5 Werkstofftechnik
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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.
Ni-base superalloy single crystals have been used in turbine blades for hot sections of gas turbines for over four decades. In order to increase the efficiency of the turbines, a continuous increase in the inlet temperature of combustion gases into the turbine has driven the design of turbine blades to complicated shapes and the presence of a complex pattern of cooling channels. These three-dimensional shapes, together with the inhomogeneous distribution of stresses along the blade, induce an also complicated triaxial stress state, which does not compare to uniaxial tests that are performed to characterize high temperature properties such as creep. A round notch on a test piece represents a simple configuration that generates a quasi-isostatic stress state across the notch. In the present contribution, the effect of a sharp round notch on the microstructural micromechanisms within the notched region cylindrical bars, loaded along [001] at 1020 °C and 160 MPa net stress, is studied. To this end, a series of interrupted creep tests is conducted on plain and notched bars and the microstructure is compared. Results are discussed in terms of degree microstructural coarsening, and dislocation activity. The effect of notch generation via grinding is also discussed in these terms. The presence of carbides evolving in from residual carbon is also shown and discussed. .Funding by the German Research Association (DFG) [grant number AG 191/1] is acknowledge
A study was made on the effect of creep loading on the precipitate radii evolution of the aluminum alloy 2618A.
The overageing process of the alloy was investigated under load at a temperature of 190 °C with stresses between 79 and 181 MPa and compared to stress free isothermal ageing. The precipitates responsible for strength were characterized using dark-field transmission electron microscopy (DFTEM). This allows the experimental Determination of radii distributions of the rod-shaped Al2CuMg precipitates and the evaluation regarding their mean precipitate radius. It was found that the mean precipitate radius enables the comparison of the different microstructural conditions of crept and uncrept samples. The mean precipitate radii of the samples experiencing creep are significantly higher than those of undeformed samples. It was shown that the acquired radii distributions are viable to determine averaged particle radii for comparison of the aged samples. A ripening process including pipe diffusion along dislocations describes the data on coarsening very well for the creep samples.