The phase diagram of Iron CarbonaExplain the transformation

The phase diagram of Iron - Carbon(a)Explain the transformation of solid phases of pure iron using Gibbs free energy plot. (b)Explain why Austenite has higher solubility than Ferrite using the change in interstitial sites for iron crystals. (c)Explain why the maximum wt% of carbon is 6.7 in typical phase diagrams of iron – carbon alloy. What does happen if the wt% of carbon increases further? (d)Why do we (mechanical engineers) use the metastable phase diagram of iron-carbon instead of the equilibrium phase of iron-carbon?

Solution

A)

G = H –TS•

A change to a lower enthalpy state (H < 0, exothermic) usually decreases the randomness (S < 0). (e.g. solidification and oxidation)•A change to a higher entropy state (S > 0) usually increases the enthalpy (H > 0, endothermic). (e.g. melting and evaporation processes)•All of these processes are characterized by a lowering of the Gibbs free energy: G = (H -TS) < 0•A crystal at equilibrium has its ………….Gibbs energy.

b)

Austenite has face centered cubic (FCC) crystal structure and ferrite has body centered cubic (BCC) crystal structure.Fe, like any crystalline solid, has 2 types of interstitial sites: octahedral & tetrahedral. In FCC (as in Austenite), the octahedral sites are significantly larger than the tetrahedral sites, which is not the case with BCC (as in ferrite), where the interstitial sites are almost same.Therfore interstitial gap in BCC is much smaller than that of FCC.This is why carbon which occupies interstitial site has higher solubility in austenite (FCC).the amont of solubility of carbon in ferrite is near 0.0218% whereas in austenite it is nearly 2%

C)

The composition of iron carbide is 6.7 wt% C is perfect because at that composition the \"equilibrium\" structure will be single phase Fe3C. The diagram of iron carbon diagram is actually Fe-Fe3C diagram. Beyond 6.7% of C there is nothing interesting related to any field has been found.after 6.7% the diagram does not contain amy Fe3C content and diagram will be of Fe-FeC diagram.At 6.7 weight %, every fourth atom in the structre is a carbon atom so we have 25% Carbon atom.

D)

In practical cases we don\'t use carbon more than 5% in an iron carbon alloy. If carbon % in the alloy will be more than that then alloy would become brittle.

Now cementite has 6.67% carbon and it is metastable. It will take many years (which is very large when compared to practical applications) for cementite to decompose into graphite. Thus we can use iron cementite diagram for practical use instead of iron graphite phase diagram.