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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.

During residual stress measurements by neutron diffraction additional information becomes available through the accompanying high-energy prompt capture gamma radiation, which in turn produces positrons within the bulk of the specimen. Plastic deformation will increase the dislocation density within the material. This leads to an increase in the mosaic spread of the diffraction reflections and to changes of the lineshape of the positron annihilation radiation. The correlation between the width of the Bragg reflections and the S-parameter, which describes the shape variation of the annihilation line was investigated as a function of the degree of plastic deformation. The experimental work was performed at the FRG-I research reactor of GKSS.

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.

A tensile specimen was machined from heat treated ferritic steel, with a small volume fraction of carbon in the form of cementite. In situ measurements of the strain response of multiple hkl lattice planes to an applied uniaxial tensile load were made using neutron diffraction, to macroscopic plastic strains of around 3%. The experimental results are compared with predictions from a self-consistent Hill–Hutchinson model modified to take into account the presence of the second phase. Good agreement was obtained between model and experimental data. The interpretation of residual stress measurements, for both single peak and Rietveld measurements is considered in light of these results.

The diffractive determination of single-crystal elastic constants using polycrystalline samples represents the inverse problem to the (`direct') experiment of conventional diffractive residual-stress analysis. Both problems are rather sensitive to the quality of the experimental data and to the applied micro-mechanical models. This especially holds for the case of samples for which texture cannot be neglected. The current methods in the field are briefly described and specific difficulties are discussed. The key relations for the BPGeo micro-mechanical model (in which a texture-weighted geometric mean is applied in the stress analysis) are given for the general case. The necessity for careful analysis of the reliability of the results of planned experiments using theoretically modelled `experimental' data obtained with the applied micro-mechanical models is demonstrated for a set of line-shift data from a uniaxial tensile experiment on steel. The possibility of resolving the so-calledd0problem using the original line-position data for a structure refinement is demonstrated.

Solid state information from coherent neutron scattering can be extracted from transmission experiments on polycrystalline materials. The transmission technique has been applied to dynamic structural and phase change investigations and will be developed to cover the fields of strain mapping and structural tomography.

The Bragg edges appearing in the neutron-transmission spectra of a polycrystalline material represent its coherent-scattering response at 2θ=180°. Structural information such as texture, phase content or strain may be derived from the intensity and position of these edges. Transmission diffractometers have been recently developed at both LANSCE and ISIS and used for studying the dynamics of phase transformations and strain imaging, respectively. The incident neutron spectra vary due to the different neutron moderators and flight paths used in the two instruments. This paper reports a comparative study of the two instruments and their analysis software.