SMALL ANGLE NEUTRONS SCATTERING IN MATERIALS SCIENCE

P. Strunz

Nuclear Physics Institute, 25068 Řež near Prague, Czech Republic

1. SANS characteristics

Specific properties of small-angle neutron scattering (SANS) method with respect to small-angle scattering of X-ray and synchrotron radiation (SAXS) are presented. The neutron sources are of much lower luminosity than synchrotron ones and even substantially lower than standard X-ray sources. On the other hand, higher penetrability of neutrons through majority of materials facilitates their use as a probe for bulk material and for in-situ studies at extreme conditions (low/high temperatures, mechanical loading, pressure).

As scattering amplitude of neutrons does not depend in a systematic way on the atomic number and can differ even for isotops, light elements can provide a significant scattering contrast and contrast variation technique can be relatively easily employed, too. Moreover, their magnetic moment enables to investigate magnetic nanostructures. A usually easier preparation of SANS samples should be mentioned as well: in certain cases we can talk even about non-destructive testing.

Generally, SANS is particularly useful for microstructural investigations where X-ray cannot deliver needed information either for lack of scattering contrast or due to strong absorption (in the sample or in the sample-environment windows).

Complementary arrangements of SAS experiment known for X-ray (pin-hole, double-crystal) are used for neutrons as well (an example pin hole SANS facility can be found in [1]). Nevertheless, the low absorption of neutrons allows the use of neutron-diffraction optics to improve the performance of double-crystal SAS setting. In Fig. 1, such device equipped with analyzer (bent perfect Si crystal) in fully asymmetric geometry is displayed [2].

2. Typical SANS investigation of solid materials

To develop materials with physical properties suited to a particular application and to optimize the technology of their processing, it is essential to know the physical mechanisms taking place in the material under different external conditions. Particularly, the presence of microscopic pores or precipitates is a characteristic feature of many types of solid materials. Here, a broad field of applications exists for SANS. Some typical examples will be presented in the talk.

Selected SANS studies performed on V4 pin-hole facility in HMI Berlin using various sample environments will be reported. Examples taken from the research of Ni-base superalloys (technologically important materials for high-temperature applications) are focused on the in-situ solutionizing of gamma prime precipitates [3], presence of TCP phase, rafting [4] and influence of heat-treatment conditions on the precipitate morphology [5]. The 2D map in Fig. 2 shows typical example of SANS pattern coming from dense system of gamma prime precipitates in single-crystal Ni-base superalloy. Size, distance, misorientation of precipitates and their volume fraction can be determined through a model fitting to such pattern.

Study of porosity in plasma-sprayed ZrO₂ thermal-barrier coatings is shown as well. The use of these materials on Ni-base superalloy blades enables appreciable increase of the temperature in the turbine combustion chamber. Evolution of their pore microstructure (strongly determining the physical properties of the material) during high-temperature exposition is discussed.

Superplasticity of ceramics is an interesting phenomenon with promising practical applications. The Y-TZP ceramics can be made into a product having a required shape and size by means of plastic workings. Double-crystal SANS was used to determine volume fraction of voids in dependence on the superplastic strain [6]. The evaluation of 2D pin-hole SANS patterns provide information on anisotropy of pore morphology as well as on coexistence of two different populations of pores at high strain rates.

[1] http://www.hmi.de/bensc/instrumentation/instrumente/v4/v4.html

[2] http://omega.ujf.cas.cz/CFANR/k13.html

[3] P. Strunz, D. Mukherji, R. Gilles, A. Wiedenmann, J. Rösler And H. Fuess: J. Appl. Cryst. 34, (2001), 541-548.

[4] P. Strunz, G. Schumacher, W. Chen, D. Mukherji, R. Gilles and A. Wiedenmann: Applied Physics A (ICNS 2001 Proceedings), accepted

[5] R. Gilles, D. Mukherji, P. Strunz, S. Lieske, A. Wiedenmann, R.P. Wahi: Scripta Materialia 39, 715-721 (1998)

[6] S. Harjo, N. Kojima, Y. Motohashi, J. Šaroun and V. Ryukhtin: submitted to the Annual meeting of Japan Institute of Metals (2002).