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.

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.

Cu-Nb composites are characterized by some special properties, which were discussed since a long time by many different authors [1, 2, 3, and 4]. For manufacturing linear accelerator units it is a great advantage that Cu-Nb don’t form intermetallic compounds. One of the basic questions during application is the influence of the thermal expansion of copper and niobium. Thermal expansion of Cu-Nb was widely discussed by Nadeau and Ferrari [5]. Our research program consists of investigations on Cu50%-Nb50% composites and on Cu-Nb tubes, which on one hand have different textures and on the other hand the grain boundaries are much different in the composite with a curling microstructure and in co-extruded tubes. The present paper will concentrate on thermal stresses and the texture behavior in the temperature range 4K -1273K.

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 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 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 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 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.

Recent progress in neutron scattering for industrial purposes is reviewed. The relevant fields of interest are small-angle scattering, high-resolution diffraction, texture determination, reflectometry and radiography. Potential users are encouraged to address themselves to one of the neutron scattering centers given in the Appendix.

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.

Time resolved phase transition and strain experiments have been performed on the millisecond time scale using a Bragg-edge transmission technique that has been developed at the Los Alamos National Laboratory. The precision with which lattice parameters can be determined from edge positions is sufficient to perform high-resolution strain measurements in uniaxial stress.

Lattice constants often depend on the concentration of solutes in materials. The analysis of the gamma radiation following neutron capture can be used to detect variations of chemical composition. The measurements can be performed simultaneously with the strain measurements. Prompt gamma analysis (PGA) can detect hydrogen in the 30 to 50 ppm range. Electron-positron pairs are formed in the bulk material and positron annihilation radiation can be used to characterize plastic deformation.

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.