Iron interstices

Diffusion of Carbon. When carbon atoms are deposited on the surface of the austenite, these atoms locate in the interstices between the iron atoms. As a result of natural vibrations the carbon atoms rapidly move from one site to another, statistically moving away from the surface. Carbon atoms continue to be deposited on the surface, so that a carbon gradient builds up, as shown schematically in Figure 5. When the carbon content of the surface attains the equihbrium value, this value is maintained at the surface if the kinetics of the gas reactions are sufficient to produce carbon atoms at least as fast as the atoms diffuse away from the surface into the interior of the sample. 

Figs. 20.45fl and b, respectively. It follows from the diagrams that Ni and C are austenite (7-phase) stabilisers while Cr is a ferrite (a-phase) stabiliser. The fact that the interstices in an f.c.c. structure are bigger than those in a b.c.c. structure accounts for the fact that C is much more soluble in f.c.c. iron (austenite) than in b.c.c. iron (ferrite) (Fig. 20.44). 

Steel is an alloy of about 2% or less carbon in iron. Carbon atoms are much smaller than iron atoms, and so they cannot substitute for iron in the crystal lattice. Indeed, they are so small that they can fit into the interstices (the holes) in the iron lattice. The resulting material is called an interstitial alloy (Fig. 5.48). For two elements to form an interstitial alloy, the atomic radius of the solute element must be less than about 60% of the atomic radius of the host metal. The interstitial atoms interfere with electrical conductivity and with the movement of the atoms forming the lattice. This restricted motion makes the alloy harder and stronger than the pure host metal would be. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Erosional transport of iron stones may have led to a mechanical concentration of these spherical bodies in alluvial sediments or in marine depressions and caused their breakdown Trummererze). These deposits may be recemented by Fe oxides, predominantly goethite, formed in situ in the interstices. 

If austenite is cooled slowly toward ambient temperature, the dissolved carbon in excess of 0.022 weight % comes out of solid solution as cementite, either in continuous layers of FeaC (pearlite) or as layers of separated FeaC grains (bainite). In either case, the iron is soft and grainy, as with cast iron. If, on the other hand, the hot austenite is cooled quickly (i.e., quenched), the 7-Fe structure goes over to the a-Fe form without crystallization of the interstitial carbon as cementite, and we obtain a hard but brittle steel known as martensite in which the C atoms are still randomly distributed through the interstices of a strained a-Fe lattice. Martensite is kinetically stable below 150 °C above this temperature, crystallization of FesC occurs in time. 

Steel is different. Most forms of steel are made by alloying iron with carbon. High-carbon steels, which contain up to 1.7% carbon, are stronger and harder than either of their constituents, iron and carbon (in the form of coke or charcoal). This change in properties is similar to that produced by adding tin to copper, but the structure of the alloy is entirely different. Iron has an atomic radius of 140 pm, but that of carbon is only 67 pm. So small is the carbon atom in relation to iron, that it cannot replace iron in the metallic-bonded lattice. Instead, the carbon atoms slip into the interstices between the iron atoms. This type of alloy is called—not surprisingly—an interstitial alloy. 

There are also many oxides that are nonstoichiometric. These commonly consist of arrays of close-packed oxide ions with some of the interstices filled by metal ions. However, if there is variability in the oxidation state of the metal, nonstoichiometric materials result. Thus iron(II) oxide generally has a composition in the range FeO0.90 to FeOo.95, depending on the manner of preparation. There is an extensive chemistry of mixed metal oxides. 

Fe(OH)2 is prepared from Fe° solutions by precipitation with alkali. When freshly precipitated under an inert atmosphere (in a Schlenck apparatus for example) Fe(OH)2 is white (Bernal et al., 1959). It is, however, readily oxidized by air or even water upon which it darkens. Fe(OH)2 has the CdL type structure with hep anions and half of the octahedral interstices being filled with Fe ions. The crystals form hexagonal platelets. In solution Fe(OH)2 transforms by a combination of oxida-tion/de-hydration/hydrolysis reactions to other iron oxides and hydroxides. The end product depends both upon the order in which these processes occur and upon their rates. 

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