Possibilities of X-ray diffractometric methods by study of ultrafine-grained and nanostructured metal materials
|Margarita Isaenkova , Yuriy Perlovich , Vladimir Fesenko , Olga Krymskaya|
Moscow Engineering Physics Institute (MEPhI), Kashirskoe shosse, Moscow 115409, Russian Federation
By the present boom in the field of ultrafine-grained and nanostructured metal materials it is meant that there are reliable experimental methods, allowing to classify all materials from the standpoint of their grain size. However, the real situation is more complicated and requires additional methodical elaborations. When studying deformed metal materials, coherent domains and separate grains practically can not be distinguished by the X-ray line profile, since in both cases we deal with fragments of undistorted crystalline lattice, though differing in boundaries with neighbors. Methods, based on the X-ray line profile analysis, give the direct information on the size of coherent domains, but allow only indirect judgments concerning the size of grains in deformed metal materials. The profile of X-ray line (hkl), obtained by the standard geometry of diffractometric measurement, characterizes only some group of reflecting grains, corresponding to the single texture component, which in the general case can be insignificant and non-representative. Advantages of the new method of generalized pole figures (GPF) by X-ray certification of the material condition are considered. The GPF method includes reconstruction of profiles of the same X-ray line (hkl) for grains of all possible orientations and characterizes textured metal materials by distributions of substructure parameters instead of their single values.
In the case of metal materials with FCC or BCC crystalline lattice these distributions characterize relative fractions ν of axes <hkl>, along which the corresponding substructure parameter is equal to that or another value. In the case of metal materials with HCP lattice the similar histograms, constructed by X-ray reflections from basal planes (0001), show distributions of volume fractions ν of grains, characterized by different values of substructure parameters, measured along basal axes.
The most effective X-ray method to ascertain formation of ultrafine-grained or nanostructured condition under SPD bases on the quantitative texture analysis. Operation of crystallographic deformation mechanisms (slip and twinning) results in regular grain reorientation and development of textures, depending on the used deformation scheme. Since crystallographic deformation mechanisms can not operate in nano-dimensional grains, the alternative non-crystallographic mechanism of grain slippage along boundaries becomes active and results in their accidental rotations as well as in scattering of texture maxima. Measurement of this scattering and relative estimation of volume fractions, deformed by means of crystallographic and non-crystallographic mechanisms, characterizes the structure condition of material under SPD. The consideration is illustrated by numerous results, obtained by X-ray studies of SPD metal materials.
Presentation: Oral at 11th European Powder Diffraction Conference, Microsymposium 13, by Margarita Isaenkova
See On-line Journal of 11th European Powder Diffraction Conference
Submitted: 2008-04-24 18:07 Revised: 2009-06-07 00:48