Recently I involve in this field and I did some experimental investigations which resulted in ultrafine/nano grained austenitic matrix in 304L steel with average grain size of ~ 480 nm (initial grain size was ~10 micron). I want to get a estimation of the lower limit of grain refinement, theoretically. My method was advanced thermomechanical of martensite reversion.
Hi. First, thanks for your answer but apparently you didn't perceive my purpose exactly.
Hello,
As far as I know there is no theoretical limit to the size of the grains when you create them, except the bare minimum dictated by the atomic configuration of grain boundaries (let's say 1-10 nm). There might be, of course, limits due to a specific protocol.
However, small grains are thermodynamically unstable, so the material tends to recrystallize spontaneously: bigger grains grow out of small grains. The question is, of course, how many time they take to do so (polycrystalline steel is unstable in itself, but 10µm grains will take forever to coalesce at room temperature). You could do the energy-barrier-crossing thermodynamics rough calculations to find out what is the size of grains with lifetime ~1 year at 300K (for instance).
I would guess though that this will give a much bigger size than what is observed in reality. Even for small fractions of alloying elements, kinetic effects such as Zener pinning increase the nucleation time by significant factors. (This is related to the fact that low-alloy steels have widely varying behaviours under quench, which is basically recrystallization of austenite into alpha phases.)
http://en.wikipedia.org/wiki/Recrystallization_%28metallurgy%29
http://en.wikipedia.org/wiki/Zener_pinning
I suggest that Gibbs-Thompson effects will enter into the thermodynamical equation for
your specimens.
The shear transformation of austenite to martensite in low and medium carbon steels produces extremely fine crystals (grains), mostly thinner than 0.5 microns.
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