In addition to the metal-metal phase diagrams discussed to this point, the other large class of phase diagrams are the metal-nonmetal systems. In particular, the most important nonmetal is oxygen. Of all the metal/oxygen systems, the Fe/O phase diagram, shown in Fig. 8.15, has received the closest study. On the diagram, the concen-
tration units are the O/Fe atom ratio rather than the mole fraction. The diagram begins at O/Fe = 1 ends at O/Fe = 1.5. The latter corresponds to Fe2O3, with the highest oxidation state of the metal 3+ (ferric). The other valence of iron is 2+ (ferrous). The diagram does not continue to O/Fe = 0 because there is no change from the two-phase regions listed on the left-hand side. Heavy horizontal lines in the diagram separate two-phase regions. The dashed lines represent isobars of the oxygen pressure in equilibrium with each point in the diagram. Single-phase regions are shaded
Other features of the diagram include:
• The 2+ valence state of iron should result in the compound FeO. However what actually exists is a broad (wide range of O/Fe) single-phase oxide called wustite, which is FeO deficient in Fe2+. This oxide is symbolized by "FeO", even though it is not a stoichiometric FeO (i.e., it does not have an oxygen-to-iron ratio of unity). Alternatively, wustite is written as Fei_xO, x indicating the degree of hyperstoichiometry (meaning O/Fe > 1) To maintain electrical neutrality, some of the iron must be in the 3+ state.
The single congruently-melting (meaning that the solid and the liquid into which it transforms have the same O/Fe ratio) compound magnetite, Fe3O4. Since ions cannot have a fractional valence, this compound is a mixture of 2/3 Fe3+ and 1/3 Fe2+. The average valence is 8/3 which, multiplied by 3, balances the 8 negative charges from the four O2- ions in the compound.
The single-phase magnetite region of the phase diagram consists of the iron-deficient compound Fe3-yO4. This O/Fe ratio requires that the fraction of Fe3+ be greater than the value of 2/3 in the stoichiometric compound Fe3O4.
The highest oxide, hematite, has a very narrow single-phase region above 1200oC. This oxide is oxygen-deficient, with the formula Fe2O3-x.
Two liquids exist above 1535oC: metallic iron and a liquid oxide.
• The diagram shows two eutectics,
- an oxygen-rich one at point A at the confluence of the magnetite and hematite solids and a liquid oxide
- another at point B where the two solids are wustite and y-Fe and the liquid is again an oxide.
• Point C is a eutectic-like invariant point involving a-Fe, magnetite and wustite.
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