Фазовая диаграмма системы Fe-O

К оглавлению: Другие диаграммы (Others phase diargams)

Fe-O (Iron-Oxygen) H.A. Wriedt The stable solid phases of the Fe-O system at 0.1 MPa are (1) the terminal bcc solid solution with a narrow range of composition denoted ferrite, (aFe) or ( dFe), with the designations used below 912 and above 1394 C, respectively; (2) the terminal fcc solution denoted austenite or (gFe), with a narrow range of composition extending approximately from 912 to 1394 C, the stable temperature range of gFe; (3) the fcc oxide, denoted FeO, Fe1-xO, FexO, FeO1+x, or FeOx (sometimes with specific x values), wustite, wuestite, wЃstite, or iozite, with a broad range of compositions, which may possibly be subdivided into regions with differing types or degrees of order; (4) Fe3O4 or magnetite, which is monoclinic and almost stoichiometric below -149 C and is fcc above -149 C, with a range of composition considerably broadened at high temperatures; and (5) the rhombohedral oxide Fe2O3 or hematite, which is almost stoichiometric at low temperatures, but which has an appreciably broadened range of composition at high temperatures. The assessed phase diagram essentially follows that of [46Dar]. As in other assessments [Hansen, Kubaschewski] that retained all the main features of that diagram, details of the univariant and invariant positions were modified because of perceived improvements in measurements. The temperatures of the allotropic transformations, Curie point, and the melting point of Fe are from [ 82Swa]. The experimental investigations of the (Fe) boundaries in the Fe-O system prior to 1955 are summarized in [Hansen] and [62Vol]. Most of these results are discredited from application to the high-purity Fe-O system, because they were made before the use of zone-refined Fe. [58Sif] and [59Sey] showed that apparent O solubility values measured in impure Fe specimens exhibited gross errors that were not due simply to precipitates of known oxides of reactive impurities. The compositions of (dFe) coexisting with L2 that were reported by [66Hep] are adopted, but may be lower than the actual values. The revised value of 0.029 at.% O [67Swi] for monotectic (dFe) at 1528 C is adopted; a linear solidus is depicted in the assessed diagram. The [67Swi] composition for (dFe) coexisting with (gFe) and L2 (0.019 at.% O) is also depicted. Because there are apparently no other reported data, the experimental [67Swi] values for the compositions of (gFe) coexisting with wustite are adopted. The assessed compositions of (gFe) coexisting with (aFe) and wustite at 912 C, with L2 and wustite at 1371 C and with (dFe) and L2 at 1392 C, are those proposed by [67Swi]: 0.0007, ~0.0094, and ~0.0098 at.% O, respectively. The compositions stable at 0.1 MPa hydrostatic pressure range from 51.2 at.% O at about 912 C to 54.6 at.% O at 1424 C. The stoichiometric composition "FeO" is outside the range. At its Fe-rich boundary compositions, wustite coexists with (aFe) from 570 to 912 C, with (g Fe) from 912 to 1371 C, and with liquid from 1371 to 1424 C. At its O-rich boundary compositions, wustite coexists with Fe3O4 between 570 and 1424 C, which are the temperature limits of its stable range. Except at its eutectic and peritectic termini, the solidus apparently was investigated directly only by [31Pfe]. The curve shown in the assessed diagram is that derived by [46Dar] by combining their experimental solidus temperatures and gas compositions with their thermodynamic data relating gas and solid compositions. Wustite (W) participates in four invariant equilibria of the condensed Fe-O system at 0.1 MPa. In its stable range, wustite exhibits no first-order transformations, but metastable wustite cooled below about -80 C undergoes antiferromagnetic ordering. Controversy surrounds claims that stable wustite exhibits second-order transformations, with boundaries separating several discrete fields in X-T space [89Val]. These transformations are assumed to be associated with changes in defect ordering. Of the four invariants involving wustite, that of (gFe) + W = (aFe) may be placed quite accurately at 912 C because of the very small O solubility in ( Fe). The assessed composition of wustite, 51.2 at.% O, is that reported by [ 45Dar] and confirmed by others. The chosen value for the temperature of the eutectoid equilibrium W = (aFe) + Fe3O4 is 570 C, at 51.4 at.% O, in agreement with the [83Kna] nonexperimental assessment. The third invariant, the eutectic equilibrium L2 = (gFe) + W, is located at 1370 C [24Tri, 31Pfe], 1380 C [32Bow], or 1371 C [46Dar]. There is excellent agreement in the eutectic wustite compositions 51.2 and 51.3 at.% O reported by [31Pfe] and [46Dar], respectively; the [46Dar] values are adopted. The fourth invariant, the peritectic equilibrium L2 + Fe3O4 = W, was located at 1430 C by [31Pfe] and at 1424 C by [46Dar]. Disagreement between [ 31Pfe] and [46Dar] on the indicated compositions of peritectic wustite (53.8 and 54.6 at.% O, respectively) is appreciable; the [46Dar] values are adopted. The existence of a fifth invariant point at the presumably peritectoid equilibrium, (aFe) + W + Fe3O4, is implicit in the claim of [84Liu] that stoichiometric FeO is stable relative to (aFe) and Fe3O4 below 465 C. This claim has not been adopted in the preparation of the assessed diagram. According to [58Ark1] and [58Ark2], elevation of the hydrostatic pressure lowers the eutectoid temperature of wustite, displaces the eutectoid composition to higher O concentrations, and shifts its Fe-rich and O-rich boundaries to higher Fe and O concentrations, respectively. At <301> 3.6 GPa, the Fe-rich boundary at 770 C is at about the composition of stoichiometric " FeO" [67Kat]. According to thermodynamic calculations of [75Kur] for 700, 1000, and 1300 C, which contradict part of the [58Ark2] conclusions, both the Fe- rich and O-rich boundaries are shifted by increasing pressure until they reach a limit at the "FeO" composition (50 at.% O). Higher pressures (different for each boundary) are required to reach this limiting composition as temperature increases; above about 30 GPa, stable wustite is essentially a line compound with 50.0 at.% O at all temperatures. It was indicated that, at pressures above about 18 GPa, Fe3O4 is unstable at all temperatures and O-saturated wustite coexists stably with Fe2O3. The eutectoid temperature was reported to decrease by 64 C/GPa [75Kur], 13.5 C/GPa [83She], or 45.5 C/GPa [84Liu]. According to [83She] and [84Mcc], Fe-saturated wustite approaches stoichiometric "FeO" in composition as pressure increases up to about 10 GPa ( depending on temperature), then retreats to higher O concentrations at still greater pressures. Very high pressure (>70 GPa) induces a transformation, possibly to the B2 (CsCl) structure [80Jea]. Application of hydrostatic pressure to Fe3O4 induces a transformation at room temperature [70Mao]. The equilibrium pressure for coexistence of the low- pressure (cubic) phase (LPM) and the high-pressure phase (HPM) has not been evaluated accurately because of hysteresis. Transformation to HPM at room temperature requires 22 to 27 GPa [70Mao, 74Mao, 75Syo, 86Hua], but reversion to LPM does not occur above 5 [70Mao] or 3.4 GPa [86Hua]. Severe hysteresis persisted to 600 C; from consideration of the experimental transition pressures (increasing and decreasing), a value of -68 C/GPa for the temperature dependence of the actual boundary pressures and a value near 21 GPa at 25 C were estimated [86Hua]. On its Fe-rich side, Fe3O4 coexists with ( aFe) below 570 C, with wustite from 570 to 1424 C, and with L2 from 1424 C to its congruent melting point at 1596 C. Although lower O concentrations have been reported in metastable Fe3O4, e.g., 56.657 at.% O at 245 C [67Col], the stable lower boundary for coexistence with (aFe) is quite precisely at the stoichiometric composition 57.143 at.% O [ 46Dar]. No data showing deviations are available. On its O-rich side, Fe3O4 is in equilibrium with aFe2O3 at lower temperatures. In the condensed system without O2 pressure restriction, this boundary terminates at 1539 C [71Cro] in a eutectic equilibrium, where Fe3O4, aFe2O3, and L2 coexist. From 1539 to 1596 C, the upper boundary of the Fe3O4 phase field is its solidus. In instances where the system is restricted to O2 pressures of 1 atm (0.1013 MPa), the upper boundary between 1457 and 1582 C corresponds to this O2 isobar, intersecting the curve for coexistence with aFe2O3 and the solidus at these respective temperatures. On its Fe-rich side, aFe2O3 is in equilibrium with Fe3O4. Reported boundary compositions of aFe2O3 coexisting with Fe3O4 lie between 59 and 60 at.% O, but the deviations from 60 at.% O vary by more than a factor of 10, with no two unrelated sets agreeing, except for partial agreement between [41Sch] and [ 70Roe]. More recent sets of measurements, with a range of >200 C [61Sal, 67Kom], differ by a factor of 2 to 3, although the quality of experimentation appears comparable. The tentative adoption of the [67Kom] data (and their extrapolation [80Gul]) in the assessed diagram is arbitrary. According to [ 78Spe], the composition of aFe2O3 coexisting with Fe3O4 at 1457 C and 0.1 MPa O2 is 59.82 at.% O, in good agreement with the [80Gul] value of 59.79 at.% O. On the O-rich side of aFe2O3, no higher oxide has been observed in stable coexistence, even at O2 pressures exceeding 0.1 MPa. Up to 1447 C, aFe2O3 equilibrated with O2 at 0.1 MPa exhibits no detectable deviation from the stoichimetric composition (60.0 at.% O) [78Spe]. Moreover, [71Dra] observed no excess O in aFe2O3 equilibrated with O2 at 0.1 GPa and 500 to 700 C. Above 1447 C, aFe2O3 at 0.1 MPa O2 is substoichiometric [78Spe], reaching the composition 59.8 at.% O and saturation with respect to Fe3O4 at 1457 C [46Dar] or 1455 C [69Sch]. The [46Dar] value is adopted, consistent with the [78Spe] and [Kubaschewski] assessments. The O concentrations of L1 in equilibrium with (dFe) between 1538 C and the monotectic point were shown experimentally to be linear with temperature [68Kus, 70Kus]. The critical point of the miscibility gap has not been observed; [84Oht] roughly estimated its location from the [78Fis] data at about 2830 C and 47 at.% O. Compositions of L2 on the O-rich side of the miscibility gap were measured only by [71Dis]. They reported three points at 1785, 1880, and 1960 C, which when extrapolated yield the composition 50.51 at.% O at 1528 C, in excellent agreement with the monotectic L2 composition, 50.48 at.% O, reported by [46Dar] for 1524 C. The only experimental values for compositions of L2 on the wustite liquidus at 0.1 MPa are apparently those of [31Pfe] and [46Dar]. The former obtained two rough points on this liquidus segment. He extrapolated a fitted curve to its L2 = (gFe) + W (1370 C and 50.72 at.% O) terminus and to intersect his experimental Fe3O4 liquidus at 1430 C and 53.78 at.% O. His convex upwards curve lies higher than the shallowly inflected (almost straight) curve that [ 46Dar] drew through their six experimental points between 1371 C at 50.92 at.% O and 1424 C at 54.19 at.% O. The Fe3O4 liquidus was studied experimentally by [31Pfe], [38Whi], and [46Dar]. Only the last of these observed the curve on both sides of the maximum at the congruent melting point of Fe3O4 (57.14 at.% O); the [31Pfe] and [38Whi] data were only for lower or higher O concentrations, respectively. There apparently are no reliable data for the Fe2O3 liquidus compositions. The equilibrium gases over solids or liquids of the Fe-O system, according to the composition of the condensed phase, may contain significant fractions of the following molecular species: Fe, O2, O, FeO, and FeO2. The existence of FeO2(G) was first reported by [75Hil] in gas over Fe2O3; [84Smo] recognized its presence in their analysis of gas thermodynamics. Molecular FeO2 is not known to be the dominant gaseous species in any conditions, but Fe, FeO, O2, or O may be dominant. According to [78Shc], the congruently vaporizing composition of the condensed phase is displaced to lower O concentrations as temperature increases. Below about 1360 C, the congruently vaporizing solid is Fe3O4; for a short interval above 1360 C, it is wustite. At still higher temperatures, the congruently vaporizing condensed phase is liquid. [84Smo] showed that the widely quoted value for O concentration, 52.74 at.% O, in congruently evaporating liquid at 1600 C [46Dar] is greater than the actual value. When quenched below about 200 C, wustite can be retained without transformation to (aFe) and Fe3O4 or other metastable phases for indefinite periods. The quenched wustite may not have retained exactly the defect structure of the original at a higher temperature. Three types of metastable wustite - P›, P››, and P››› - were reported [68Man]. [66Her] reported a possible miscibility gap in supercooled wustite from electron microscopy and XRD studies with boundaries at 300 C of 50.5 and 52.1 at.% O. Metastable wustite with the composition of stoichiometric "FeO" was reported to occur as a decomposition product of annealing at 225 C wustite quenched from the stable region [70Hen]. In another study of decomposing quenched wustite, various transient wustite compositions were reported [59Hof]. With low-temperature heat capacity measurements, [29Mil] detected a transition in metastable wustite at about -90 C. Subsequent studies, including magnetic susceptibility measurements, showed that the anomaly was due to a change from paramagnetic to antiferromagnetic [70Mic]. A change in crystal structure from cubic to rhombohedral wustite (LT) at this N‚el point, TN, was discovered by [ 50Tom]. The deviation from cubic is almost undetectable at the O-rich compositions; the deviation of a from 60 C increases with Fe content. There is considerable variation among measured values of TN, which depends on composition (O/Fe) and apparently is sensitive to other factors, such as thermal history and impurity content. Different methods of measurement also yielded slightly different values [67Koc]. Measurements of the composition effect on TN [67Koc, 68Fin, 68Mai, 70Mic, 84Sri] are not concordant, but most indicate that TN increases about 8 to 12 C/at.% O and that TN is near -80 C in Fe-rich (metastable) wustite. According to [67Oka], increasing pressure up to 0.6 GPa raises TN by 6.5 C/GPa. The effect of dissolved O on the Curie point of (aFe) is unknown, but because of the very small O solubility, the displacement from 770 C is probably not detectable. No magnetic changes occur in the stable range of wustite. The ferrimagnetic-paramagnetic transition in Fe3O4 was observed at 576 [84Has] and 580.3 C [87Hau1, 87Hau2]. Data on the effect of O concentration are unavailable, but the width of the composition range is very slight at 580 C, which is the adopted value. Increasing pressure raises this transition temperature [69Sam, 79Leb, 82Gov] by about 20 C/GPa. Pure, annealed, coarse-grained aFe2O3 at 0.1 MPa pressure and zero external magnetic field exhibits magnetic transitions at -10.5 с 1.5 C (first order) [ 63Mor, 64Ise, 65Fla, 71Jac, 81Gie, 87Ami] and at 688 с 7 C (second order) [ 62Fre, 63Gil, 64Ise, 65Hil, 65Lie, 67Sch, 75Gro, 75Hon, 87Nov]. Values of the lower transformation temperature are denoted by TM, with M referring to Morin, who rediscovered this transition [50Mor, 71Jac]. Below TM, aFe2O3 is antiferromagnetic, with the spins parallel (and antiparallel) to the hexagonal c axis; above TM, the spins are in the hexagonal basal plane, but their slight deviations from perfectly balanced antiparallelism confer weak ferromagnetism with the basic antiferromagnetism. 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KronmЃller, and F. Haberey, Phys. Status Solidi (b), 144, 411-422 (1987). 87Hau2: M. Haug, M. F„hnle, H. KronmЃller, and F. Haberey, J. Magn. Magn. Mater., 69, 163-170 (1987). 87Nov: L. Novakovic, A. Sreckovic, J. Dojcilovic, and M. Napijals, High Temp. - High Pressures, 19, 437-442 (1987). 89Val: P. Vallet, Bull. Alloy Phase Diagrams, 10(3), 209-218 (1989). Submitted to the APD Program. Complete evaluation contains 2 figures, 7 tables, and 370 references. Special Points of the Fe-O System