Why is the carbon solubility so different in ferrite vs austenite?

A phase diagram of Fe-C shows right away that austenite can withstand up to 2.14wt% C, whereas the solubility of C in ferrite is a measly 0.022wt% C. Why the big difference? Even more crazy, consider that ferrite is BCC which is only 68% densely packed while FCC is 74% packed. Why should the more densely packed structure be able to fit more carbon in the lattice? It has to do with the size of the interstitials! You can actually calculate the size of the interstitials in both and you'll see that austenite has larger interstitials so more carbon can fit in these without straining the lattice which eventually limits the solubility.

[@FearNoSteel]

Well, that was a wonderful explanation, I was always curious about how to visualize the ability of FCC to hold more carbon despite the fact that the BCC arrangement looks like it has more space. Ah, I see, the specific spots where the carbon attaches are bigger in the FCC, bravo.

[@رشيدالسامرائي-م8ب]

Thank you so much 🌹❤️

[@skydivenext]

this is fun and not sad like my teacher that has phd and master and cant explain he just shrugged it off like look the graph is more right so this graph more right it gets more carbon right so wonderfull right? well next chapter, while everyone looked dumfounded, it was all the semester like this, we know he knows but cant explain anything is ridicolous

[@TaylorSparks]

So sorry you had a bad experience with a teacher. He was probably trying his hardest. Either way, I'm glad this video helped clarify things. Please share with your fellow students.

[@joshtargo6834]

If you put 1 carbon atom in each interstitial, the solubility numbers are WAY too high. What actually limits the solubility if it's not the total number of interstitial voids?

[@TaylorSparks]

Strain! Each carbon adds to the strain of the lattice.

[@joshtargo6834]

@@TaylorSparks so essentially, the carbon doesn't quite fit, so it stretches the iron bonds in the lattice? And you can only fit a few carbon within a certain volume of crystal before the stretched bonds become too tight to allow any more?

[@TaylorSparks]

@@joshtargo6834 yup!!

[@joshtargo6834]

@@TaylorSparks are there other elements you can add to the iron matrix that are attracted to the iron stronger than the iron is attracted to itself? Something fully soluble? Oxygen to make iron oxide?

[@TaylorSparks]

@@joshtargo6834 good question. I don't think so, but I'm not expert in this field.

[@lakshaybhatt7643]

Great video.... Btw why does Iron change its structure from BCC to FCC when heated ?

[@lucaspatiri]

Because iron has mutiple allotropic forms, based on temperature it changes its properties and even its cristaline structure, that's the reason. You can also see allotropy in carbon for example , which changes from graphite, diamond and fullerene

[@Zucsebe]

@@lucaspatiri But fullerene doesn't occur naturally as compared to graphite and diamond. right?

[@vascodealmeidaflamengo1739]

Thank u

[@TaylorSparks]

my pleasure!!

[@vascodealmeidaflamengo1739]

Maybe u can answer me... To obtain martempering you have yo pass the line of MF(final transformation of martensite)

[@TaylorSparks]

@@vascodealmeidaflamengo1739 ya, but tempered martensite requires a subsequent reheating to low temperatures.

[@vascodealmeidaflamengo1739]

Really thank you so we have to do a tempering after a quenched

[@TaylorSparks]

@@vascodealmeidaflamengo1739 yes. If you just quench martensite it's actually brittle. There's a volume expansion from fcc to bct and it slightly cracks the material. however, if you temper it then you actually form very very small dispersions of the carbide phase which gives it its excellent properties.