Hans Georg Priesmeyer

FSS - A Novel RTOF-Diffractometer Optimized for Residual Stress Investigations

Abstract 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.

Neutron Diffraction for Non-Destructive Strain/Stress Measurements in Industrial Devices

Abstract High resolution neutron diffraction is becoming widely used as a non-destructive tool to investigate three-dimensional stress fields in polycrystalline metals and ceramics. Lattice strains can be derived from Bragg peak positions measured with an accuracy better than 1 : 104. Both crystal and time-of-flight diffractometry are used to perform the measurements, as is demonstrated by examples. New instrumental developments like the European PREMIS project at ISIS, England, the Fourier correlation spectrometer FSS at Geesthacht, Germany and the energy-dispersive Neutron Transmission Stroboscope at LANSCE, United States, are introduced. Recent results on industrial devices are presented.

Investigation of the austenite-bainite transformation in gray iron using real time neutron transmission

Abstract In globulitic gray iron (3.6 wt.% C, 2.4 wt.% Si and 0.45 wt.% Mn) the decomposition of austenite into bainite is investigated in real time using a new method based on neutron scattering. During the phase transformation transmission spectra of thermal neutrons are continuously recorded containing specific Bragg edges. Thus, the transformation is followed in-situ time resolved by evaluating the relative (micro) structural fractions of the transforming phases from the heights of the corresponding Bragg edges. The progression of the transformation is described and the results are discussed. The observed late time deviations from the Avrami law are explained by residual stresses of the second kind (homogeneous microstresses), which occur during the transformation because of the different specific volumes of the phases.

Real time neutron transmission investigation of the austenite-bainite transformation in grey iron

Abstract 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.

Optimization of detectors in time-focusing geometry for RTOF neutron diffractometers

Abstract 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.

BRAGG-EDGE TRANSMISSION AS AN ADDITIONAL TOOL FOR STRAIN MEASUREMENTS

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.

On the shape of the diffraction peaks measured by Fourier reverse time-of-flight spectrometry

Abstract 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.

Progress in single-shot neutron transmission diffraction

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.

In situ Investigation of Plastic Deformation by Neutrons and Positrons—a Novel Approach

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.

Single-shot neutron transmission diffraction

Abstract 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.