Фазовая диаграмма системы Fe-N

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Fe-N (Iron-Nitrogen) H.A. Wriedt, N.A. Gokcen, and R.H. Nafziger The established equilibrium solid phases of the Fe-N system at 0.1 MPa hydrostatic pressure are (1) the terminal bcc solution, (aFe) or (dFe), designations used below 912 C and above 1394 C, respectively; (2) the terminal fcc solution, (gFe); (3) the fcc nitride Fe4N; (4) the cph nitride, e; and (5) the orthorhombic nitride, Fe2N. In addition, ferrous and ferric azides (possibly FeN6 and FeN9) were reported; if actually occurring, they may be equilibrium phases. Experimental data for phases containing more than 0.5 at.% N are limited to temperatures below 810 C. The assessed phase diagram is based primarily on the work of [30Eis], [30Leh], and [50Par]. At elevated pressure, the terminal cph solution (eFe) is stable. The nonequilibrium phases that have been observed are (1) the bct nitride Fe16N2, (2) bcc martensite, and (3) bct martensite. At its N-rich phase boundaries (aFe) coexists with (gFe) from 912 to 592 C [ 75Hil] and with Fe4N below 592 C. The corresponding phases coexisting with ( dFe) are (gFe) from 1394 C to a peritectic equilibrium [80Gab] at about 1495 C and with L from 1495 to 1538 C. At its lowest temperature for stable existence in the condensed Fe-N system at 592 C, (gFe) coexists with (aFe) and Fe4N in a eutectoid equilibrium. At its Fe-rich boundary from 592 to 912 C and from 1394 to 1495 C, (gFe) coexists with (aFe) and (dFe), respectively. The equilibrium compositions of (gFe) coexisting with (dFe) have not been determined experimentally. At its N-rich boundary, (gFe) coexists with Fe4N between the 592 and 650 C eutectoids, above which it coexists with liquid and with e above 650 C up to a high temperature. (gFe) may be retained below its stable range by quenching. In the metastable state, when held isothermally at temperatures just below the stable range, it transforms to stable products. Further cooling of metastable (gFe) with <301> 0.7 at.% N causes martensitic transformation, with a bcc product at <2.5 at.% N and a bct product at >2.7 at.% N. These martensites are not stable phases. The martensite start temperature falls with increasing N content and reaches room temperature at 8.6 с 0.4 at.% N. Therefore, (gFe) with more than 8.6 at.% N is prepared easily and retained at ambient conditions. The compositions of Fe4N coexisting with (aFe) have been measured only above 400 C. The N concentration at this boundary decreases with increasing temperature. For Fe4N coexisting with (gFe), there is no change of N concentration between 592 and 650 C. Along its Fe-rich phase boundary above 650 C and along all of its O-rich phase boundary, Fe4N is in equilibrium with e. The Fe-rich boundary of e at its coexistence with Fe4N exhibits a temperature maximum at 680 C, where it is congruent with Fe4N. It decreases on the Fe- rich side of this maximum to the 650 C eutectoid equilibrium with (gFe) and Fe4N and on the N-rich side continuously to the limit of observation at about 350 C. Above 650 C, also extending from the eutectoid e composition, is the Fe-rich boundary of e for equilibrium with (gFe). At its highest N concentrations, e coexists with Fe2N, but this boundary has been observed only below about 500 C because of practical limitations in retention of the phases at the extremely high N2 fugacities involved. The Curie point temperature in the stable (aFe) range is approximately that of pure aFe (770 C). (gFe) is paramagnetic in its stable range. No Curie point has been observed in metastable (gFe) retained by quenching. The Curie point temperature of Fe4N varies from 480 to 508 C, between its Fe-rich and N-rich limits, respectively. Fe16N2 and the martensites are ferromagnetic in the ranges of temperature at which they have been observed. 30Eis: O. Eisenhut and E. Kaupp, Z. Elektrochem., 36(6), 392-404 (1930) in German. 30Leh: E. Lehrer, Z. Elektrochem., 36(7), 460-473 (1930) in German. 50Par: V.G. Paranjpe, M. Cohen, M.B. Bever, and C.F. Floe, Trans. AIME, 188, 261-267 (1950). 75Hil: M. Hillert and M. Jarl, Metall. Trans. A, 6, 553-559 (1975). 80Gab: R.M. Gabidullin, B.A. Kolachev, and E.V. Krasnova, Izv. V.U.Z., Chernaya Metall., (7), 5-9 (1980) in Russian; TR: Steel USSR, (7), 348-350 ( 1980). Published in Bull. Alloy Phase Diagrams, 8(4), Aug 1987. Complete evaluation contains 12 figures, 17 tables, and 125 references. Special Points of the Fe-N System