Mechanochemical synthesis of Fe$_{1-x}$Cr$_x$ (x=0,2-0,5) nanocrystalline powders and its consolidation by means of magnetic-pulse compaction

The bcc Fe-Cr alloys are the basis of ferritic steels and considered as a candidate materials for shells of fuel elements of fast neutron reactors operating under irradiation and at temperatures up to 700 $^{\circ}$C. However, as it was shown in the second half of the last century, the structural temperature stability and, accordingly, embrittlement of these alloys limits their practical application. In the period from 1990 to the present time, considerable attention is drawn to the production of nanocrystalline Fe-Cr alloys by means of mechanical alloying with a Cr concentration from 20 to 50 %. Studying the formation of $\sigma$-phase in the heat-treated nanocrystalline Fe$_{56}$Cr$_{44}$ alloy showed that $\sigma$-phase is formed at 480 $^{\circ}$C, and at 522 $^{\circ}$C the formation of $\sigma$-phase is completed after heating up treatment for 100 hours, i.e. at significantly lower temperatures and shorter times than in case of macrocrystalline samples. The question arises whether or not the formation of $\sigma$-phase nano-inclusions in a two-phase region at lower Cr concentrations can prevent grain growth during heat treatment, and thus maintain a high level of mechanical properties, that is characteristic for nanocrystalline alloys. To answer this question, one have to carry out a detailed study of the mechanical alloying (MA) process in the samples with the concentration of chromium C$_{Cr}$ $\le$ 44 %. The second challenge was to obtain bulk samples by magnetic pulse compaction provided the conservation of nanocrystalline state. As a matter of course it has been shown by means of X-ray diffraction and Mössbauer spectroscopy on $^{57}$Fe nuclei that mechanical alloying of Fe$_{1-x}$Cr$_x$ (x $\le$ 0.3) system leads to dissolution of Cr atoms in Fe lattice throughout the whole process of mechanical treatment. In case of x $>$ 0.3, the initial stage is also characterized by dissolution of Cr in iron, and then starting from a certain time of mechanical treatment the reverse process of Fe dissolution in Cr matrix is observed. Magnetic pulse compaction technique have been used to obtain consolidated nanocrystalline samples ($\langle L \rangle$ $\approx$ 20 nm), with the density being equal to 67–90 % of the theoretical value. To improve the characteristics of compacts the temperature of pressing needs to increase up to 500 $^{\circ}$С.