Papers by Author: Lorenz Singheiser

Abstract: The VHCF behavior of age hardened 2024 and 7075 aluminum sheets was studied. The experiments were performed at frequencies of ≈ 20 kHz with fully reversed axial loading (R = -1). Special focus was put on the influence of AA 1050 claddings and riblet-like surface structures, which are used in aerospace applications to reduce aerodynamic drag. The fatigue life and fatigue limit of the AA 2024 bare material are – compared to the non-structured case – significantly reduced by the stress concentrations induced by the riblet structure. However, the fatigue behavior of the clad AA 2024 material is less sensitive to the surface structure. In this case, we obtained a sharp transition from HCF failure up to 5x10⁶ cycles to run-outs at ≥ 2x10⁹ cycles. This threshold value for failure differs with cladding thickness as well as with riblet geometry. We attribute this to the modified stress distribution near the interface (cladding/substrate) as well as to a locally reduced thickness of the cladding in the riblet valleys. Fatigue cracks are – even in the case of run-outs – always initiated at the surface of the clad layer and grow easily to the substrate. Samples only fail, if the threshold for further crack growth into the substrate is exceeded. Both Alclad 2024 and 7075 show the same failure mechanism.

Abstract: In aeronautics, economic and environmental aspects become increasingly important. As those are very much influenced by the frictional drag of the airplane, a reduction of skin friction which causes a major portion of total aerodynamic drag is desirable. One possible approach for passive drag reduction is the application of riblets small longitudinal grooves orientated in flow direction. Through an adapted rolling process, riblets can be brought into metal sheets on a large scale. For this process a thin high-strength steel wire is wound around a work roll to structure it with the negative riblet imprint. In a subsequent step the riblet profile is rolled into the sheet material. Different parameters can influence the process and the quality of the resulting riblet structure. Those parameters that depend on the sheets sheet thickness, material strength, and composition of the sheet are discussed in this paper. Form filling is used as an indicator for riblet quality. It is found that decreasing sheet thickness is beneficial for form filling, but a process dependent minimum sheet thickness exists for which this effect will reverse. Material strength is found to have a much smaller influence on form filling. Nevertheless, harder alloys seem to need a slightly smaller thickness reduction, but higher rolling forces and pressures to achieve desired form filling. Using clad instead of bare materials has a positive influence on form filling and riblet structuring. Furthermore, riblet rolling does not reduce the fatigue strength of the clad material.

Abstract: In the present study the oxidation behaviour of a number of candidate alloys for heat exchanging components was investigated in model gas mixtures containing high amounts of CO₂ and/or water vapour in the temperature range 550-700°C up to exposure times of 1000 h. During exposure in Ar/CO₂ and Ar/CO₂/H₂O base gas mixtures at 550-650°C the oxidation rates and scale compositions of martensitic 9-12%Cr steels were similar to those previously observed in steam environments. Thin and protective Cr-rich oxide scales which are commonly found during air oxidation was observed locally on the specimens surfaces after oxidation in Ar-(1-3%)O₂-CO₂. The tendency for protective chromia base scale formation increased when 3% oxygen was added, especially for the 12%Cr steel. When iron base oxide scales were formed on the metal surface, the martensitic steels tended to exhibit carburisation whereby the extent was reduced by increasing the water vapour and oxygen contents. All three studied austenitic alloys exhibited very slow scale growth rates at 550°C, however, at and above 600°C the steels with lower Cr content started to form two-layered iron rich surface oxide scales whereby the outer oxide was prone to spallation upon thermal cycling. The high-Cr austenitic steel 310N and the nickel base alloy 617 formed very thin, Cr-rich oxide scales at all used test temperatures and atmospheres. For those two materials the oxidation behaviour in gases containing water vapour in combination with intentionally added oxygen was affected by formation of volatile chromium oxyhydroxide.

Abstract: One possibility to run a zero CO2-emission power plant is the oxyfuel combustion process. An efficient technology to separate oxygen from air are processes with oxygen transport membranes. It is important that these membranes have a high permeability additional to high chemical stability under power plant conditions. La0.58Sr0.4Co0.2Fe0.8O3- (LSCF) is a mixed ionic electronic conducting material (MIEC) which is considered to have good permeation properties and a high stability. In this work LSCF based membranes are investigated. Data from a TGA (Thermo-Gravimetric Analyser) have been used to determine the diffusion coefficient of oxygen vacancies in LSCF perovskite type oxides for different temperatures.

Abstract: For a number of chromia and alumina forming high temperature alloys and coatings, recent studies revealed, that in some cases the specimen/component or coating thickness may substantially affect the growth rates of the surface oxides. For the alumina formers the thickness dependence is mainly governed by depletion of oxygen active elements such as Y, Zr, Hf, Mg which are either intentionally added alloying elements or manufacturing related alloy impurities. In the case of the chromia forming materials, which tend to exhibit a more substantial dependence of oxidation rate on specimen/component thickness, depletion of minor alloying additions is also an important factor to be considered. However, for these alloys relaxation of oxide growth stresses by plastic deformation of the metallic substrates seems to be the dominant parameter which governs the observed behaviour.

Abstract: The oxidation behaviour of binary Fe-Cr alloys containing 10 and 20 mass % Cr, respectively, was studied in Ar-20%O2, Ar-7%H2O and in Ar-4%H2-7%H2O at temperatures between 800 and 1050°C. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the surface oxide scale, the present results reveal that the presence of water vapor also affects the transport processes in the alloy. The enhanced internal oxidation, which is likely the result of water vapor increasing the solubility and/or the diffusivity of oxygen in the alloy, explains the frequently observed effect that Fe(Ni)Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective oxidation in dry gases but breakaway type oxidation in steam. The temperature dependence of the change from protective to non-protective behaviour in Ar-H2O differs quantitatively, but not qualitatively from that in Ar-O2.

Abstract: The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.

Abstract: To evaluate the performance of plasma facing materials (PFM) and components for future thermonuclear fusion devices under the expected operation conditions, an in-depth material characterization and extensive high heat flux simulation tests are performed routinely in electron beam test facilities. These experiments cover both, thermal fatigue tests with power densities up to approx. 20 MWm-2 and thermal shock loads with deposited energy densities of several MJm-2. In addition, irradiation experiments have been performed in material test reactors to investigate the neutron induced material degradation.