Small-angle neutron scattering (SANS)
Small-angle neutron scattering ranks among several techniques (e.g. X-ray small angle scattering, electron microscopy) widely used for investigation of condensed matter structure. The SANS technique is concerned with the measurement of structures within the size range 30-105 Å. This size range corresponds to momentum transfer Q values ranging from 105 to 0.3 Å-1. For measurements in the range of small Q values, we employ the double-crystal (DC) nondispersive setting using elastically bent perfect crystals.
We keep the priority in employing elastically bent perfect crystals in the DC setting for measurement in the Q range from 10-4 to 10-2 Å-1 (medium resolution range). This experimental arrangement makes possible easy variation of the resolution according to the experimental requirements [3.1]. Feasibility of this instrument has been demonstrated in the practical use since the year 1983 [3.2, 3.3]. At the beginning of the year 1993 we introduced into a routine operation an improved version of the DC diffractometer when employing the second crystal (analyzer) in the fully asymmetric diffraction geometry. In such a way the angular deviation of the scattered neutrons is converted to the linear spatial scale [3.5, 3.6] which in fact enables us to use a linear position sensitive detector (PSD) and to acquisite efficiently the SANS spectrum [3.7, 3.8]. Promising properties of the new device has been proved by investigations of porous structure of Vycor glasses [3.9] and of a morphology of precipitates in the creep exposed CMSX2 single crystal [3.10] (in collaboration with University of Ancona, Italy).
In the last years, we use a complementarity of DC arrangement and conventional SANS facilities (HMI Berlin, KFKI Budapest) in material research of single-crystal nickel-based superalloys [3.11,3.12] and of plasma sprayed materials which are both technologically very important materials.
The two-phase microstructure consisting of g� precipitates growing in the gamma phase matrix is the basic feature which determines a high creep-resistance of nickel-base superalloys, the materials frequently used in aircraft and land-based turbines. Outstanding high-temperature mechanical properties of these materials strongly depend on the morphology of the precipitates and thus also on the applied heat treatment. The morphology is conventionally studied by transmission electron microscopy, however, this and other standard methods of materials science do not usually characterize the microstructure completely. SANS, which provides bulk-averaged information, has been found to be a powerful tool for investigation of microstructural inhomogeneities in single-crystal alloys. A number of studies documents an applicability of SANS particularly to investigation of precipitation in single-crystal Ni-base alloys.
Recently, our research has been focused to investigation of precipitate microstructure in ZS26, CMSX3, SC16 and SCA superalloys. Anisotropic 2D SANS curves provide information on the average shape of the ordered cuboidal g�-precipitates [3.13,3.14,3.15,3.16] (the asymptotic region of a SANS curve) as well as on the precipitate dimension and distances between them [3.12,3.14,3.15] (central part of the scattering curve). The original data evaluation procedure [3.17,3.18] (numerical modeling and fitting) allows - in many cases - to estimate well the volume fraction of the precipitates even without the knowledge on the scattering contrast. The SANS results are usually related to the used heat treatment procedures or creep expositions. In the case of SC16, the SANS measurement revealed the presence of well oriented additional phase [3.19, 3.20, 3.21] which could not be effectively characterized by the other experimental methods. Detection of this additional phase is important even it occupies only small volume fraction because it can cause initialization of cracks in the material.Our investigations were also recently extended to non-destructive studies of lamellar precipitates resulting from high-temperature creep-exposition of superalloys.
The plasma-sprayed materials find various applications in chemical, ceramic, aerospace industries, medicine and other fields in the form of coatings or even self-supported components, which can improve substantially the thermo-mechanical, chemical or electrical properties of final products. These materials are characterized by the presence of wide spectrum of pores and cracks, which mainly determine the material properties. The knowledge of porosity characteristics and their dependence on the conditions of matarial preparation has therefore essential importance. SANS is one of the techniques suitable for the characterization of porosity in mesoscopic size range. However, evaluation of SANS measurements is typically complicated by the presence of strong multiple scattering. The measurements at conventional (�pinhole�) instruments were mostly limited to the determination of specific surface of pores, which can be evaluated from outer parts of scattering curve, usually not affected by multiple scattering. The double-crystal (DC) diffractometer (DN-2) makes high-resolution SANS measurements possible at short neutron wavelengths, where the influence of multiple scattering is much weaker. In 1998, we have finished the development of the software for SANS data evaluation in multiple scattering regime. This program combines standard indirect Fourier transform method with formalism for calculation of multiple scattering and allows to fit model microstructure (e.g. polydisperse system of spheres) on the data measured at DC instruments. By this method, we could first time evaluate size distribution of pores in plasma-sprayed alumina in the size range from 100 Å to 1 mm and to study evolution of porosity in dependence on the thermal treatment of the samples. In the overlapping regions of scattering vectors, the results are in good agreement with measurements on identical samples performed at pinhole instruments at KFKI Budapest and NIST Gaithersburg [3.11, 3.22-3.24].
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