There two types of voids i.e., tetrahedral and octahedral site. My question is at which site C atoms preferer to go in Fe for (ferrite phase) BCC structure and (austenite phase) FCC structure, and how does it affect its properties. Tell me about Martensite as well and how it effect its properties.
You can refer to the following article:
“Carbon dissolution and diffusion in ferrite and austenite from first principles”
By: De-en Jiang and Emily A. Carter
Phys Rev B, June 2003
Dear Malik Awais ,
What is very important to you to keep in mind as a metallurgists, when analysing Fe based alloys, as known as steels for lower contents of carbon and silicon or cast irons for higher contents of carbon and silicon, take the Fe-C equilibrium diagram and check the solubility limits of carbon in both ferrite and austenite.
You will realise that the solubility limit of carbon in ferrite or alpha phase (BCC Fe crystal), is approximately 0.02% at eutectoid temperature (The eutectoid reaction describes the phase transformation of one solid into two different solids. In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C. The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma phase).
The solubility limity of carbon in austenite (FCC Fe crystal) is 2.14% at eutectic temperature...
It means that the solubility limit of carbon in austenite (FCC) is more than 100 times bigger than in ferrite (BCC)!
When you really understand it, a lot of "misterious" phase transformations like the metastable formation of martensite will be very clear and easy to understand! Realise that there is no martensite in the Fe-C equilibrium diagram! So why if you quench a carbon steel you may find martensite at the microstructure???
The answer is 100% linked with what I wrote before... All that carbon in solid solution in the austenite, if the steel is chill out slowly, will difuse to form ferrite and cementite (FeC3) through an eutectoid reaction (austenite forming ferrite + cementite at 723°C). But if the steel is quenched, or chill out fast, the carbon will be trapped in the austenite crystal once the diffusion will not occur properly at lower temperatures. The solution found by the universe is the formation of martensite that is a BCT crystal, very hard but brittle....
I would suggest you one of my presentations:
Presentation BASICS OF STEEL
I hope it helped you!
Keep on touch!
André
Respected Sir, We know Fe lattice(ferrite/austenite) has octahedral and tetrahedral sites. I need to know, whether C atom occupy octahedral site or tetrahedral site in all the three crystals.
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