Analyse der Verformungsmechanismen nanokristalliner Palladium-Gold-Legierungen mittels in-situ Röntgenbeugung und Modellierung der Streuintensitäten

Ziel dieser Arbeit war die Untersuchung des Verformungsverhaltens von nanokristallinem Pd90Au10 in Abhängigkeit von Dehnrate, Spannungszustand und Relaxationszustand, sowie die Identifikation dabei aktiver Plastizitätsmechanismen. Dazu wurden Verformungsexperimente an entsprechend präparierten Shear-Compression-Specimens (SCS) bei verschiedenen Dehnraten durchgeführt, wobei die Verformung mittels optischer Dehnungsmessung erfasst wurde. Simultan dazu wurde die Mikrostruktur mittels Transmissionsröntgenbeugung untersucht. Die Analyse der Röntgendaten erfolgte mittels Whole Powder Pattern Modeling, wodurch eine detaillierte und richtungsabhängige Analyse der Mikrostruktur über die gesamte Verformung hinweg ermöglicht wurde. Zusammen mit den makroskopischen Kraft- und Dehnungsdaten ergibt sich damit eine umfassende sowie durchgängig konsistente Beschreibung des Verformungsverhaltens nanokristalliner Pd90Au10 SCS. In der kristallinen Phase konnten Versetzungsgleiten, Coupling und Kornrotation als aktive und miteinander wechselwirkende Mechanismen gezeigt werden. Zusätzlich sind in den Korngrenzen weitere Plastizitätsmechanismen aktiv, die in Scherprozesse und volumenabbauende Relaxationsprozesse unterschieden werde können. Das gesamte Verformungsverhalten ist das Resultat des Zusammenwirkens und Konkurrierens aller Einzelmechanismen, welches maßgeblich durch Relaxation, Dehnrate, Spannungszustand und Verformungshistorie beeinflusst wird.The objective of this work was to investigate the deformation behaviour of nanocrystalline Pd90Au10 as a function of strain-rate, stress-state and relaxation-state, and to identify active plasticity mechanisms. Deformation experiments were carried out on accordingly prepared shear-compression-specimens (SCS) at different strain-rates, with the deformation being recorded by optical strain measurement. Simultaneously the microstructure was investigated by transmission X-ray diffraction. The analysis of the X-ray data was performed by Whole Powder Pattern Modeling, which allowed a detailed and directional analysis of the microstructure throughout the entire deformation. Together with the macroscopic force and strain data, this results in a comprehensive and consistent description of the deformation behaviour of nanocrystalline Pd90Au10 SCS. In the crystalline phase, dislocation slip, coupling and grain rotation were shown to be active and interacting mechanisms. In addition, further plasticity mechanisms are active in the grain boundaries, which can be distinguished into shear processes and volume-reducing relaxation processes. The overall deformation behaviour is generated by the interaction and competition of individual mechanisms, which are significantly in uenced by relaxation, strain-rate, stressstate and deformation history

Structural relaxation of nanocrystalline PdAu alloy: Probing the spectrum of potential barriers

A commonality between nanocrystalline metals and metallic glasses is their dependence of structure and properties upon preparation history and postprocessing. Depending on preparation conditions, stored excess enthalpy and volume—relative to the crystalline ground state—can vary significantly. Annealing of material states of elevated enthalpy or volume induces structural relaxation and concomitant depletion of excess energy and volume. We analyzed the kinetics of volume relaxation in nanocrystalline PdAu alloys by partitioning the overall process into a set of independent and parallel reactions for arbitrary time-temperature protocols. The obtained spectra of kinetic parameters imply a complex relaxation behavior that violates time-temperature superposition and time aging-time superposition. The analysis will enable to reconstruct the effective energy landscape underlying the relaxation dynamics

Structural relaxation of nanocrystalline PdAu alloy: Mapping pathways through the potential energy landscape

Preparation history and processing have a crucial influence on which configurational state material systems assume. Glasses and nanocrystalline materials usually reside in nonequilibrium states at room temperature, and as a consequence, their thermodynamic, dynamical, and physical properties change with time—even years after manufacture. Such changes, entitled aging or structural relaxation, are all manifestations of paths taken in the underlying potential energy landscape. Since it is highly multidimensional, there is a need to reduce complexity. Here, we demonstrate how to construct a one-dimensional pathway across the energy landscape using strain/volume as an order parameter. On its way to equilibrium, we map the system’s release of energy by calorimetry and the spectrum of barrier heights by dilatometry. The potential energy of the system is reduced by approximately B during relaxation, whereas the crossing of saddle points requires activation energies in the order of 1eV/atom relative to the energy minima. As a consequence, the system behaves as a bad global minimum finder. We also discovered that aging is accompanied by a decrease in the non-ergodicity parameter, suggesting a decline in density fluctuations during aging

Structural relaxation of nanocrystalline PdAu alloy: Probing the spectrum of potential barriers

A commonality between nanocrystalline metals and metallic glasses is their dependence of structure and properties upon preparation history and postprocessing. Depending on preparation conditions, stored excess enthalpy and volume—relative to the crystalline ground state—can vary significantly. Annealing of material states of elevated enthalpy or volume induces structural relaxation and concomitant depletion of excess energy and volume. We analyzed the kinetics of volume relaxation in nanocrystalline PdAu alloys by partitioning the overall process into a set of independent and parallel reactions for arbitrary time-temperature protocols. The obtained spectra of kinetic parameters imply a complex relaxation behavior that violates time-temperature superposition and time aging-time superposition. The analysis will enable to reconstruct the effective energy landscape underlying the relaxation dynamics

Synthesis of Hydroxyapatite Substrates: Bridging the Gap between Model Surfaces and Enamel

Hydroxyapatite substrates are common biomaterials, yet samples of natural teeth do not meet the demands for well-defined, highly reproducible properties. Pellets of hydroxyapatite were produced via the field assisted sintering technology (FAST) as well as via pressureless sintering (PLS). The applied synthesis routes provide samples of very high density (95%–99% of the crystallographic density) and of very low surface roughness (lower than 1 nm when averaged per 1 μm²). The chemical composition of the raw material (commercial HAP powder) as well as the crystalline structure is maintained by the sintering processes. These specimens can therefore be considered as promising model surfaces for studies on the interactions of biomaterial with surfaces of biological relevance, as demonstrated for the adsorption of BSA proteins