The distribution of carbon between the ferrite and grain boundary segregations in the model nanocrystalline mechanical synthesized powder steels based on the fe - 1 mas. % C, doped by Cr, Mn, Ni, Si, N, B
Abstract:
The influence of alloying elements like as Cr, Mn, Si, Ni and N, B on the formation of nanostructures in the nanocrystalline powder steels based on the Fe - 1 mas. % C, obtained by mechanical alloying, was investigated. In the nanocrystalline steels carbon is distributed between the volumes of ferrite nanograins and the grain boundary segregations. The studies of materials structure were carried out using the X-ray diffraction methods. The grain size of the ferrite was determined using CMWP program by the broadening of the diffraction lines. The concentration of carbon in ferrite was determined by the change of the ferrite lattice parameter. The influence of carbon and alloying elements were taken into account for this. The amount of carbon in ferrite was calculated from the carbon concentration in ferrite. It was believed that the rest of the carbon is distributed in the grain segregations. The concentration of carbon in the grain boundary segregation was calculated as the amount of carbon per unit surface area of the grain boundaries (mole/m$^2$). The concentration of carbon in the ferrite varies in the range of 0.1 - 0.37 at. % and depend on the alloying elements. Cr, Si and B increase the concentration of carbon in ferrite, Mn, Ni reduce this concentration, N not changes it. The concentration of carbon in the grain boundary segregation changes in the range of (1.1 - 2.1)$\cdot$10$^{-5}$ mole/m$^2$. Ni and  increase the concentration of carbon in the grain boundary segregation, Ñr reduce this concentration, Mn, Si, N not change it. The concentration of carbon in segregations slightly depend on its concentration in ferrite. The carbon concentration in the grain boundary segregations mainly depend on the size of ferrite grains which form in the process of mechanical alloying. The correlation between the concentration of carbon in the grain boundary segregations and grain size caused by the fact that segregation effectively reduce the energy of grain boundaries. This fact promotes the formation of new boundaries and grain refinement in process of mechanical alloying. Carbon promotes greater grain refinement in contrast with the pure iron. The alloying elements like as Mn, Cr, Si, N just a little reduced grain size as compared to the binary Fe-C alloy. Other alloying elements (Ni and B) result in increasing of grain size. Large boundaries area corresponds to a smaller grain size and carbon, not included in the composition of the ferrite, is distributed on this area.
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