat treatment. Alloy Nb-18Si-15Cr (JG5) was studied to clarify the disputed three-phase fields (Nbss-1-0144- alphaNb5Si3 or Nbss-CrNbSi-l-alphaNb5Si3) near the Nb corner. The C14 Laves formed in both the as cast and heat treated samples. The results confirmed that the C14 Laves phase in the binary Nb-Cr system was stabilised by Si to lower temperatures.

No description available.

In this work the constitutive behaviour and the influence of casting defects on the failure of Ni-base single crystal (SC) superalloy components is investigated. It is well known that the presence of casting-related porosities can lead to the nucleation of microcracks under both creep and fatigue conditions and thus, ultimately produce component failure. A rate dependent crystallographic formulation is introduced to describe the inelastic deformation behaviour of the latest generation of single crystal superalloys. The evolution of the current dislocation and obstacle network is described through appropriate slip resistance and internal or back stress variables for each slip system. Good correlations are obtained between experimental data and numerical predictions within the 750◦C .- 950◦C temperature range and for < 001 > and < 111 > crystallographic orientations. The formulation is then numerically implemented into the FE method and used to investigate the deformation of a representative material volume containing a spherical void of approximately 20 micrometers diameter. The functional dependence of the void growth rates in terms of material anisotropy, stress state, temperature and interaction with a free surface is determined. It is shown that the rate of growth of casting defects in an infinite single crystal medium is strongly dependent on the applied triaxiality and relative orientation between the crystallographic axes and the applied stresses. Furthermore, it has been found that, for the acceleration of the defect growth rate as the result of its proximity to a free-surface to be non-negligible, the void needs to be within two diameters of the free surface. Based on the above results, a framework is proposed to describe the growth of embedded casting defects within superalloy single crystals under a given applied multiaxial stress state. The framework provides an explicit link between the mesoscopic (at the level of the voids) and the macroscopic length scales. A number of blunt notch bar creep specimens and thermo-mechanical cyclic stress-strain specimens were tested to validate both the rate dependent crystallographic formulation and the micro-mechanics void growth model. In addition microstructural analysis of the blunt notch bars provided necessary data for the development of a micro-crack initiation criterion. A life assessment methodology combining these models is developed and applied in the analysis of an actual gas turbine blade. It is expected that the understanding of defect growth kinetics and the corresponding damage accumulation will be beneficial in the design and life prediction of superalloy components.

La présente étude s’inscrit dans le cadre d’un projet de recherche portant sur le soudage par friction malaxage (Friction Stir Welding, FSW) d’une configuration bimétallique d’un alliage d’Aluminium 6061-T6 (Al) et d’un alliage de Magnésium AZ31B (Mg). Le but de cette investigation est d’optimiser les paramètres de soudage FSW pour l’amélioration des propriétés mécaniques des structures soudées. Le travail préliminaire a ciblé la détermination des propriétés microstructurales et mécaniques des deux matériaux de base utilisés. A ce niveau, l’influence de la température et de la vitesse de déformation a été analysée via des essais de traction monotone. Une étude paramétrique a permis un couplage entre le procédé de soudage FSW et la qualité des joints soudés en termes de microstructure et de propriétés mécaniques. Ainsi, les La morphologie des joints soudés a été mise en évidence en vue de mieux comprendre le flux de matière lors du procédé FSW d’un multi matériau. De plus, des observations fines microstructurales ont permis de mettre en évidence les évolutions de la taille des grains, de la micro dureté et de la texture des différentes zones définissant un joint soudé. Une analyse micrographique des faciès de rupture, réalisée à l’aide d’un microscope électronique à balayage (MEB), a contribué à mieux cerner les mécanismes d’endommagement. / This study is part of a research project initiated by Texas A&M University in Qatar and the American University of Beirut on characterizing the mechanical and microstructural properties of friction sit welded bimetallic joints. The current researched work focused on two of the materials proposed in the initial project, namely, the 6061-T6 aluminum alloy (Al) and an AZ31B magnesium alloy (Mg). Therefore, the purpose of this investigation is to optimize the friction stir welding parameters of similar and dissimilar materials in the aim to improve the mechanical properties of welded structures. The preliminary work focused on the investigation of the microstructural and mechanical properties of the two studied base materials. In what followed, the properties of the welded parts were studied by analyzing the influence of the temperature and the strain rate via monotonous tensile tests and by analyzing the microstructural and textural properties.

Thesis (M.Sc. (Physics)) --University of Limpopo, 2008 / We present first principles structural, electronic and optical properties investigation of PdS, which are carried out using density functional theory under plane wave pseudopotential method within the local density approximation. We used ultrasoft- pseudopotentials to carry out our calculations. Calculated lattice parameters of the system show excellent agreement with the experimental values. The lattice parameters were observed to decrease linearly with increasing pressure. The density of states and optical properties of PdS have been computed under hydrostatic pressure. The actual size of the band gap remains constant with increasing pressure, whilst the peaks just below and above the Fermi energy moves to the left and to the right respectively. We also investigated the effect of compositional variation on our reflectance by calculating the reflectivity of Pd4-xPtxS4 and Pd4-xNixS4. Since we have different positions for the same concentration, we used the heats of formation to determine the most stable structures and these structures were used to study the effect of compositional variation on our reflectance spectrum. We studied the equation of state (EOS), structure under hydrostatic pressure, and deduced the bulk modulus. It is important to study these properties under such extreme conditions of pressure and temperature as they tend to occur below the earth's surface. Investigation of stability and mechanical properties of binary and ternary compounds from PtS to PdS have been carried out, were the presence of the miscibility gap is still uncertain. We investigate stability of these compounds by studying the heats of formation, elasticity and electronic properties. Our results show no miscibility gap but continuum solid solution between these compounds. A shift of the Fermi energy towards the conduction band is observed at a 50% concentration of Pd and Pt. All the information obtained on PdS is intended to assist in fitting interatomic potentials to enable studies of systems with many atoms.

Dans les alliages multiphasés, la cohérence des interfaces entre des phases en désaccord paramétrique génère des champs élastiques internes à longue distance et généralement anisotropes. L'interaction de ces champs affecte fortement la cinétique des transformations de phase diffusives, et influence la forme et l'arrangement spatial des précipités. Dans la microstructure des superalliages monocristallins à base de nickel, obtenue par précipitation de la phase γ’ ordonnée L12 dans la matrice CFC γ, l'élasticité conduit à la formation d'alignements quasi-périodiques des précipités γ’ cuboïdaux. La microstructure γ/ γ’ possède cependant des défauts systématiques d'alignement des précipités: des branches, des macro-dislocations et des motifs en chevrons. Nous nous intéressons à l'origine de ces défauts d'alignement. Nous conduisons des analyses de stabilité de l'arrangement périodique de précipités en interactions élastiques. Contrairement à la stabilité attendue, les calculs semi-analytiques ont révélé l'instabilité de la distribution périodique de précipités γ’ cubiques, vis-à-vis de certains modes de perturbation. Les principales instabilités sont le mode longitudinal [100] et le mode transverse [110], et leur domaine d'instabilité est analysé vis-à-vis de l'anisotropie élastique. Le développement de ces modes instables est étudié par une méthode de champ de phase classique, en simulant l'évolution de microstructures périodiques soumises à des légères perturbations initiales. Nous montrons que l'expression des instabilités d'arrangement procède essentiellement par l'évolution de la forme des précipités, et conduit à la formation de motifs qui ont pu être reliés à des microstructures expérimentales. En particulier, le mode transverse [110] conduit à la formation de motifs en chevrons. Nous étudions l'influence du taux de phase γ’ et de l'inhomogénéité du module élastique C’, et nous montrons le rôle qu'ils jouent dans la stabilisation de l'arrangement périodique. Dans des simulations réalisées dans des études antérieures, la dynamique des défauts est analysée au moyen de paramètres topologiques issus de la phénoménologie des structures hors-équilibre. Au cours d'un recuit isotherme, nous observons que les branches et les macro-dislocations migrent dans la microstructure selon des mécanismes de montée et de glissement. Nous utilisons ensuite une nouvelle formulation des modèles de champ de phase, intrinsèquement discrète, dans laquelle les interfaces sont résolues essentiellement avec un pas de grille sans friction de réseau et avec une invariance par rotation précise. Cette approche, appelée Sharp Phase Field Method (S-PFM), est implémentée sur une grille CFC, et avec une description des quatre variants de translation des précipités γ’. Nous montrons que la S-PFM permet la modélisation de microstructures à grande échelle, avec plusieurs milliers de précipités à deux et trois dimensions, et donne ainsi accès à des informations statistiques sur l'évolution de la microstructure et sur la dynamique des défauts d'alignement. Nous discutons finalement la perspective de modéliser l'évolution de la microstructure γ/γ’ à une échelle supérieure par une description de la dynamique des défauts d'alignement des précipités. / In multiphase alloys, internal elastic fields often arise as a result of a coherently adjusted misfit between the lattices of coexisting phases. Given their long-range and usually anisotropic nature, the interaction of these fields is known to significantly alter the kinetics of diffusion-controlled phase transformations, as well as influence the shapes and spatial arrangement of the misfitting precipitates. In the microstructure of single-crystal nickel-base superalloys, obtained by precipitation of the L12-ordered γ’ phase in the FCC γ matrix, elasticity leads to the formation of nearly periodic alignments of the cuboidal γ’ precipitates. However, the γ/γ’ microstructure systematically displays defects in the precipitate alignment: branches, macro-dislocations and chevron patterns. We first address the question of the origin of these alignment defects. Stability analyses of the periodic arrangement of elastically interacting precipitates are carried out. Contrary to the expected stability, the semi-analytical calculations revealed the periodic distribution of cubic γ‘ precipitates to be unstable against specific perturbation modes. The main instabilities are the [100] longitudinal mode and the [110] transverse mode, and their instability range is analyzed with respect to the elastic anisotropy. The consequences of these unstable modes are investigated using a classic phase field method, by modeling the evolution of periodic microstructures undergoing small initial perturbations. We show the expression of the instabilities mainly proceeds by the evolution of the precipitate shapes, and leads to the formation of patterns which were related to experimental microstructures. Specifically, the [110] transverse instability is responsible for the formation of chevron patterns. The effects of the volume fraction and of an inhomogeneity on the C’ shear modulus on the stability of the arrangement are studied, and we show the role they play in the partial stabilization of the periodic distribution, though the [100] longitudinal mode always remains unstable. In phase field calculations carried out in previous studies, the dynamics of alignment defects are analyzed by means of topological parameters derived from pattern formation theory. During annealing, branches and macro-dislocations were observed to migrate in the microstructure according to climbing and gliding mechanisms. We then use a new formulation of phase field models, intrinsically discrete, in which the interfaces are resolved with essentially one grid point with no pinning on the grid and an accurate rotational invariance. This approach, known as the Sharp Phase Field Method (S-PFM), is implemented on a FCC grid and accounts for the four translational variants of the γ’ precipitates. We show that the S-PFM allows for the modeling of large-scale microstructures, with several thousand precipitates both in two and three dimensions, and provides access to statistical information on the microstructure evolution and on the the dynamics of alignment defects. We finally discuss the perspective of modeling the evolution of the γ/γ’ microstructure at the macroscale by means of a description of the defect dynamics in the precipitate alignments.