Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
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The LEK94 is a Ni-base superalloy single crystal (SX) of the second generation, a materials class which is important for its high temperature creep resistance (>1000°C) in first stage blades of low-to-medium pressure gas turbines. Monocrystalline Ni-base superalloy SXs have a two-phase microstructure consisting of small cubes (' phase with the ordered L12 crystal structure, cube edge length: 500 nm), separated by thin channels (γ phase with fcc solid solution structure, channel width: 20 nm). The microstructural evolution during high temperature and low stress tensile creep has been thoroughly investigated previously, mainly for [001] loading, both in terms of dislocation activity (filling of γ channels, formation of dislocation networks, cutting of the γ’ phase) as well as phase coarsening (rafting, topological inversion). Other loading geometries have received less attention.
The present work studies high temperature and low stress creep deformation of the superalloy LEK 94 at temperatures around 1000°C, where rafting occurs. Differences between loading under different uniaxial, biaxial and triaxial stress states are discussed. Stereo-microscopy and g∙b analysis in the scanning transmission electron microscopy mode (STEM) are combined for microstructural analysis. The focus is set on the role of dislocation interactions with the aging microstructure. Both development in STEM characterization methods, as well as the roles of phase coarsening, γ channel filling, microstructural heterogeneity and γ’ phase cutting are discussed.
Complex materials (e.g., porous ceramics) need high resolution techniques at multi-scale levels. Microstructure is intimately linked to properties and performance. One technique (or even a few techniques) is not enough. X-ray refraction and CT are very powerful tools. Non-destructive characterisation and quantitative image analysis techniques (2D and 3D) are necessary to understand the anisotropy.
Several restoration projects of stained-glass windows have been performed in Lower Silesia (Poland) since 2010. The aim of the projects was to protect stained-glass windows against environmental impact of industial pollutants and acid rain by installing a protective glazing. The usefulness of protective glazing has been proved by climate measurements and determination of environmental impact before and after installation.
The presentation shows how the three-dimensional quantification of dislocations and their characteristic features, e.g. Burgers vector, line direction, dislocation density, is carried out at the transmission electron microscope in scanning mode (STEM) at Division 5.1 at BAM. Exemplarily, the methods are shown for Ni-base superalloy single crystals, for which a short introduction is given using further TEM techniques. Additional examples on low angle grain boundaries, nucleation of oxides at dislocations and interaction of dislocations and carbides are shown.
The content of the presentation was addapted, aiming at scientists who work within the DFG Priority Programme 1713 "Strong coupling of thermo-chemical and thermo-mechanical states in applied materials".
In Germany the damages on concrete motorways have increased during the past 15 years. This led often to a reduction of service life from 30 to 10 years. In spite of comprehensive research the reasons for these damages have not been clarified yet. But it is known that in this context the ASR plays a decisive role. Against this background ASR performance tests have been developed. However, these tests do not take into account the effects of mechanical loading due to cyclic traffic and climatic impact. Therefore in Germany the ASR research group 1498 was founded. The research group consists of three experimentally- and two multi-scale modelling oriented subprojects, which are working closely together. Selected findings of BAM’s experimental subproject SP4 are content of this contribution. The aim was to investigate the impact of a cyclic mechanical pre-damage on moisture transport. The pre-damaging was carried out on large-format beams using a cyclic four point bending test with a total number of 5 million load cycles. Thereby the damage process was monitored by using different NDT-techniques. After the mechanical pre-damaging, smallsized specimens were taken from loaded as well as unloaded areas of the large format beams. Absorption tests were carried out on these specimens using water and NaCl solution. Thereby moisture distribution was measured with Nuclear Magnetic Resonance (NMR) and Time Domain Reflectometry (TDR). Additionally, moisture suction was quantified integrally by means of gravimetric measurements. The analysis of the first test data shows that the mechanical loading influences the moisture transport.
The concrete prism test at 60°C facilitates accelerated testing of alkali-silica reactivity of aggregates within five months. Manual measurement is usually conducted to determine expansion of test specimen every 28 days. This conventional procedure gives only limited insight to expansion behaviour of ASR-affected prisms. Due to the large time intervals, expansion exceeding the critical value of 0.3 mm/m is likely to be noticed late. Continuous recording applied in this work resolves this problem and delivers beneficial information on the shape of the expansion curve.
Different types of aggregate have been tested applying continuous and manual measurement to the concrete specimens.
Models of ASR-induced expansion, available in the open literature, so far are validated with discontinuous data hardly tracing the modelled curve. Continuous measurement improves validation and further model development.
The martensitic steels X30CrMoN15-1 and 100Cr6 did not show any hydrogen enhanced wear in sliding friction tests against alumina balls in hydrogen gas environment. However, depending on the purity of the environment, unexpected high wear occurred at the alumina ball. The results of a tests series with a broad variation of test parameters are presented.
The novel heat treatment concept, “quenching and partitioning” (Q&P) has been developed for high strength steels with enhanced formability. This heat treatment involves quenching of austenite to a temperature between martensite start and finish, to receive a several amount of retained austenite. During the subsequent annealing treatment, the so called partitioning, the retained austenite is stabilized due to carbon diffusion, which results in enhanced formability and strength regarding strain induced austenite to martensite transformation. In this study a Q&P heat treatment was applied to a Fe-0.45C-0.65Mn-0.34Si-13.95Cr stainless martensite. Thereby the initial quench end temperature and the partitioning time were varied to characterize their influence on microstructural evolution. The microstructural changes were analysed by dilatometer measurements, X-ray diffraction and scanning electron microscopy, including electron back-scatter diffraction. Compression testing was made to examine the mechanical behaviour. It was found that an increasing partitioning time up to 30 min leads to an enhanced formability without loss in strength due to a higher amount of stabilized retained and reversed austenite as well as precipitation hardening.