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Apologies in advance - feel free to PM me instead...
Laymanic:
Consider the diagram below. Above T_eq, the solid phase has a lower (gibbs free) energy, so the system will minimise this and exist as a liquid. Above T_eq, the liquid phase has a lower energy so that becomes the preferred phase. These two lines are different because there are energy pros and cons to each phase. Liquid = high entropy = awesome. Solid = regular strong bonds = lots of energy released forming these = awesome.
Now considering our system (ie the iron). It can lower its free energy by changing phase above 912 degC. This is because the gamma/FCC phase has a more space per iron atom, so it can wiggle around more (increased temperature = increased wiggling = larger radius); that increases its entropy. However, it moves the atoms further apart, which costs energy. It's only able to afford this energy cost when the temperature is high enough to compensate through the increase in entropy.
In a bit more detail:
The gibbs free energy is a combination of the enthalpy (the net energy released in the reaction) and the energy required to adjust the level of entropy. It's why endothermic reactions (ones that get colder, like instant ice-packs) can take place, because they massively increase entropy - enough to compensate for their taking in energy. As an equation, Gibbs free energy (G) = Enthalpy (H) - Temperature (T,in K) x Entropy (S). G can be evaluated for any chemical, liquid, crystal etc. That 'times T' is why entropy increases are the stronger driving force for transitions at higher T. Note that the diagram above didn't show straight lines (as you would expect from G=H-TS). This is because H and S are both dependent on T as well, but they vary more slowly so you still see this downward trend with increasing T.TLDR;
Magnets.
hamrack-
Excellent discussion.
Does FCC really allow more wiggle room per atom? I'd have thought the lower packing efficiency of BCC would do that?
In that case, the wiggle room argument might help to explain the reappearance of BCC at high temperature, while we have to go looking for other effects at low temperature.
I've also seen it said that BCC has smaller spaces for interstitial atoms, which doesn't seem to line up with the lower density either. Is it that BCC has both more space and more spaces (points maximum distance from atoms, counted per atom), so the individual spaces come out smaller?
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Thanks, I think I half get most of that. In the second and third sentence is confusing me though. did you mean higher:
Above T_eq, the solid phase has a higher (gibbs free) energy, so the system will minimise this and exist as a liquid. Above T_eq, the liquid phase has a lower energy so that becomes the preferred phase.
moth
HHC
thanks,
so to paraphrase the top idea in laymanic:
The energy of the gamma phase iron is sufficient to maintain the shorter therefore higher energy bonds in FCC structure however as the temperature drops the gibbs free energy of the iron drops and it cannot maintain the FCC structure and goes to the BCC structure?
Apologies if I've missed the plot entirely.
for the second one, why does the iron radius change?