Search

The automotive industry and aerospace industry are two of the most important industrial sectors using γ-TiAl based alloys — a group of intermetallic materials with tetragonal structure. γ-TiAl based alloys offer advantages for use at working temperatures range between 500°C and 850°C and lead to weight reductions of up to 40% because of their low density, advanced mechanical properties and high oxidation and creep resistance. By reducing weight, especially in oscillating or rotating components of engines and turbines in the high temperature range, efficiency and power can be increased significantly. In current work the elastic properties of single phase γ-TiAl polycrystalline material were investigated at ambient and elevated temperature. The hkl dependent lattice strain was measured in situ at uni-axial compressive load using RTOF high-resolution neutron diffraction.

When characterizing residual stresses by neutron diffraction, neutron absorption is always a competing process. It leads to the emission of high-energy, so-called prompt gamma radiation, which in turn produces positrons within the bulk of the specimen. The decay of these antiparticles and the resulting annihilation radiation are influenced by material properties like defects. Plastic deformation and resulting increase of the dislocation density leads in the neutron scattering case to an increase in the mosaic spread of the BRAGG reflections, while on the other hand, the 511 keV line of the annihilation radiation is also measurably broadened. Suitable collimation can assure that the information gathered by neutron diffraction comes from about the same gauge volume as the information from positron annihilation. It is the aim of this project to investigate the correlation between the width of the BRAGG reflections and the S-parameter, by which the width of the annihilation line is described as a function of the degree of plastic deformation. In this way, more can be learned about a material, since during strain measurements by diffraction additional information is available through simultaneous gamma-ray spectroscopy.

The main problem with cast austenitic—ferritic duplex stainless steels is the formation of cracks after the solidification. The crack sensitivity is caused by volume differences between the microstructural components and coupled to an extreme embrittlement. Therefore the σ phase precipitation becomes important. It binds Fe, Cr and Mo and degrades the corrosion resistance. In addition, it decreases the toughness of the steel because of its embrittling character and by precipitating at grain boundaries. Both the absolute amount of σ phase and its distribution have an influence. During casting of components, the σ phase precipitation should be kept as low as possible. This is achieved by rapid cooling through the critical temperature range from 1000 to 600°C. Since the main cause of crack formation lies in microstructural stresses and incompatibilities, this process must be better understood. Neutron time-of-flight diffractometry can detect strains in different phases simultaneously from the same vo...

The influence of phase errors on the accuracy of the neutron diffraction peak parameters is discussed. It is shown that a constant phase error leads to both a distortion of the shape and a shift of the center of gravity of a reflection peak. A method was developed which takes the phase error into account and corrects for the peak shift.

Three different modes of operation have been realized for the Fourier-RTOF diffractometer FSS at the GKSS Research Center which produce peak shapes that are either a Gaussian or the first or second derivative of it. The triangular shape of the modulation function of the neutron intensity as well as the rectangular shape of the optical pickup signal cause deviations from the ideal shapes of the reflection peaks. Distortions caused by the constraints on the upper limit of the modulation frequency cause specific oscillations, which are superimposed on the diffraction peaks. The characteristic properties of the three modes of operation are discussed.

The quality of a neutron time-of-flight diffractometer can be significantly improved by setting up its detectors in time-focusing geometry. In order to fully exploit the time-focusing method, the nominal resolution of the spectrometer must be thoroughly investigated by analysing the contributions of several different factors. In this paper analytic formulae are given, which describe the different components contributing to the resolution function and the conditions which optimize the resolution are derived. It is shown how the analytic integral equations can be solved numerically. The programs allow iterative computations to optimize a detector system composed of many individual detector elements, considering the specimen size and instrumental resolution. The flow diagram of the programs to calculate numerically the detector system is briefly discussed and the precision of the individual optimal detector position coordinates is given.

The first successful application of a new method to investigate phase transformations in real time, like the decomposition of austenite into bainite in grey iron, is described. During the ongoing transformation, transmission spectra of thermal neutrons, which contain Bragg edges corresponding to the crystal structure of the transforming phases, are recorded. By evaluating the height of these Bragg edges, which is a measure of the volume fraction of the phase, at different transformation times, the transformation can be followed in-situ in a time resolved manner. The method is compared to other previously used methods (micrographs, dilatometry, diffraction techniques); also a summary and an outlook are given.

A neutron diffractometer based on the reverse time-of-flight method and intended for the study of residual stresses is reported. The special requirements for high resolution neutron diffractometry to investigate residual stresses are discussed. The components and characteristics of the diffractometer are introduced. Finally, first results of residual stress measurements are given.

The Kiel University fast-chopper has been simulated by a Monte Carlo model taking into account the geometric outlines of the system, neutron leakage through rotor edges and neutron beam divergence. The calculated results have been verified by experimental investigations. Special emphasis was laid on the very sensitive dependence of the measured neutron time-of-flight spectra on the rotor displacement from the collimator axis, which is also well reproduced by the calculations. Therefore the model is regarded to be authentic to derive a better estimate for the resolution function of the spectrometer, which is needed for measurement tasks like neutron resonance spectroscopy and shape analysis. The FORTRAN programs permit achievement of the best spectrometer adjustment to a given experimental problem. They may be modified to calculate different system geometries.