Grain-by grain mapping the real structure of polycrystalline materials

Jaroslav Fiala, Michal Kolega

New Technology-Research Centre, West Bohemian University, Universitni 22, Plzen

X-ray diffraction analysis of polycrystalline materials is usualy performed using the Bragg-Brentano parafocusing diffractometer [1]. Millions or even billions of crystals are irradiated and diffract in such an arrangement. The collected diffraction pattern represents a superposition of the more or less overlapped diffractions of all these crystals. Therefore, it is a function of the large number of parameters describing the distribution of the size, shape, orientation and position of crystallites in the irradiated region of the analyzed sample. The number of these parameters by far exceeds the information content of such a diffraction pattern, i.e. the amount of information that can be acquired by its evaluation. And this is the main cause of all problems and the reason of the low efficiency of the x-ray diffraction analysis of the real structure („mesostructure“) of the polycrystalline materials as (when) executed in the usual way [2].

Much greater efficiency may be achieved by a topographic technique analysing resolved diffractions of individual crystals of the polycrystalline aggregate (grain-by-grain method). A narrow primary beam is used for this goal in order that the number of irradiated crystals is small [3,4]. And, the intensity of diffracted radiation is measured not only along a single curve of the reciprocal space, which is enough in case of the Bragg-Brentano parafocusing arrangement. The topographic technique requires mapping of two-dimensional areas of the reciprocal space. These are the tracts in which the plane surface of the area positlon sensitive detector applied intersects the conical surfaces of diffracted beams. And, it is through out these tracts, where the azimuthal distribution of the diffraction spots is determined [5,6].

The examination of the azimuthal diffraction line profile, i.e. the size, number and shape of individual diffraction spots that discontinuous diffraction line consists of, reveals useful information on the materials' structure which cannot be obtained by other techniques. Such an information can be to advantage used in the development and optimization of technological processes [7] as well as, in the monitoring of processes which degrade the materials structure in course of their service [8].

So we used the x-ray diffraction topography (grain-by-grain mapping) to the monitoring of changes in the internal structure of high-pressure turbine casings in the course of their long-term service. The casings serve for admission, line and outlet of expanding steam, being reckoned among the most exposed components of the stator part of turbines. Therefore, their microstructure changes throughout the continuance of service which results in the degradation of their mechanical properties. We have found that the azimuthal profile of the diffraction lines of casings, becomes really too much different during their many years´ work. From these changes of the diffraction pattern we deduced e.g. that the crystallites of the iron matrix of a high-pressure turbine casing have grown in size from some 0.1 mm to more than 10 mm in the course of about 100 000 hours service. This huge structural change (by two degrees of order) was in no way perceptible under a light microscope. The grains which are seen by using the microscope are composed of crystals (mosaic blocks) which cannot be perceived through the microscope. But x-rays diffracted by these blocks (crystallites) are incoherent and that is why we can recognize the mosaic blocks, into which some grains are broken up, on diffraction patterns. This demonstrates the usefulness of the topographic technique of x-ray diffraction (grain-by-grain mapping) in monitoring structural degradation of steels which occurs in the course of service of steam turbines made of these steels and in their residual-life prediction. That is to say, that mosaic blocks considerably influence the movement of dislocations and in this way also the strength characteristics of materials.

Acknowledgements: This paper is based upon work sponsored by the Ministry of Education of the Czech Republic under research and development project LN00B084.

[1] W.Parrish, Science 110 (1949) no. 2858, 368-371.

[2] J.Fiala, R.L.Snyder: „Introduction to defect and microstructure analysis or the analysis of real structure" in Defect and Microstructure Analysis by Diffraction (ed. R.L.Snyder, J.Fiala, H.J.Bunge), Oxford University Press, New York 1999, pp.1-15.

[3] B.Ja.Pines, Fine-Focus X-ray Tubes and Applied X-ray Diffraction Analysis (in Russian), pp.175-182, GITTL, Moscow 1955.

[4] S.Weissmann, L.H.Lee, Progress in Crystal Growth and Characterization 18 (1989) 205-226.

[5] K.W.Andrews: „The interpretation of non-continuous diffraction rings" in X-ray Diffraction by Polycrystalline Materials (ed. H.S.Peiser, H.P.Rooksby, A.J.C.Wilson), The Institute of Physics, London 1955, pp.454-461.

[6] J.Fiala, S.Nemecek, Advances in X-ray Analysis 44 (2001) 24-31.

[7] J.Fiala, R.Medlin, S.Nemecek, NDT World Review 19 (2003) no.1, 54-55.

[8] V.Mentl, F.Hnilica, S.Nemecek, R.Medlin, J.Fiala, NDT Welding Bulletin 13 (2003) no.2, 21-30 .