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Stainless steel: diffusion and phase transformations in nanocrystals

The influence of grain size reduction from the usual um sized to the nm-size range of austenitic-ferritic stainless steels is of great interest. It is especially the physical-mechanical and the corrosive properties that are improved by such a reduction of grain sizes. The project targets development of a method to produce this kind of nanocrystalline (nc) materials by a powder metallurgical route based on the pressure sintering of ultrafine powders.

Nanocrystalline compacts have been prepared from stainless steel powders with particle size 10-100 nm and composition 22 % chromium, 11 % nickel, 0.6 % titanium. The a-g-phase transformation was studied basically by using X-ray analysis, transmission electron microscopy, scanning electron microscopy, Mossbauer spectroscopy and thermal dilatometry. A method for diffusion studies using Rutherford backscattering is also proposed. A new structural size effect is found. A stainless steel in the nanocrystalline state is identified as an austenitic-ferritic steel, while a casted steel of the same chemical composition is of austenitic type. The largest particles have exclusively ferritic structure while the smaller ones have basically austenitic structure. Mossbauer and thermal analysis have shown that the distribution of the species in the ferritic particles is very inhomogeneous. The role of diffusion, especially the role of intercrystalline (surface) diffusion, on the distribution of species and the phase transformation behaviour is discussed. Tin is used as a diffusion species. By Rutherford backscattering it is shown that the diffusion rate along nanograin boundaries is considerably larger than in massive course-grained materials of the same composition.

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