Universität Stuttgart
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Item Open Access Thermodynamics and kinetics of the oxidation of amorphous Al-Zr alloys(Stuttgart : Max-Planck-Institut für Intelligente Systeme (ehemals Max-Planck-Institut für Metallforschung), 2015) Weller, Katharina; Mittemeijer, Eric J. (Prof. Dr. Ir.)The present thesis presents a comprehensive investigation of the thermal oxidation of amorphous Al-Zr alloys. The oxide composition and microstructure, as well as the oxidation kinetics and oxidation mechanism of amorphous Al-Zr alloys upon thermal oxidation at relatively low oxidation temperatures of 350 - 400 °C and at high oxidation temperatures of 500 - 560 °C have been investigated. The phase and microstructural development upon oxidation of amorphous Al-Zr alloys, as well as the oxidation-induced changes in the alloy substrate, were investigated by a combinatorial experimental approach using X-ray diffraction (XRD), cross-sectional (analytical) transmission electron microscopy (TEM), Auger electron spectroscopy (AES) sputter-depth profiling and spectroscopic ellipsometry (SE). Furthermore, the corrosion behavior of as-deposited and oxidation-treated amorphous Al-Zr alloys have been investigated as function of the alloy composition and oxidation conditions by employing an electrochemical microcapillary technique.Item Open Access Kinetics of phase transformations(2015) Rheingans, Bastian; Mittemeijer, Eric Jan (Prof. Dr. Ir.)In this thesis, the kinetics of heterogeneous solid-state phase transformations in different prototype experimental systems are investigated with focus on the development of new strategies for kinetic modelling using mean-field kinetic models. Topics cover the interrelation between the kinetic model description and the amount of available experimental information, the interpretation of kinetic model parameters determined upon model fitting and the coupling of kinetic models to external (thermodynamic) input data. Experimental studies include the crystallisation kinetics of metallic glasses and precipitation kinetics in supersaturated alloys.Item Open Access Formation of lath martensite(2015) Löwy, Sarah; Mittemeijer, Eric Jan (Prof. Dr. Ir.)In this thesis the formation of different lath martensites was investigated upon cooling, particularly with regard to the mechanisms contributing to the transformation process. Upon very slow cooling of different Fe-Ni alloys and a maraging steel, all forming lath martensite, a discontinuous transformation behaviour was observed. This modulation of the transformation rate is ascribed to the interplay of chemical driving force, developing strain energy and its relaxation upon slow cooling. It is proposed that the modulation is caused by simultaneous formation of blocks in different martensite packages. Additionally, the influence of the Ni content on the transformation behaviour is presented as well as the influence of an externally applied force.Item Open Access Precipitation of nitrides in iron-based binary and ternary alloys; influence of defects and transformation-misfit stresses(2015) Akhlaghi, Maryam; Mittemeijer, Eric Jan (Prof. Dr. Ir.)The initial microstructure of the unnitrided specimen has a significant influence on the nitriding response of binary Fe-Me (Me: Mo or Al) alloys specimens. This effect was not investigated until now for the case of nitrided ternary Fe-Me1-Me2 alloys, the role of the initial microstructure was studied upon nitriding Fe-4.1 at.% Cr-7.9 at.% Al specimens. To this end, the recrystallized and cold-rolled specimens were nitrided at low nitriding temperature of 400 °C. Upon precipitation of misfitting coherent nitrides during nitriding of thin-foils of binary Fe-Me (Me: Cr and V) alloys, a hydrostatic tensile lattice-stain component results from the elastic accommodation of volume misfit of nitrides and ferrite matrix. The change of the ferrite-matrix lattice parameter can be traced upon precipitation of the nitrides by X-ray diffraction measurements. The theory originally developed for the case of imperfections (by Eshelby) in solids can be applied for quantitatively describing the lattice-parameter changes of the matrix, the nitrides and the aggregate (matrix+ nitrides) as function of volume fraction and type of nitrides.Item Open Access Transient dynamics of a colloidal particle driven through a viscoelastic fluid(2015) Gomez-Solano, Juan Ruben; Bechinger, ClemensWe study the transient motion of a colloidal particle actively dragged by an optical trap through different viscoelastic fluids (wormlike micelles, polymer solutions, and entangled λ-phage DNA). We observe that, after sudden removal of the moving trap, the particle recoils due to the recovery of the deformed fluid microstructure. We find that the transient dynamics of the particle proceeds via a double-exponential relaxation, whose relaxation times remain independent of the initial particle velocity whereas their amplitudes strongly depend on it. While the fastest relaxation mirrors the viscous damping of the particle by the solvent, the slow relaxation results from the recovery of the strained viscoelastic matrix. We show that this transient information, which has no counterpart in Newtonian fluids, can be exploited to investigate linear and nonlinear rheological properties of the embedding fluid, thus providing a novel method to perform transient rheology at the micron-scale.Item Open Access Strukturelle und spektroskopische Eigenschaften epitaktischer FeMn/Co Exchange-Bias-Systeme(2015) Schmidt, Mathias; Goering, Eberhard (PD Dr.)Das Thema dieser Arbeit bestand in der Präparation und Charakterisierung des Exchange-Bias Systems FeMn/Co. Hierbei wurden mittels Molekularstrahlepitaxie zwei sich durch ihre kristalline Orientierung unterscheidende FeMn/Co-Probensysteme auf (100)-orientiertem Magnesiumoxid hergestellt. Zur Erzeugung einer flachen Schichtstruktur mit einer ausgeprägten kristallinen Ordnung war der Einsatz zweier Pufferschichten (Pt,Cu) notwendig. Bei Substrattemperaturen oberhalb von 900 K eine (100)-orientierte Schicht erschaffen, bei niedrigeren Temperaturen um 670 K entstand eine (111)-Orientierung. Untersuchungen der Kristallstruktur ergaben ein epitaktisches Wachstum des (100)-orientierten Probensystems (HTPt) mit großen Kristallitstrukturen, während für das (111)-orientierte System (NTPt) eine vierfach entartete Untergitterstruktur mit kleineren Kristalliten entstand. Es entstand ein wohldefinierter und reproduzierbarer Herstellungsprozess, bei dem sämtliche Schichtparameter gezielt verändert werden konnten. Anschließend wurden magnetometrische Untersuchungen der Probensysteme durchgeführt. Mittels SQUID-Magnetometrie wurde die Temperaturabhängigkeit dieser beiden Parameter untersucht. Es stellte sich eine stärkere Temperaturabhängigkeit des NTPt-Probensystems heraus, die der kleineren Kristallitgröße und der höheren Aktivierung von Pinnningzentren in den Korngrenzen zugeschrieben werden konnte. Dann wurde die magnetische Anisotropie der Probensysteme untersucht, dies geschah mittels eines MOKE-Systems. Es zeigte sich eine stärkere magnetokristalline Anisotropie des HTPt-Systems verglichen mit dem NTPt-System, die mit der ausgeprägteren kristallinen Ordnung in den magnetischen Schichten korreliert. Für eine ausführlichere Charakterisierung wurde auf die FORC (First Order Reversal Curves)-Methode zurückgegriffen. Dieses Verfahren erbrachte den Nachweis der asymmetrischen Magnetisierungsumkehr nicht nur parallel und antiparallel zur Feldkühlrichtung sondern auch für identische Projektionen auf die Feldkühlrichtung. Dieses Verhalten lässt auf eine nicht parallele Anordnung der leichten Richtungen von Ferromagnet und Antiferromagnet schließen. Neben der Asymmetrie der Magnetisierungsumkehr konnte auch das unterschiedliche Ummagnetisierungsverhalten beider Probensysteme analysiert werden. Die Auftrennung der irreversiblen und reversiblen Magnetisierungsbeiträge mittels FORC erbrachte für das HTPt-System irreversible Anteile über den gesamten Winkelbereich, während für das NTPt-System über nahezu den gesamten Winkelbereich reversible und somit rotationsbasierte Mechanismen identifiziert werden konnten. Zuletzt wurden die Probensysteme Röntgenabsorptionsmessungen unter Ausnutzung des Röntgenzirkulardichroismus (XMCD) unterzogen. Einerseits wurde ein Vergleich beider Probensysteme erstellt, andererseits auch Veränderungen der magnetischen Eigenschaften durch gezielte Manipulationen der antiferromagnetischen Struktur untersucht. Diese bestanden in Veränderungen der Dicke der Cu-Pufferschicht sowie in der Änderungen der Zusammensetzung des Antiferromagneten. Die Ergebnisse der Absorptionsmessungen wurden mit Hilfe der Summenregeln analysiert, um die Beiträge von magnetischem Spin- und Bahnmoment zu separieren und quantitativ zu untersuchen. Sowohl Eisen als auch Mangan zeigen ein XMCD-Differenzsignal von unkompensierten rotierbaren magnetischen Momenten. Die Magnetisierung findet sich nahe der Grenzfläche, in den tiefensensitiveren TFY-Messungen konnten keine unkompensierten Momente nachgewiesen werden. Verglichen mit den für Volumenproben reiner Elemente erhaltenen Ergebnissen wurde eine Zunahme des Bahndrehmomentes an beiden Kanten festgestellt. Die Menge an unkompensierten rotierbaren Spinmomenten nimmt bei Reduktion der magnetokristallinen Anisotropie des Antiferromagneten zu, dies ist besonders an der Eisenkante der Fall, auch wenn der Effekt ebenfalls an der Mangankante feststellbar ist. Durch eine auf der Intensität des Absorptionssignals basierende Abschätzung wurde die effektive Dicke der rotierbaren Schicht ermittelt, sie beträgt je nach Probensystem bis zu drei Monolagen für vorliegenden Exchange-Bias. Zuletzt wurde aus den gemessenen Spektren das sogenannte „Branching Ratio“ ermittelt, das Rückschlüsse auf den Erwartungswert der Spin-Bahn-Kopplung in den untersuchten Probensystemen zulässt. Hierbei ergibt sich für Mangan generell ein höherer Wert der Spin-Bahn-Kopplung verglichen mit den Messungen an der Eisenkante. Die Summe der beobachteten Effekte legt zur Erklärung der nahe der Grenzfläche im Antiferromagneten stattfindenden Abläufe ein Wechselspiel der globalen magnetokristallinen Anisotropie der Probensysteme mit einer lokal erhöhten Anisotropie an Stellen mit gestörter Kristallsymmetrie wie Fehlstellen oder Korngrenzen nahe. Letztere führt zum Ausbildung von gepinnten magnetischen Momenten, die durch den Feldkühlprozess eine unidirektionale Ausrichtung erhalten und den Exchange-Bias verursachen.Item Open Access Ion beam lithographic and multilayer fresnel zone plates for soft and hard X-rays: nanofabrication and characterization(2015) Keskinbora, Kahraman; Schütz, Gisela (Prof. Dr.)X-ray microscopy has become an important analytical characterization method for a plethora of applications in materials science, physics, chemistry and biology, thanks to the emergence of modern synchrotron radiation facilities. These facilities enable high brilliance, energy tunable, variable polarization X-rays which gives access to mass density, elemental, chemical, electronic and magnetic properties of materials. In the soft X-ray energies nearly all elements can be probed by spectromicroscopic methods. Another important property of synchrotron radiation is the time structure in the ns to ps range, which can be utilized for sophisticated time resolution studies. These opportunities can be combined with high spatial resolution which is determined by the focusing method and the optic. Focusing of X-rays has historically been a difficult task due to strong absorption and weak phase shift of X-rays within matter. The required phase shift of X-rays, which depends on the real part of the complex refractive index, differs from 1 (the vacuum refractive index) only on the order of 10^-2 to 10^-6 and conventional lenses do not work. One very successful X-ray optic is the Fresnel Zone Plate (FZP), a diffractive optic that act as a lens under certain conditions and can focus X-rays to nanometer sized spots. The resolution of the FZP depends on the width of the outermost zone and is highly correlated with the smallest feature that can be fabricated. Conventionally, the e-beam lithography (EBL) is used for production FZPs which could resolve up to 10 nm structures with serious limitations. One difficulty of EBL is its ever increasing complexity for many-step fabrication of smaller features or intricate geometries. Therefore, EBL is mostly constrained to planar, binary geometries with moderate efficiencies strongly decreasing with energy and not effective for hard X-rays. Special 3D geometries in the form of kinoform lenses can theoretically have 100 % focusing efficiencies. Attempts to approximate these geometries via EBL increased the number of process steps even further. The smallest FZP feature size even for low aspect ratios achievable via EBL is fundamentally limited due to the proximity effect which is the interaction and spread of electrons within the resist material. We addressed these issues by focusing our research on alternative FZP fabrication techniques as high-speed ion beam lithography (IBL), and gray scale ion lithography to realize efficient kinoforms. Another approach towards full-material multilayer FZPs with infinite aspect ratio was based on atomic layer deposition (ALD) with subsequent ion beam slicing. Each of these three methods targets specific challenges faced by the e-beam lithography based FZP fabrication techniques. All the fabricated FZPs were tested for their resolution and efficiency performances at a state of the art scanning transmission X-ray microscope at BESSY for soft X-rays and/or at optical test stations at ESRF and PETRA III for hard X-rays. Using IBL the rapid preparation of a 110 nm thick Au FZP with 50 µm diameter and 50 nm ∆r in less than 13 minutes is demonstrated. Employed for X-ray microscopy, the FZP clearly resolved 28.5 nm features with a cut-off of 24.3 nm at ~1120 eV. Additional process improvements were made towards smaller zones with higher zone quality. They allowed the preparation of a FZP with 30 nm outermost half-period remarkably, in about 8 min. This FZP was shown to clearly resolve 21 nm features on a multilayer test object with large room for improvement. This high through-put FZP production route is of special interest not only concerning the low cost and easy availability. A large array of these optical components is attractive, for experiments such as one-shot ultra-high brilliance FEL investigations due to the radiation damage or for instance for coded-aperture arrays for high-angle resolving X-ray astronomy. Towards fabrication of kinoforms for high efficiency X-ray focusing, we have performed various materials optimization studies in order to achieve a high surface quality optic. After various trials the materials were finally optimized and the fabricated lenses achieved more than 14 % absolute diffraction efficiency that is almost 90 % compared to the theoretical prediction. This confirms how closely we were able to replicate the ideal three dimensional surface relief structure for the first time. It was possible to carry out imaging with these lenses with half-pitch resolutions down to 60 nm. The kinoform lenses were tested at the soft X-ray range where a significant absorption is present in materials. These results also potentially pave the way for very high efficiency hard X-ray focusing which can in principle be utilized in laboratory based X-ray sources, X-ray astronomy and the new rising field of X-ray ptychography. To fabricate high resolution ML-FZPs, Al2O3/Ta2O5multilayers, deposited on a smooth glass optical fiber via atomic layer deposition using non-dedicated instruments were carefully cut-out, sliced and polished to a high quality surface finish using focused ion beams. Following the transfer of the slice to a TEM grid as holder the slices were polished to a high surface finish quality, also via a focused ion beam. Fabricated ML-FZPs were synchrotron tested using an in-house constructed 2-axis tilt stage specially designed for aligning ML-FZP with respect to the X-ray optical axis. The results showed that it was possible to resolve 21 nm features in direct imaging at 1200 eV and sub-30 nm focusing at 8 keV. This is the highest demonstrated resolving power for a multilayer type FZP, to date to the best of our knowledge. Results exhibit the potential for high-resolution hard X-ray focusing where this type of optics are especially efficient. For ultra-high resolution hard and soft X-ray imaging, with potentially achievable ∆r of a few nm is well below what can be achieved through any lithography method available today.Item Open Access Nitriding behavior of Ni and Ni-based binary alloys(2015) Fonovic, Matej; Mittemeijer, Eric J. (Prof. Dr. Ir.)Gaseous nitriding is a prominent thermochemical surface treatment process which can improve various properties of metallic materials such as mechanical, tribological and/or corrosion properties. This process is predominantly performed by applying NH3+H2 containing gas atmospheres serving as the nitrogen donating medium at temperatures between 673 K and 873 K (400 °C and 600 °C). NH3 decomposes at the surface of the metallic specimen and nitrogen diffuses into the surface adjacent region of the specimen whereas hydrogen remains in the gas atmosphere. One of the most important parameters characterizing a gaseous nitriding process is the so-called nitriding potential (rN) which determines the chemical potential of nitrogen provided by the gas phase. The nitriding potential is defined as where and are the partial pressures of the NH3 and H2 in the nitriding atmosphere. In contrast with nitriding of alpha-Fe where the nitriding potential is usually in the range between 0.01 and 1 atm-1/2, nitriding of Ni and Ni-based alloys requires employing nitriding potentials higher than 100 atm-1/2 and even up to infinity (nitriding in pure NH3 atmosphere). This behavior is compatible with decreased thermodynamic stability of the 3d-metal nitrides with increasing atomic number. Depending on the nitriding conditions (temperature, nitriding potential and treatment time), different phases are formed at the surface of the Ni-based alloys. By applying very high nitriding potential, formation of hexagonal Ni3N at the surface of the specimen (known as external nitriding) leads to the development of a compound layer, which may improve tribological properties. Underneath the Ni3N compound layer, two possibilities exist: (i) alloying element precipitation within the nitrided zone (known as internal nitriding) and/or (ii) development of metastable and precipitate-free microstructure known as expanded austenite or S-phase, which can enhance surface hardness, fatigue properties and corrosion properties.Nitriding of multicomponent Ni-based alloys is usually applied in the industry. Nevertheless, the understanding of nitriding is mostly based on phenomenological research and experience. Thereby there is still absence of complete understanding of nitriding of Ni-based alloys, which requires further detailed investigations. Since studying the nitrided multicomponent alloys is complicated, in this thesis fundamental investigations were performed on pure nickel and binary Ni-based model alloys.This thesis focuses on the nitriding behavior of pure nickel, which will result with an thermodynamic evaluation of the Ni-N system. Furthermore, deeper insights in the nitriding behavior of the binary Ni-based alloys is obtained upon nitriding Ni-4 wt.% Ti and Ni-2 wt.% Ti (Ni-5 at.% Ti and Ni-2.5 at.% Ti) alloys. Thereby, the development of large residual macrostresses parallel to the surface of the specimen is related with the N concentration gradient in the nitrided zone.Item Open Access Stress relaxation mechanisms in thin Sn films and its alloys : Sn whisker formation and its mitigation(2015) Stein, Jendrik; Mittemeijer, Jan Eric (Prof. Dr. Ir.)The present thesis project deals with the build-up of stresses and stress relaxation mechanisms in thin Sn(-based) films electro-deposited on pure Cu or Cu-alloy substrates. In particular, the spontaneous formation of Sn whiskers, i.e. the growth of single crystalline, needle-like filaments on the Sn film surface, and its mitigation, is taken into focal point of this study. Thereby, new-developed and well-known methods and analysis techniques have been applied in order to investigate i) the specimen composition, ii) the film- and substrate microstructure and iii) the state of the film stress in the initial state as well as during storage at room temperature. Further, the crystallographic growth directions of Sn whiskers have been investigated and a model, on basis of the concept of periodic bond chains as proposed by Hartmann and Perdok, has been derived for explanation. In chapter 2, an in-situ two-dimensional (2D) detector X-ray diffraction technique has been applied on aging pure Sn films electro-deposited on Cu substrates in order to trace local microstructural changes in residually stressed Sn films. Changes of diffraction spots in the 2D-diffraction patterns were observed, as e.g. emergences, disappearances or migrations as well as increases and/or decreases of intensities of diffraction spots. These diffraction spots on the 2D diffraction patterns originate from (usually) only single Sn grains located within the Sn film microstructure. All observed changes of diffraction spots have been assigned to local microstructure changes in the Sn film: Grain rotation, grain growth and grain dissolution processes could be observed during and even before the onset Sn whisker growth. In chapter 3, the impact of alloying Ag to pure Sn films on the formation of Sn whiskers has been explained. In the as-deposited Sn,Ag films (containing silver of about 6 wt.%), a film microstructure with partially columnar and partially equiaxed Sn grains as well as isolated Ag3Sn precipitates along the Sn/Sn grain boundaries have been observed During room temperature storage of these specimens, Sn whisker growth did not occur even though the formation of the IMC Cu6Sn5 took place at the film/substrate interface in an irregular manner with a similar growth rate as observed in whiskering pure Sn/Cu specimens. The Sn,Ag film microstructure allows global stress relaxation since inclined, with respect to the specimen surface, grain boundaries of equiaxed grains are found at many locations within the film. In chapter 4, pure Sn films electro-deposited on Cu substrates containing Zn as alloying element have been investigated. In particular, the impact of the Zn (content) in the substrate on the interfacial reaction between Cu and Sn and its consequences for the formation of Sn whiskers has been in focal point in this chapter. The Cu6Sn5 formation rate was greatly slowed down in Sn/Cu(Zn) specimens as compared to Sn/pure Cu specimens since the driving force for the Cu6Sn5 formation at the film/substrate interface was lower in the Sn/Cu-alloy specimens (as compared to Sn/pure Cu specimens). In chapter 5, two independent X-ray diffraction methods have been employed on specimens mounted on a newly developed device, consisting of a custom-made glass cylinder provided with a connection piece for vacuum tubes, a needle valve, a vacuum pump and a pressure gauge. This method, the so-called wafer-curvature method, provides an easy and controllable way for imposition of defined strain/stress states in thin films, deposited on (Si-)wafer substrates. This wafer curvature method has been applied on an aging (potentially whiskering) Sn film of 3 µm thickness with a predominantly columnar microstructure. A homogeneous compressive stress in the Sn film, close to the yield limit of Sn, has been induced which, however, did not lead to formation of Sn whiskers or hillocks on the film surface. This finding confirmed that stress gradients are essential for inducing whisker and/or hillock formation. In Chapter 6, a model has been derived describing the probability for Sn whiskers for growing along certain crystallographic growth directions in the crystal structure. Therefore, the concept of periodic bond chains (PBC, i.e. uninterrupted chains of strong bonds), originally proposed by Hartman and Perdok, has been applied in a modified way. The experimental findings of this study (frequently found whisker growth directions were: <100>, <101> and <111>) and also most of the results presented in the literature fit well to the model derived: Sn whiskers tend to grow along a direction parallel to a PBC vector and the smaller the number of strong bonds necessary to form a building unit for a PBC, the more frequently Sn whiskers grow along the corresponding PBC vector.Item Open Access Non-equilibrium work distribution for interacting colloidal particles under friction(2015) Gomez-Solano, Juan Ruben; July, Christoph; Mehl, Jakob; Bechinger, ClemensWe experimentally investigate the non-equilibrium steady-state distribution of the work done by an external force on a mesoscopic system with many coupled degrees of freedom: a colloidal crystal mechanically driven across a commensurate periodic light field. Since this system mimics the spatiotemporal dynamics of a crystalline surface moving on a corrugated substrate, our results show general properties of the work distribution for atomically flat surfaces undergoing friction. We address the role of several parameters which can influence the shape of the work distribution, e.g. the number of particles used to locally probe the properties of the system and the time interval to measure the work. We find that, when tuning the control parameters to induce particle depinning from the substrate, there is an abrupt change of the shape of the work distribution. While in the completely static and sliding friction regimes the work distribution is Gaussian, non-Gaussian tails show up due to the spatiotemporal heterogeneity of the particle dynamics during the transition between these two regimes.