5 Werkstofftechnik
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Hierarchical microstructures are created when additional γ particles form in γ’ precipitates and they are linked to improved strength and creep properties in high-temperature alloys. Here, we follow the formation and evolution of a hierarchical microstructure in Ni86.1Al8.5Ti5.4 by in situ synchrotron X-ray diffraction at 1023 K up to 48 h to derive the lattice parameters of the γ matrix, γ’ precipitates and γ particles and misfits between phases. Finite element method-based computer simulations of hierarchical microstructures allow obtaining each phase's lattice parameter, thereby aiding peak identification in the in situ X-ray diffraction data. The simulations further give insight into the heterogeneous strain distribution between γ’ precipitates and γ particles, which gives rise to an anisotropic diffusion potential that drives the directional growth of γ particles. We rationalize a schematic model for the growth of γ particles, based on the Gibbs-Thomson effect of capillary and strain-induced anisotropic diffusion potentials. Our results highlight the importance of elastic properties, elastic anisotropy, lattice parameters, and diffusion potentials in controlling the behavior and stability of hierarchical microstructures.
A variety of materials of technological interest change their properties through contact with reactive media. Solid-gas reactions lead to a variety of reaction products on the surfaces and internal interfaces. The observation of nucleation and growth processes in the environment where they occur (in situ) from a chemical-structural perspective is especially challenging for aggressive atmospheres. The talk presents innovative approaches to study corrosion mechanisms using advanced X-ray methods. Using energy dispersive X-ray diffraction and X-ray absorption spectroscopy in different tailor made environmental reaction chambers, valuable insights into high temperature oxidation and sulfidation processes were gained. Fe-based alloys were exposed to hot and reactive atmospheres containing gases like SO2, H2O and O2 at 650°C. During the gas exposure the tailor made reaction chambers were connected to a high energy diffraction end station at the synchrotron. The crystallization and growth of oxide and sulfide reaction products at the alloy surfaces were monitored by collecting full diffraction pattern every minute. Careful examination of shape and intensity of phase-specific reflections enabled to a detailed view on growth kinetics. These studies showed, oxides are the first phases occurring immediately after experimental start. As soon as reactive gas media enter the chamber, the conditions change and different reaction products, such as sulfides start to grow. A comparison of different gas environments applied, illustrated the differences in the type of reaction products. The in situ observation of high temperature material degradation by corrosion made it possible to study the contribution of phases, which are not stable at room temperature. For instance, wuestite (Fe1-xO), was frequently observed at high temperatures in humid gases on Fe with 2 wt.% and 9 wt.% chromium, but not at room temperature. The strength of the occurrence of this phase additionally explains why, despite a higher Cr content, ferritic alloys with 9 wt.% Cr in a challenging atmosphere prevent the intrinsic formation of protective layers. The in situ observations were supplemented by careful considerations of thermodynamic boundary conditions and detailed post characterization by classical metallographic analysis. Additionally, the structure and chemistry of the dominant oxide layers were evaluated using X-ray absorption near edge structure spectroscopy. The talk will give an overview about chances and challenges for studying high temperature corrosion phenomena by advanced X-ray methods.
Konventionelle und innovative Hochtemperaturwerkstoffe sind im technischen Betrieb einer Vielzahl an Belastungen ausgesetzt. Neben mechanischen Beanspruchungen stellt die chemische Belastung durch das Vorhandensein von aggressiven und korrosiven Medien (SO2, CO2, O2, H2O…) bei hoher Temperatur einen Schlüsselfaktor dar, der die Langlebigkeit der Materialien im Einsatz beeinflusst. Neben konventionellen, mikroskopischen Analyseverfahren bieten röntgenbasierte Techniken Möglichkeiten, die Grenzflächenreaktionen zwischen dem Werkstoff und den Medien während der chemischen Belastung (in situ) zu untersuchen. Neuartige Einblicke in die zugrunde liegenden Prozesse der Schädigungsmechanismen werden anhand von klassischen ferritischen Legierungen beschrieben. Das Verständnis von Schlüsselfaktoren zur Induzierung eines Schutzmechanismus ist insbesondere für die Entwicklung von neuartigen Materialklassen wie Hochentropie- und chemisch komplexe Legierungen notwendig. Erste Einblicke in Schädigungsmechanismen durch Gaskorrosion dieser innovativen Materialsysteme werden vorgestellt.
A femto-second pulsed laser ablation in liquid (PLAL) procedure for the generation of titanium oxide nanoparticles (NP) is reported with the purpose of understanding morphology and structure of the newly generated NPs. Ablation duration was varied for optimization of NP generation processes between 10 and 90 min. Surface morphology of NPs as well as their size and shape (distribution) were analysed by various complementary electron microscopy techniques, i.e. SEM, TSEM and TEM. The crystalline structure of titanium oxide particles was investigated by XRD and HR-TEM. Concentration of generated titanium oxide NPs in liquid was analysed by ICP-MS. A mix of crystalline (mainly anatase), partly crystalline and amorphous spherical titanium oxide NPs can be reported having a mean size between 10 and 20 nm, which is rather independent of the laser ablation (LA) duration. A second component consisting of irregularly shaped, but crystalline titanium oxide nanostructures is co-generated in the LA water, with more pronounced occurrence at longer LA times. The provenance of this component is assigned to those spherical particles generated in suspension and passing through the converging laser beam, being hence subject to secondary irradiation effects, e. g. fragmentation.