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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.

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...

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.

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 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.

Integrated substructure analysis in polycrystals is often based on the interpretation of physical diffraction-line broadening. In the present work the case of high-resolution neutron diffraction by means of RTOF is considered in comparison with the angular-dispersive neutron diffractometry. First experimental results indicate that RTOF can successfully be used for the determination of physical line-broadening.

Low-energy neutron transmission diffraction has been successfully demonstrated to be a powerful experimental method to investigate changes in crystal structure development down to the millisecond range. Bragg edge diffraction measurements thus can be the basis for future experiments like strain radiography, determination of the strain-rate dependent elastic properties of engineering materials, temporal development of phase changes, structural properties of materials under extreme environmental conditions like high pressure or temperature states. At high instantaneous neutron bursts current-mode detection can be used, which enables both transient and stroboscopic measurements. Progress has been made in rapid data acquisition and analysis as well as temperature measurement by resonance doping.

Phase transformations in engineering materials are inevitably related to mechanical behavior and are often precursors to residual stress and distortion. Neutron scattering in general is a valuable tool for studying their effects, and pulsed neutrons are of special value, because of the inherently comprehensive crystallographic coverage they provide in each measurement. At the Manuel Lujan neutron scattering center several different research programs have addressed the relationships between phase transformation/mechanical behavior and residual strains. Three disparate examples are presented: 1) stress induced transformation in a NiTi shape memory alloy, 2) cryogenically induced transformation in a quenched 5180 steel, and 3) time resolved evolution of strain induced martensite in 304 stainless steel. In each case a brief description of the principal result will be discussed in the context of using neutrons for the measurement.

The neutron intensity transmitted through polycrystalline material exhibits sudden well defined increases whenever the neutron wavelength ceases to fulfil the Bragg coherent scattering condition for a specific subset of lattice planes. Since in the transmitted beam all diffracted neutrons are absent, the so-called Bragg edges build up very rapidly, because diffraction of neutrons into the full solid angle contributes to their presence. High peak-intensity neutron bursts may be detected using the current-mode technique, opening up the possibility to do both transient and stroboscopic measurements. This new method is relevant to different research areas, like transient phase transformation, melting and solidification, dynamic stress influence or materials under extreme environmental conditions. Progress has been made in rapid data acquisition and analysis as well as temperature measurement by resonance doping.

High-resolution neutron diffraction has become a powerful method to determine strains caused by residual stresses in polycrystalline materials on a microstructural scale, measuring distorted lattice plane distances to within an accuracy of better than 0.5 X 10-4. The method implies very good knowledge of the equivalent undistorted (stress-free) lattice parameters in order to avoid systematic errors. Lattice parameters can also be affected by material composition, so that one can arrive at false values even under stress-free conditions. This has led the authors to use neutrons to measure both lattice distances by neutron time-of- flight (TOF) and chemical composition by prompt-gamma analysis non-destructively at the same time. Aluminum-copper alloys with copper contents between 0.08 and 4.98 weight% have been investigated as model substances, because the aluminum lattice contracts with increasing copper solute.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Fourier type correlation spectrometers at reactors allow strain measurements by neutron time-of-flight diffractometry. For instruments of this type intensity and resolution are independent. Data acquisition is done by on-line correlation, which is possible in the reverse-timeof-flight Fourier (RTOF) mode. A spectrometer designed for strain measurements is now operating at the 5 MW FRG-1 research reactor at GKSS Geesthacht.

The transmission of polycristalline materials in a white neutron spectrum shows sharp edges as a function of neutron wavelength or time-of-flight. They correspond to the breakdown of the Bragg interference condition for those sets of lattice planes which scatter by 180 degrees. Materials stresses as well as texture will influence the position and shape of the edges. Transmission measurements may be favourable for a number of experimental tasks.

Nondestructive characterization of residual stresses in materials by neutron time-of-flight using a novel correlation technique has been supplemented by a high-resolution gamma ray spectrometer to detect radiation from prompt neutron capture simultaneously with the stress measurement. Sensitivities and detection limits for important alloy constituents from characteristic gamma ray spectral lines as well as the composition of a number of alloys have been determined by this method. It can also be used for nondestructive determination of hydrogen contents. The interaction of the high-energy capture radiation with the material generates electron-positron pairs. Positron annihilation radiation is thus produced with intensities sufficiently high to investigate dislocation densities from plastic deformation.

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.

Non-destructive neutron diffractometry for stress analysis will be a powerful experimental tool in material science research performed at the GKSS 5 MW reactor FRG-1. Arguments which show the advantages of the time-of-flight method are given and a suitable high-resolution neutron-efficient type of spectrometer is introduced. First results derived from this method are presented.

A lithium glass detector bank for the fast chopper facility at the Geesthacht research reactor (FRG-1) is described.