G. D. Bokuchava

Correlation Fourier diffractometry: 20 Years of experience at the IBR-2 reactor

The high-resolution Fourier diffractometer (HRFD) was commissioned at the IBR-2 pulsed reactor at FLNP JINR in 1994. The specific feature of the HRFD design is the use of fast Fourier chopper for modulating the primary neutron beam intensity and the correlation method of diffraction data acquisition. This allowed to reach with HRFD extremely high resolution (Δd/d ≈ 0.001) over a wide range of inter-planar spacings at a relatively short flight path between chopper and sample (L = 20 m). Over time, a lot of diffraction experiments on crystalline materials, the main goal of which was to study their atomic and magnetic structures, were performed at HRFD. Successful implementation of the Fourier diffractometry technique at the IBR-2 reactor stimulated the construction of yet another Fourier diffractometer intended for internal mechanical stress studies in bulk materials (FSD, Fourier Stress Diffractometer). In this paper the experience of using this technique at the IBR-2, which is a long-pulse neutron source, is considered, the examples of HRFD studies are given, and possible solutions for existing technical problems of using correlation diffractometry and ways of increasing the intensity and resolution of HRFD are discussed.

Neutron Fourier diffractometer FSD for residual stress studies in materials and industrial components

Neutron diffraction study of residual stresses in materials became widely used in the world due to high penetrating power of neutrons. Therefore, to study residual stresses, the FSD (Fourier stress diffractometer) was developed at the IBR-2 reactor channel (Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Russia), which, due to a special correlation technique (a fast Fourier chopper for modulating the primary neutron beam intensity and the RTOF method for data acquisition) makes it possible to obtain high-esolution diffraction spectra Δd/d = 4 × 10-3. This diffractometer was developed taking into account world experience in the study of residual stresses in materials; experience in the development of such devices in Russia and abroad was also used. The FSD diffractometer itself and its current state are described.

Evolution in the dislocation structure of austenitic 16Cr-15Ni-3Mo-1Ti steel depending on the degree of cold plastic deformation

High-resolution neutron diffraction is employed to determine the main structural parameters and microstrain levels of several austenitic 16Cr-15Ni-3Mo-1Ti steel samples subjected to different degrees of cold plastic deformation and estimate the dislocation densities thereof. It is demonstrated that the anisotropic diffraction peak broadening observed in the neutron experiment is caused by variations in the dislocation-contrast factor and can be satisfactorily described with the help of the used model. Comparative analysis of the results obtained by neutron diffraction and transmission electron microscopy is performed.

Neutron radiography and tomography facility at IBR-2 reactor

An experimental station for investigations using neutron radiography and tomography was developed at the upgraded high-flux pulsed IBR-2 reactor. The 20 × 20 cm neutron beam is formed by the system of collimators with the characteristic parameter L/D varying from 200 to 2000. The detector system is based on a 6LiF/ZnS scintillation screen; images are recorded using a high-sensitivity video camera based on the high-resolution CCD matrix. The results of the first neutron radiography and tomography experiments at the developed facility are presented.

Reverse time-of-flight neutron diffraction study of residual stresses in perforator's striker

Abstract In this paper we present the results of residual stress measurements on a perforators striker, which is used in the mineral resource industry. The stresses in this component arise due to various processes of heat treatment. The residual stress distribution in the material after isothermal hardening and after carburising was investigated. Three strain tensor components were measured at various depths. The microstress level was estimated from diffraction peak broadening. The experiments were carried out on the HRFD diffractometer at the pulsed reactor IBR-2 (Dubna, Russia).

Application of neutron stress diffractometry for studies of residual stresses and microstrains in reactor pressure vessel surveillance specimens reconstituted by beam welding methods

The high-resolution neutron-diffraction technique is used to determine the residual stresses and microstrains in unirradiated reactor pressure vessel surveillance specimens reconstituted by means of different welding methods. Comparative analysis of the results demonstrates that the lowest level of residual stresses is observed in the specimens reconstituted via electron-beam welding. The level of microstrains thereof is maximum, indicating a high dislocation density in the material.

Investigation of microstrain in dispersion-strengthened steels

Microstrains in three series of the samples made of various stainless austenite dispersion-strengthened steels, which are used to produce different construction parts in reactor engineering, have been studied by high-resolution neutron diffraction. The influence of the temperature and duration of thermal treatment on the precipitation of particles of the dispersion-strengthening phase and on the variation in parameters and microstrains of the crystal lattice has been investigated. An increase in microstrain with the drop of coherence has been observed for all studied steels.

Microstrain in dispersion-hardened steels

Using high-resolution neutron diffraction, microstrain was investigated in three series of samples of stainless austenitic dispersion-hardened steels, which are used as various structural reactor components. The effect of temperature and duration of heat treatment on the precipitation of dispersion-hardened phase particles, as well as on lattice parameter changes and microstrain, was studied. An increase in microstrain upon coherence failure was observed in all the steels.

Neutron Time-of-Flight Stress Diffractometry

Over recent decades, the diffraction of thermal neutrons has become a powerful tool for solving various actual problems of materials science. To carry out scientific investigations on this theme, a neutron time-of-flight Fourier diffractometer FSD was developed and has been successfully operated for many years at the IBR-2 pulsed reactor in the Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna. To ensure high resolution of the instrument, a special correlation technique is used, i.e., a fast Fourier chopper for modulation of the primary-neutron-beam intensity and the reverse time-of-flight method for data acquisition. The current state of the FSD diffractometer and its capabilities are described and examples of performed experiments are given.

Neutron diffraction studies of laser welding residual stresses

The residual stress and microstrain distribution induced by laser beam welding of the low-alloyed C45 steel plate was investigated using high-resolution time-of-flight (TOF) neutron diffraction. The neutron diffraction experiments were performed on FSD diffractometer at the IBR-2 pulsed reactor in FLNP JINR (Dubna, Russia). The experiments have shown that the residual stress distribution across weld seam exhibit typical alternating sign character as it was observed in our previous studies. The residual stress level is varying in the range from -60 MPa to 450 MPa. At the same time, the microstrain level exhibits sharp maxima at weld seam position with maximal level of ∼4.8·10-3. The obtained experimental results are in good agreement with FEM calculations according to the STAAZ model. The provided numerical model validated with measured data enables to study the influence of different conditions and process parameters on the development of residual welding stresses.