An Important Phase Diagram - Fe-C

Text under construction - to be completed 5/14

One of the most technologically relevant phase diagrams of all is the Fe-C phase diagram, shown in Figure 10.10.1. This is no different than other phase diagrams you've seen, but we add some terminology that is specific to iron and steel. Note immediately that this is only a small region of the full Fe-C phase diagram: we cut it off at 6.7 wt% C, as it is only carbon compositions less than this that are relevant to iron- and steel-based materials.

The most relevant area of this phase diagram is between about 0.02 wt% C - 2.14 wt% C, which are termed "steels", which are traditionally very strong, and resistant to fracture due to the presence of carbon atoms in the iron lattice and the formation of microstructures containing carbon. This, combined with the extremely low cost and great abundance of terrestrial iron and carbon, makes steel one of the commonly used structural materials. As such, hundreds of years of technological development has led to the optimization of steel properties through fine control of the material's microstructure. More on this in Chapter 12.

Above 2.14 wt % C we have "cast irons", which are useful as well. However, in comparison to most steels, most cast irons are not malleable, even when hot, and cannot be worked or shaped easily. However cast irons - perhaps unsurprisingly - are easily cast: melted into a liquid and poured into a suitable mold. This has allowed for useful cast iron components to be fabricated for millenia. Common items made from cast irons are pipes, cookware, machine and automotive components, and bridges.

The $\alpha$ Phase Field

Let's explore the details of the phase diagram a bit, starting with pure iron (0 wt% C). At $400 ^{\circ}\text{C}$ we find ourselves in a very thin phase field labeled the $\alpha$ phase, which is a BCC structure. Because this phase (and some others) is so important we give it a special name: ferrite. The maximum solubility of C in this phase is quite low: only about 0.02 wt% C at $727 ^{\circ}\text{C}$. Carbon sits on the octahedral interstitial site in the BCC lattice in $\alpha$ iron. (Essentially because it induces less strain, overall, than the tetrahedral site.)

The $\gamma$ Phase Field

As we increase temperature of pure iron we undergo a polymorphic phase transition at $912 ^{\circ}\text{C}$. Here, the $\alpha$ phase converts to the $\gamma$ phase, which also has a special name: austenite. Austenit is FCC and has a much large maximum solubility of carbon of 2.14 wt% C (or 8 at%). $100\times$ that of what can be tolerated in the BCC phase. Interestingly, carbon is much more soluble in the more densly-packed FCC lattice because the octahedral site is relatively much larger in the FCC crystal structure compared the the interestitial sites in BCC, and so it can accommodate much more carbon! I told you those interestial sites were important!

It is, in part, this extensive phase field that makes steel such a superlative material. We can process the Fe-C mixture into the austenite phase field and ultimately utilize the eutectoid transformations that occur at $727 ^{\circ}\text{C}$ to control microstructural details. Without this single phase field with high carbon solubility, we wouldn't have had fine control over final microstructures. (Austenite is an important phase in some steels in it's own right, but that's a topic for a later course.)

The $\delta$ Phase Field

As we continue to raise the temperature of pure iron above $1394 ^{\circ}\text{C}$, we enter another low-carbon-solubility phase field, the $\delta$ phase. This region isn't so technologically important (although it does have impact on steels performing at high temperatures). It is therefore not named - although some call it $\delta$ ferrite - and doesn't even have its own Wikipedia page). It is interesting because it has the same BCC structure as ferrite! So - if it is the same structure, why does it have a different Greek letter? There are two reasons: it has its own phase field completely unconnected to the $\alpha$ phase, and it actually has distinct magnetic structure.

The $\beta$ Phase Field??

You might be asking - where is the $\beta$ phase? It's a good question - because phases are typically discovered and notated in order: $\alpha$, $\beta$, $\gamma$, etc. Well, there used to be one, but it was later determined to be the "same" atomic arrangement as the $\alpha$ phase, just with different magnetic ordering. This interesting paper from 2018 brings the topic up again, begging the question of "what's a phase", but the discussion and implications (while convincing, in my opinion) are pretty academic.