Фазовая диаграмма системы Mn-N

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Mn-N (Manganese-Nitrogen) N.A. Gokcen The assessed Mn-N phase diagram is based on review of the experimental data [ 29Hag, 33Sch, 51Zwi, 54Bas, 55Bri, 57Juz, 62Lih, 63Sch, 75Kud, 75Rab, 78Kor, 79Jar, 79Pom] and has been obtained by thermodynamic modeling. The liquid-gas boundary is based on the data of [78Kor]. With increasing N2 pressure, this boundary shifts to the right; at 10 atm, it shifts to the vicinity of 30 at.% N. The melting point of pure Mn is 1246 C [Melt]. The liquidus is estimated up to 4 at.% by using the thermodynamic equation for the depression of freezing point [75Gok]. Because most interstitials lower the melting temperature, it is assumed that this would lead to a eutectic of L <259> d + g. Furthermore, an increase in concentration of nitrogen increases the melting temperature of the alloy according to [32Och], who found that a 12.8 at.% N alloy was not molten at 1260 C and it melted in the vicinity of 1300 C. [61Gok] observed that an alloy saturated with N2(g) at 1 atm was molten above 1270 C. The liquidus in this vicinity was estimated by assuming that two peritectic type reactions occur, i.e., L + z <259> g and L + N2 <259> z. The dashed lines in the assessed diagram above 1100 C and 0 to 20 at.% N are estimates based on the foregoing assumptions and on a remote resemblance to the Mn-C system [ Brandes]. Furthermore, this portion of the diagram is in partial agreement with [51Zwi] and [67Mam], but the latter has a point of intersection of five boundary lines at 11.5 at.% N, which is difficult to reconcile with the phase rule. The solidus curves in equilibrium with the liquid are estimates. The compositions of terminal solid solutions b and d have not been investigated. The eutectoid temperature of 650 C for b <259> a + g is based on 645 C from [79Pom] and on the estimates of 650 C from [67Mam]. The a phase is stable below 720 C and contains probably no more than 0.5 at.% N according to [29Hag], who reported that the solubility of N in b is much higher than in a. The g phase extends from 605 C to about 1250 C. The eutectoid temperature for g <259> a + e varies considerably in earlier publications [57Juz, 75Kud, extrapolated]. The assessed value of 605 C from [79Pom] is the most reliable. The portion of the g phase field below 800 C is similar in shape to that in the diagrams by [67Mam] and [84Bur]. The lower and upper limits of solid solubilities of N in the z phase from 790 to 1250 C are from thermodynamic data of [79Jar] coupled with the phase equilibria of [75Kud]. The eutectoid composition of z›(=z) as 12.9 at.% is in accord with [75Kud] and the correlation by [79Jar]. The phase boundary curve for z› on the right side is quite similar to that in the Fe-N system by [87Wri] . If that curve for the Fe-N is extrapolated down 100 C from Fe4N to correspond to the eutectoid temperature of the Mn-N system, then approximately 13 at.% N is obtained for the eutectoid point, which should correspond to the eutectoid composition of the Mn-N system for z›. A value of 15.4 at.% N by [ 79Pom] would take the phase boundary curve on the right and the left to incompatible locations with the diagram of [75Kud] and the correlation by [ 79Jar]. However, the assessed eutectoid temperature of 790 C is much closer to the value of 782 C of [79Pom], whose "tectoid" transition temperatures obtained by thermal analysis are more reliable than those of other investigators. The upper limit of z boundaries would shift somewhat with the N2 pressure. For example, at 800 C and 100 atm, this phase would contain about 30.2 at.% N, i. e., about 20% higher than the value at 1 atm. However, at 1200 C, the corresponding increase is about 57% higher than 15.2 at.% N at 1 atm. Mn4N and its composition limits are quite similar to Fe4N [87Wri]. [29Hag] showed that the homogeneity range of this phase below 400 C is 20.0 to 21.4 at.% N, but above 550 C, the lower limit tends to 12.5 at.% N, which is now known to be too low. The limits of concentration of this phase were closely determined by [33Sch] by equilibrium with N2 at various pressures and temperatures; the results from a plot of composition versus pressure are 19.5 and 20 at.% N at 540 C and 18.8 to 21.1 at.% N at 740 C, the result for 800 C being inconclusive. More recent values of [62Lih] were obtained by preparing Mn amalgams, nitriding in N2(g) and NH3(g), and determining the upper phase boundary limits by X-ray and chemical analyses. Their results, which agree with the upper limits of [29Hag], are taken to be the upper limits of existence for the e phase. The lower limits at 400 and 600 C are those of [ 29Hag] and [33Sch], and at 790 C, those of [75Kud] and [79Pom]. The lower limit of 17.4 at.% N and the upper limit of 20.0 at.% N at 790 C are the best values selected from [75Kud] and [79Pom], with greater emphasis on [79Pom]. The e phase congruently transforms to the z phase at 890 C and 19.0 at.% N. This region is quite similar to the Fe-N system [87Wri]. Mn3N2 begins to decompose at about 600 C [29Hag, 51Zwi, 62Lih]. At approximately 710 C, it forms a type of peritectoid with N2 [51Zwi, 62Lih, Hansen]. The pressure of N2(g) in equilibrium with the q phase is not known, because it was formed by reaction with ammonia [62Lih]. It is certain that the pressure of N2(g) in equilibrium with q is very high. All of the solid-state phase boundaries were obtained by X-ray analysis and by gas-solid equilibria, each coupled with chemical analysis. In both methods, the experimentally measured property plotted versus composition indicated a discontinuity or a break. The resulting phase boundaries were then plotted by all the investigators without the individual data points. The discontinuities by gas-solid equilibria were not always sufficiently sharp. Furthermore, X-ray data on quenched samples may not always represent the equilibrium concentrations at high temperatures. A great deal of work remains to be done to establish the phase boundaries on a much firmer basis. The magnetic transformation (Curie temperature) was found to be 465 to 475 C by measurements on samples nitrided with N2 [48Gui]. However, [57Juz] found that Mn nitrided with ammonia exhibited different upper temperatures depending on the history of preparation, possibly because of the presence of dissolved hydrogen. The sample prepared at 400 C yielded 485 C as the upper temperature, whereas all the samples prepared at various temperatures yielded about 475 C as the lower temperature. The average value of 480 C for the upper temperature and that of 470 C for the lower temperature are shown in the assessed diagram. It should be noted that the e phase is the only ferromagnetic phase in this system. Mn(N3)2 was prepared by [34Fra] by reacting Mn with hydrazoic acid. It is not a product in equilibrium with N2(g). No physical data are available for this compound. Martensitic transformation of g phase in the compositional vicinity of Mn8N is a possibility. In fact, a tetragonal distortion of this phase was reported to be an equilibrium phase by numerous investigators, until it was shown by [ 75Kud] that the quenched g phase is not an equilibrium phase. It is quite likely that metastable Mn-N alloys can be obtained by splat quenching, because other Mn-base metastable alloys have been obtained by this technique. 29Hag: G. H„gg, Z. Phys. Chem. B, 4, 346-370 (1929) in German. 32Och: R. Ochsenfeld, Ann. Physik, 12, 353-384 (1932) in German. 33Sch: R. Schenck and A. Kortenbr„ber, Z. Anorg. Chem., 210, 273-285 (1933) in German. 34Fra: E.C. Franklin, J. Am. Chem. Soc., 56, 568-571 (1934). 48Gui: C. Guillaud and J. Wyart, Rev. Met., 45, 271-276 (1948) in French. 51Zwi: U. Zwicker, Z. Metallkd., 42, 274-276 (1951) in German. 54Bas: Z.S. Basinski and J.W. Christian, Proc. Roy. Soc. (London), A223, 504- 560 (1954). 55Bri: C. Brisi, Metall. Ital., 47, 405-408 (1955) in Italian. 57Juz: R. Juza, H. Puff, and F. Wagenknecht, Z. Electrochem., 61, 804-809 ( 1957) in German. 61Gok: N.A. Gokcen, Trans. Met. Soc. AIME, 221, 200-201 (1961). 62Lih: F. Lihl, P. Ettmayer, and A. Kutzelnigg, Z. Metallkd., 53, 715-719 ( 1962) in German. 62Tak: W.J. Takei, R.R. Heikes, and G. Shirane, Phys. Rev., 125, 1893-1897 ( 1962). 63Sch: H. Schenck, M.G. Frohberg, and W. Kranz, Arch. EisenhЃttenwes., 34(11), 825-830 (1963) in German. 67Mam: G.Sh. Mamporiya, Soobshch. Akad. Nauk Gruz. SSSR, 45(3), 663-667 (1967) in Russian. 75Gok: N.A. Gokcen, Thermodynamics, Techscience Inc., Hawthorne, California ( 1975). 75Kud: H. Kudielka and H.J. Grabke, Z. Metallkd., 66(8), 469-471 (1975) in German. 75Rab: A.V. Rabinovich, V.E. Nemogai, M.I. Tarasev, S.B. Vukelich, V.G. Rizun, and V.P. Perepelkin, Metall. Koksokhimiya Resp. Mezhved Nauk - Tekn. Sb., (44), 68-72 (1975) in Russian. 78Kor: G.J.W. Kor, Met. Trans. B, 9, 97-99 (1978). 79Jar: M. Jarl, Met. Trans. A, 10, 511-512 (1979). 79Pom: R. Pompe, Scand. J. Metall., 8(2), 51-54 (1979). 84Bur: A. Burdese, D. Firrao, P. Rolando, and M. Rosso, Chim. Ind., 66(7-8), 456-460 (1984) in Italian. 87Wri: H.A. Wriedt, N.A. Gokcen, and R.H. Nafziger, Bull. Alloy Phase Diagrams, 8(4), 355-377 (1987).