ИСТИНА

For intermetallic compounds, the fact of decreasing of the unit cell parameters during the transition from a disordered to an ordered state is well known. The reasons for this effect are still under discussion, mainly on the basis of models involving its energy or geometric mechanisms (see review article [1]). Theoretical calculations by means of the Bragg–Williams model based on embedded atom method potentials led the authors of [2] to the conclusion that in the case of a structural transition of the second order, changes in the cell parameter are not linear. We have studied the processes of atomic ordering in Fe-Al(Cr) and Fe-Ga alloys by neutron TOF-diffraction at the HRFD instrument (IBR-2 pulsed reactor, JINR, Dubna) in wide temperature range (20 – 850) °C. For instance, in the Fe-25Al-9Cr composition it was found that during the second order transition from the partially ordered B2 state to the ordered D03 state the relative decrease in the atomic volume is about 0.4%. The high sensitivity of the method made it possible to clearly observe a similar change in the lattice parameter down to 0.08% in others Fe-Al(Cr) alloys, as well as in the Fe-xGa compositions. Earlier, it was found that the microstructure of both types of studied alloys is organized in the form of a disordered matrix and clusters of nanoscopic size with an ordered atomic structure dispersedly embedded in the matrix (see [3] for details). In contrast to the conclusions of [2], our results indicate that the relationship between the cell parameter and the ordering factor of the structure is linear. 1. R. W. Cahn, Intermetallics 7 (1999) 1089. 2. G. Chen, X. Ni, T. Nsongo, Intermetallics 12 (2004) 733. 3. A.M. Balagurov, I.A. Bobrikov, S.V. Sumnikov, I.S. Golovin, Phys. Rev. Materials 3 (2019) 013608 (9).