The conventional approach for x-ray radiography is absorption contrast. In recent years a new approach that eliminates the usual requirement for absorption and allows the visualization of phase based on refractive index features in a material has been demonstrated. This so-called “phase contrast imaging” has now been applied using a wide range of radiation and samples. In this work we are motivated by the need to find optimal conditions for achieving high quality phase contrast images. We consider image formation using the free space propagation of x-rays from a point source passing through a sample. This imaging model is a lens-less configuration and, as such, is very useful for x-ray wavelengths where lenses are difficult to fabricate. Although no lenses are used, image magnification is still achieved due to the expansion of the wavefront as it propagates from the point source illumination. The short wavelength and penetrating power of x-rays make them ideal for non-destructive studies of microscopic samples. However, these techniques are also important for investigating larger, non-microscopic samples.

Universiẗat-Gesamthochsch., Diss.--Paderborn, 1997.

Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographical references (p. 83-85).

Ulm, Univ., Diss., 2007.

Th.--Sci. phys.--Grenoble--I.N.P., 1980. N°: 6.

Ces travaux de thèse, réalisés dans le cadre du programme de recherche ASZTech, portent sur l’étude de deux réfractaires électrofondus : un matériau Alumine-Zircone-Silice (AZS) et un produit à Très Haute Teneur en Zircone (THTZ), destinés à l’industrie verrière. La zircone étant non stabilisée, ces réfractaires subissent donc, lors de l’étape de recuisson (refroidissement), la transformation quadratique (Q) à monoclinique (M) de la zircone qui, de par sa forte expansion volumique, est une source d’endommagement. L’objectif de cette étude a consisté à caractériser, au cours de traitements thermiques représentatifs, le comportement thermomécanique de ces matériaux et à établir des corrélations avec les évolutions microstructurales rencontrées. En étudiant leurs comportements sous sollicitations thermiques par des techniques acoustiques, l’impact de la transformation Q-M de la zircone sur le développement de l’endommagement a été mis en évidence. En s’intéressant particulièrement au comportement mécanique au passage de cette transformation, la présence d’un phénomène de plasticité de transformation (TRIP) a été confirmée. Outre le phénomène de TRIP qui participe à la relaxation des contraintes au passage de la transformation de phase, un phénomène d’endommagement diffus, offrant également une capacité d’accommodation, a été identifié pour le matériau THTZ. Au final, ces données ont permis d’améliorer le modèle de simulation numérique décrivant l’étape de recuisson de blocs industriels. En outre, la robustesse de ce modèle pourra maintenant être validée par confrontation à des mesures expérimentales de contraintes résiduelles réalisées au cours de cette étude. / This thesis, carried out under the research program ASZTech, focuses on the study of two fused cast refractories: an Alumina-Zirconia-Silica (AZS) product and a product with very High content of Zirconia (HZ), used in glass industry. These refractories containing non-stabilized zirconia are therefore subjected, during the annealing step (cooling), to the tetragonal (T) to the monoclinic (M) phase transition of zirconia associated with a very high volumetric expansion which is a source of stresses. This study aims at characterizing, during representative thermal treatments, the thermo-mechanical behavior of these materials, in correlation with the detected microstructural evolutions. Thanks to the analysis of their behavior under thermal stresses by acoustic techniques, the effect of the T-M transition of zirconia on the development of damage has been highlighted. In particular, the occurrence of a TRansformation Induced Plasticity (TRIP) phenomenon during the zirconia phase transition has been identified and characterized thanks to cooling tests under mechanical stress. In addition to this TRIP phenomenon which allows to relax the mechanical stresses induced during the phase transition, a diffuse damage phenomenon has been also identified for HZ product. Eventually, these data have led to an improvement of the numerical model which simulates the annealing step of industrial blocks. Moreover, the reliability of this model will be now checked by comparison to experimental measurements of residual stresses performed during this study.

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The current understanding of the atomic scale phenomenon associated with the influence of beta phase instabilities on the evolution of microstructure in titanium alloys is limited due to their complex nature. Such beta phase instabilities include phase separation and precipitation of nano-scale omega and alpha phases in the beta matrix. The initial part of the present study focuses on omega precipitation within the beta matrix of model binary titanium molybdenum (Ti-Mo) alloys. Direct atomic scale observation of pre-transition omega-like embryos in quenched alloys, using aberration-corrected high resolution scanning transmission electron microscopy and atom probe tomography (APT) was compared and contrasted with the results of first principles computations performed using the Vienna ab initio simulation package (VASP) to present a novel mechanism of these special class of phase transformation. Thereafter the beta phase separation and subsequent alpha phase nucleation in a Ti-Mo-Al ternary alloy was investigated by coupling in-situ high energy synchrotron x-ray diffraction with ex-situ characterization studies performed using aberration corrected transmission electron microscopy and APT to develop a deeper understanding of the mechanism of transformation. Subsequently the formation of the omega phase in the presence of simultaneous development of compositional phase separation within the beta matrix phase of a Ti-10V-6Cu (wt%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. The results of these investigations provided novel insights into the mechanisms of solid-state transformations in metallic systems by capturing the earliest stages of nucleation at atomic to near atomic spatial and compositional resolution.