abstract: Al 7075 alloys are used in a variety of structural applications, such as aircraft wings, automotive components, fuselage, spacecraft, missiles, etc. The mechanical and corrosion behavior of these alloys are dependent on their microstructure and the environment. Therefore, a comprehensive study on microstructural characterization and stress-environment interaction is necessary. Traditionally, 2D techniques have been used to characterize microstructure, which are inaccurate and inadequate since the research has shown that the results obtained in the bulk are different from those obtained on the surface. There now exist several techniques in 3D, which can be used to characterize the microstructure. Al 7075 alloys contain second phase particles which can be classified as Fe-bearing inclusions, Si-bearing inclusions and precipitates. The variation in mechanical and corrosion properties of aluminum alloys has been attributed to the size, shape, distribution, corrosion properties and mechanical behavior of these precipitates and constituent particles. Therefore, in order to understand the performance of Al 7075 alloys, it is critical to investigate the size and distribution of inclusions and precipitates in the alloys along with their mechanical properties, such as Young's modulus, hardness and stress-strain behavior. X-ray tomography and FIB tomography were used to visualize and quantify the microstructure of constituent particles (inclusions) and precipitates, respectively. Microscale mechanical characterization techniques, such as nanoindentation and micropillar compression, were used to obtain mechanical properties of inclusions. Over the years, studies have used surface measurements to understand corrosion behavior of materials. More recently, in situ mechanical testing has become more attractive and advantageous, as it enables visualization and quantification of microstructural changes as a function of time (4D). In this study, in situ X-ray synchrotron tomography was used to study the SCC behavior of Al 7075 alloys in moisture and deionized water. Furthermore, experiments were performed in EXCO solution to study the effect of applied stress on exfoliation behavior in 3D. Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, three dimensional measurements of the crack length led to a much more accurate measurement of crack growth rates. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2015

Doctor of Philosophy / Department of Physics / Daniel Rolles / Imaging the structures of molecules, understanding the molecular dynamics in onization and dissociation processes and, most importantly, observing chemical reactions, i.e. the making and breaking of chemical bonds in real time, have become some of the most exciting topics in the atomic and molecular physics. The rapid advances of experimental tools such as synchrotron radiation light sources, free-electron lasers and continuing advances of tabletop femtosecond ultrashort lasers that provide laser pulses at a variety of wavelengths have opened new avenues for understanding the structure of matter and the dynamics of the chemical interactions. In addition, significant improvements in computational techniques and molecular dynamic simulations have provided complementary theoretical predictions on structures and chemical dynamics. The Coulomb explosion imaging method, which has been developed and applied in many studies in the last three decades, is a powerful way to study molecular structures. The method has mostly been applied to small diatomic molecules and to simple polyatomic molecules. In this thesis, Coulomb explosion imaging is applied to study the structure of isomers, molecules that have the same chemical formula but different chemical structures. Specifically, by taking inner-shell photoionization as well as strong-field ionization approaches to ionize and fragment the molecules and by using coincidence electron-ion-ion momentum imaging techniques to obtain the three-dimensional momentum of fragment ions, structures of isomers are distinguished by using the correlations among product ion momentum vectors. At first, the study aims to understand if the Coulomb explosion imaging of geometrical isomers can identify and separate cis and trans structures. Secondly, in order to extend the application of the Coulomb explosion imaging method to larger organic molecules to test the feasibility of the method for identifying structural isomers, photoionization studiesof 2,6- and 3,5-difluoroiodobenzene have been conducted. In addition, using the full three-dimensional kinematic information of multi-fold coincidence channels, breakup dynamics of both cis/trans geometric isomers and structural isomers, and in particular, sequential fragmentation dynamics of the difluoroiodobenzene isomers are studied. Furthermore, for each study, Coulomb explosion model simulations are conducted to complement the experimental results. The results of the Coulomb explosion imaging reseach in this thesis paves the way for future time-resolved Coulomb explosion imaging experiments aiming to understand the transient molecular dynamics such as photoinduced ring opening reactions and cis/trans isomerization processes in gas-phase molecules.

Magnetic frustration is the underpinning theme to all of the magnetic oxide systems explored in this dissertation. The materials studied in this thesis belong to two topical families of interest in modern condensed matter physics, namely, the rare-earth titanates R2Ti2O7 and the double perovskites A2BB'O6. Chapter 1 provides the theoretical background necessary to understand the crystalline systems studied in this thesis. Chapter 2 explains the necessity of utilizing neutron scattering and x-ray experiments to tease out the key signatures which were essential to formulating the conclusions made in each study. Chapter 3 outlines the neutron scattering techniques which were employed to investigate the crystal systems. The first objective of this thesis is to understand effect of “stuffing” on the ground state anisotropy of the quantum spin liquid candidate Yb2Ti2O7 via an investigation of the crystal-field excitations in intentionally stuffed samples. The pentultimate study was performed on the monoclinic crystal systems, La2LiRuO6 and La2LiOsO6, to discern the effect of lattice distortions on the spin-orbit induced magnetic ground state of 4d3 and 5d3 double perovskites based on Ru and Os magnetic ions. The final investigation involves an inelastic neutron scattering investigation of magnetic ground states in three d2 double perovskites, Ba2CaOsO6, Ba2MgOsO6 and Ba2ZnOsO6. Here, we make the case for novel octupolar order below their respective transition temperatures T* of 50 K, 49 K and 30 K based on information provided by neutron scattering, heat capacity, muon spin relaxation and synchrotron x-ray diffraction studies. / Thesis / Doctor of Philosophy (PhD)

Multiferroic materials with a coupled ordering of electric and magnetic moments could be used to build energy-efficient, magnetic computer memory that is written with an electrical field. To understand the interaction between the magnetic and electric ordering in such materials, two examples, namely yttrium manganate YMn2O5 and neodymium iron borate NdFe3(BO3)4, are studied by means of resonant x-ray diffraction. The important role of a pure electronic contribution to the ferroelectric polarization is shown in YMn2O5. Furthermore, substitution of Fe can change the magnetic order of YMn2O5 from antiferromagnetic into ferrimagnetic, allowing the storage of easily readable magnetic information. Therefore the change of the magnetic structure upon small Fe substitution is studied. Although most of the magnetic structure of the parent compound is kept, the Fe moments have larger components along the c-direction. In NdFe3(BO3)4 the microscopic origin of the magnetoelectric coupling is addressed as the consequence of the frustration of the Fe and Nd magnetic sublattices. The application of an electrical field shifts the balance from the helical to the collinear magnetic domains, revealing again the strong magnetoelectric coupling. / Multiferroische Materialien mit einer starken magnetoelektrischen Kopplung können als energieeffizienter, magnetischer Speicher benutzt werden, welcher mit einem elektrischen Feld geschrieben wird. Um die Wechselwirkung der elektrischen mit der magnetischen Ordnung in solchen Materialien zu verstehen, werden hier zwei Beispiele, nämlich Yttriummanganat YMn2O5 und Neodymeisenborat NdFe3(BO3)4, mit resonanter Röntgenbeugung untersucht. In YMn2O5 wird die wichtige Rolle eines rein elektronischen Beitrags zur ferroelektrischen Polarisation gezeigt. Um die magnetische Struktur von YMn2O5 von antiferromagnetisch zu ferrimagnetisch zu verändern, kann Fe substituiert werden. Dies ermöglicht es, leicht zu lesende, magnetische Informationen zu speichern. Daher wurde die Änderung der magnetischen Struktur bei leichter Fe Substituierung untersucht. Auch wenn die magnetische Struktur von Fe im wesentlichen der magnetischen Struktur von Mn folgt, haben die Fe Momente größere Komponenten entlang der c-Richtung. In NdFe3(BO3)4 wird der Ursprung der starken magnetischen Kopplung als Folge der Frustration des Nd und Fe Untergitters erklärt. Das Anlegen eines elektrischen Feldes führt zur Verschiebung des Gleichgewichts von den helikalen zu den kollinearen magnetischen Domänen, welches wieder die starke magnetoelektrische Kopplung veranschaulicht.