Phase Behavior of Iron-Carbon Alloys

In general, for a mixture of two or more pure elements, there are two types of solid-solution alloys that may be obtained. Type 1 alloys are completely miscible with one another in both liquid and solid states. As long as the Hume-Rothery rules are satisfied, a random or ordered substitutional alloy will be produced. We will see many examples of these alloys for a variety of metal dopants in stainless steels. By comparison, type II alloys are only miscible in the molten state, and will separate from one another upon cooling. These alloys are usually associated with compound formation from the alloying of metals or metals/nonmetals that are too dissimilar in their reactivities (e.g., Cu and A1 to form CuAl2 precipitates). The eutectic composition represents the lowest melting point of type II alloys. 

As seen earlier, the steps used to purify iron involves carbonaceous material in order to remove the oxide-based impurities via exothermic formation of CO and CO2. Hence, carbon will be pervasive in a variety of concentrations throughout all 

Fe composition (crystal structure) Unit cell parameters (A) 

In general, the density of interstitial solid solutions is given by Eq. 15. Since the change in volume is usually more significant than the increase in number of unit cell atoms, interstitial solids usually exhibit a decrease in density, relative to the pure allotrope. For instance, the density of pure iron (7,874 kg m ) shows a significant decrease upon interstitial placement of carbon in cast irons (ca. 7,400 kg m ). 

By definition, iron containing between 0.15 and 1.4 wt.% C is typically referred to as steel.Hence, although we typically think of steel as containing chromium and other metal dopants, some y- and 5-phases of pure iron could also be considered forms of steel. Steels with a carbon concentration of 0.83 wt.% undergo a transformation from austenite into two intimately mixed solid phases at a temperature of 723°C. Although this phase transformation looks like a eutectic, the material above 

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