[thedoc (OP)]

When talking about diffusion in the metal during heat treatment, does the metal melt and then diffusion occurs as it would in a liquid? Or does it stay solid and the atoms just diffuse through it?
Asked by Android Neox, SecondLife


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[thedoc (OP)]

We answered this question on the show...



Nick -   We’re absolutely talking about it still being a solid and the atoms do move through them and there are processes that occur.  That's why we say they mix better in a liquid state because things can move more easily than when they're forcing their way through a solid lattice.  But that's why we have to give them time and temperature.  The temperature helps in getting them going, makes things move a little easier and the time is just to allow them to move the distances, but yes, they are moving through the solid state.
Ben -   And Roger, when you're designing a new metal, again obviously, you have to take into account the sort of heat treatments it’s likely to go through.  How do you model that into it as well?
Roger -   Well again, there are thermodynamic databases available for estimation of the melting temperatures and the heat treatment temperature windows that are available.  Those of course are based on thermodynamic potentials which have been estimated, and put into computer based models.  Most of the time, those models are accurate and can be used to estimate whether any given alloy has an appropriate heat treatment window to allow that to happen without the melting occurring.  Interestingly for the turbine blade alloys, they all have to be heat treated at temperatures really quite close to the melting temperature, but obviously, lower than it so that the turbine blade won't start to melt incipiently during the heat treatment process.

[CliffordK]

I'm not sure about foundries.  However, if you read about hydrogen leakage, Hydrogen can slowly diffuse through solid metals, and cause both a loss of the gas, and embrittlement of the container.  It can actually be quite difficult to contain.  Other gases and liquids may also diffuse through metal containers at a much slower rate.