We suppose thermodynamic equilibrium in a phase diagram, where cooling rates are infinitely long and diffusion in both phases can finish completely. So when we cross the solidus line and we are left with a crystal, will it have a monocrystalline structure?

Now, take for example the iron-carbon phase diagram, when cooling a hypoeutectoid steel below its upper critical temperature, ferrite will start to form around the grain boundaries of the austenite. But if there are no grain boundaries where can the ferrite start to nucleate from?

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    $\begingroup$ I'm a little confused by the question as it seems to be a little too theoretical. If you're cooling rates are infinite you would never cross the solidus. If they are not then you would almost certainly have multiple nucleation points starting at the edges (source of the cooling effect). If you were looking at a volume small enough you may be able to get a single nucleation point, but you would then likely end up with something comparable to a single grain from a larger structure. $\endgroup$
    – Kagekiba
    Nov 28 '18 at 13:13
  • $\begingroup$ +1 especially for a question that's gone unanswered for 4+ years yet with only 1 upvote! I know this is a long-shot, but since you talked about grain boundaries I wonder if you have any idea how to answer this question? It's been one of our longest-lasting unanswered questions and I'd really like to clean up the unanswered queue! $\endgroup$ Jun 22 at 2:29

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