1. Global and Local Equilibrium
|Phase Transformations, Supersaturations and Equilibrium|
|Phase transformations are everyday stuff. A new phase must form whenever something freezes or melts, evaporates or condenses, dissolves or precipitates. This new phase must nucleate - it must start somewhere and sometime as a cluster of just a few atoms or molecules. Let's look at a few examples; the first ones are the every-day kind:|
|Liquid water evaporates, the
resulting water vapor is dissolved in the air and invisible. Water vapor in
supersaturation eventually condenses to
water droplets. Now you can see it - it's a cloud. The water droplets grow and
eventually rain will fall. If the droplets freeze, we have hail or snow. There
are two phase transformations in there (solid Û liquid; liquid Û
The nucleation of the first tiny droplets needs some help, e.g. by suitable "dirt particles". That's why "rain making" is possible to some extent.
|Salt or sugar dissolves in water because at the
beginning the water is undersaturated with
salt or sugar components. The solids now have "disappeared", you
can't see them anymore. In the sun the water evaporates, the solution becomes
supersaturated, and salt / sugar crystals
"appear" again because they precipitate out of the liquid solution.
The process is far easier and produces better crystals if some nucleation help is provided, e.g., by inserting a toothpick into the solution:
|Water freezes. Watch a frozen puddle or any ice surface closely, and you see that freezing tends to start at some special points where first nucleation occurred.|
|Water boils. The steam bubbles never appear just somewhere in the water but always at "favored" nucleation points. Usually a (tiny) flaw at the pot bottom acts as nucleation center, but but also any implement you stick into the hot water will do. Rough surfaces (wood) are better than smooth surfaces (glass).|
|Now a few examples of the not-so-everyday kind:|
|Vacancies or interstitials in some crystal become supersaturated. They "condense" or agglomerate into a small void or into discs of interstitials, producing "stacking faults", respectively. Below are two examples from transmission electron microscopy.|
|Carbon atoms dissolved in steel become supersaturated. The condense or precipitate into Fe3C cementite particles.|
|A-type atoms / molecules in B-type materials (crystal / glass / polymer, you) become supersaturated. The precipitate into some AB kind of material like cementite or kidney stones.|
|The key word above is "supersaturation". What does it mean? Well - go back to the "making vacancies" module to get a first idea. Now let's go on.|
|What we do here is to look at the basic equation for point defect equilibrium once more:|
|This little beauty tells you exactly how many point defects you need for equilibrium at some specific temperature T. Chances, however, are that you have a concentration quite different from the equilibrium or "nirvana" concentration. This leaves two possibilities:|
|In both cases you are not in global equilibrium, the smallest value of the free energy that can possibly exist.|
|We have two basic kinds of point defects: intrinsic PD's, meaning essentially vacancies, and extrinsic PD's, meaning foreign atoms. The crystal can somehow make and get rid off vacancies (and interstitials as far as some are present), so there is a general possibility that equilibrium can be attained. Nevertheless, we might have supersaturations or undersaturations of vacancies, too. Let's look at that possibility first.|
|When you heat
up a crystal, the equilibrium concentration of vacancies is going up
exponentially. Since the production of vacancies takes some time, chances are
that the actual concentration is lacking
behind the one needed. We have undersaturation.
However, since heating up means it gets hotter and hotter and everything needing some energy becomes more likely to happen, it takes less and less time to produce enough vacancies for reaching equilibrium.
In other words: you cannot maintain undersaturation for very long. Nirvana is easy to reach and will eventually prevail.
is entirely different. Things get sluggish, vacancies cannot disappear as fast
as needed, and eventually you are going to "freeze in" a
supersaturation of vacancies that might be with you forever.
Nirvana or global equilibrium, the very best state of being, is unreachable forever. The time has come to look for the second-best state of being. For example, if you can't achieve nirvana, you still can hope for being reborn as Material Scientist. Send me large amounts of money and I will hope with you.
|The choices for the second-best
option for vacancies are a bit more limited:
|The catch is that precipitates of all
kinds cannot spring into full-grown existence just so. The need to grow bit by
bit - and this is a torturous process in the beginning and possibly in the end.
In the beginning the vacancies are highly mobile but need to find a hard-to-get nucleus, where agglomeration can start. In the end there are enough precipitates to go to, but you can't move anymore.
|It's a bit like the (old-fashioned) boys chase
girls. In the beginning the boys are highly mobile, able and motivated, but
they need to find one of those elusive girls before clustering can start. In
the end, there are plenty of "girls" or nuclei (woman still live
about 7 years longer than men) but the ability of the few remaining unattached
men has sadly deteriorated.
But enough foreplay by now. Let's gird our loins and look at the most simple case first: a perfect crystal with nothing but vacancies.
|Vacancies and Supersaturation|
|We are looking at a hypothetical case. We start with a perfect crystal at nirvana, i.e. a crystal with no
dislocations, grain boundaries etc., but with the global equilibrium
concentration of vacancies, since this is part of being perfect, i.e. at
nirvana. We neglect self-interstitials here for simplicities sake and because
they do not figure prominently in metals.
Then we make the crystal a bit less perfect. We do this by still having only vacancies as defects, but now with concentrations that are not the nirvana concentration. This is not a very realistic case (we only do that in our brain, after all) but it will teach us a thing or two that we can carry over to realistic cases.
Let's now make a clear distinction of what we are going to find on average at equilibrium or non-equilibrium conditions, and on what is actually going on.
|First, let's look at bit more closely at what is
going on while we have nirvana conditions. So we have exactly the right amount of vacancies on average -
but that doesn't mean that nothing is happening! Quite the opposite:
|For perfect equilibrium or nirvana we
always have c1V <<1, and that implies that
c2V << c1V obtains at all
temperatures. Since c1V decreases with decreasing
temperature, c2V decreases even faster.
|Exactly the same reasoning goes for tri-, quadro-, or simply n-vacancies (called voids if n is large enough). They will be there in some equilibrium concentration that decreases rapidly with increasing n - we actually asserted that before. In our standard Arrhenius plot this is easy to show:|
|The slope, as always, gives the formation energy. It is nE1 - EnB. The concentration of n-vacancy agglomerates are always much smaller than those of agglomerates with (n -1) vacancies. For n >> 5 or so, the concentration is zero for all practical purposes.|
|The essential message is:|
|Multiple vacancies (also called vacancy
agglomerates or clusters) exist even in global equilibrium. Their concentration
is very small, however.
An essential insight we gained is:
| Now let's look at what happens if we
generate a vacancy supersaturation. All we
need to do in an otherwise perfect crystal is to cool it down not too slowly.
The surplus vacancies can only disappear at the surface and for that they must
diffuse quite a distance. This takes time, and a supersaturation will built up,
at least for some time.
|What happens when we have a supersaturation?
Vacancies are still running around and bumping into each other to form
di-vacancies and so on. The rate for this still scales with the vacancy
concentration and their mobility.
The dissociation into di-vacancies, however, scales with their concentration and the Boltzmann factor; in the beginning of a vacancy supersaturation it is essentially the same as before.
As an immediate consequence, the di-vacancy concentration now goes up and the single vacancy concentration goes down until a new local equilibrium is reached.
|We can deduce that easily from the equations above for what we now will call local equilibrium. We are no longer going for global or universal equilibrium but only for what can be done locally, the second-best state of being under conditions where some insurmountable constraints are present. The constraint we made up here is that we cannot get rid of vacancies as quickly as we would like.|
|All we need to do is to assume that the movement of the vacancies below some temperature T1 becomes so sluggish that it takes a long time to change their concentration. Somewhat idealized, we simply assume that from some temperature T1 downwards, the concentration of single vacancies stays (approximately) constant. This will then produce a supersaturation that increases with declining temperature.|
|What is going on is still described by the
equations above. The only difference is that c1V stays
That demands that all cnV values must now increase with decreasing temperature T < T1. Below T1 we have
In the Arrhenius plot this looks like this:
|Of course, the concentration is not coming down
smoothly and then suddenly stays constant when T1 is
reached. It's a continuous process. Nevertheless, the point made is valid. What
happens doesn't change in principle, just
the details get more complicated if we make the situation more realistic.
|Here is a calculation for the case of a perfect crystal and a constant over-all concentration of the vacancies for a large and constant cooling rate (i.e. for rapid quenching) and with realistic diffusion behavior used for the calculations. You still get an increase of n-vacancy cluster.|
|Note that the the di-vacancy concentration and so on would keep growing according to our simple equation. In the not-so-simple computer simulations, everything peters out below about 200 oC because nothing can move anymore.|
|Impurity Atoms and Supersaturation|
|Looking at impurity atoms, the
situation is entirely different. The over-all concentration has a more or less
constant value, given by the history of the material. Equilibrium with respect
to atomically dissolved impurity atoms would only be reached at some particular
temperature if at all.
For pretty much all temperatures, atomically dissolved impurities are therefore either undersaturated or supersaturated.
|What is the crystal going to do
about that? For small impurity
concentrations it follows pretty much the same strategy it used for vacancies:
|The remarks to what could happen at
interfaces are important for what we do with steel. What is really going on at
the surface can be rather complicated. The word "encouragement" here
simply refers to rates again. If there are more impurity atoms inside the
crystal than required for happiness, the rates with which they reach the
surface is simply larger than the rates with which they go in again. The net
effect is that the concentration in surface-near region goes down. For
undersaturation, it is the other way around.
The complications come from the fact that a lot of things are usually going on at the surface; we don't have just some impurity atoms sitting there. For example, if carbon atoms from the interior are immediately burnt off upon reaching the surface because your iron is kept in the oxidizing part of a flame, the concentration will always go down, we have de-carburization. If the iron is kept in the reducing (CO-rich) part of a flame, the opposite will happen. The surface near regions become richer in carbon and we are now setting the stage for case hardening.
|What Did We Learn?|
|What did we learn by all of this for our nucleation? Admittedly not much by this module alone - but quite a bit as soon as we put it in perspective with what is to follow. The messages (to get a little bit ahead of myself) are:|
|Even equilibrium thermodynamic teaches us that a supersaturation of point defects encourages the formation of point defect clusters. We knew that qualitatively all along - but now we have numbers.|
|Equilibrium - global or local - results from some
competition of (at least) two competing processes. In the example here it is
rate of vacancy generation by the decay of vacancy clusters, and the rate of
vacancy removal be the forming of those clusters. Equilibrium is just another
word for the equality of the two rates.
If one or both of those rates changes for some reason, the concentrations change until a new equilibrium is reached. This happens as long as not everything is just "frozen in" because it gets too cold.
|The shortcoming of the brain model here is that we kept the total vacancy concentrations constant. That allows for easy math but is not realistic. The crystal, in fact, wants pretty much all of its vacancies gone for good at some lower temperatures.|
|Moreover, we conveniently forgot about a new energy term that must come into consideration as soon as we don't just have di- or triple-vacancies but real voids with an internal surface and thus a surface energy.|
|It is time for the Homogeneous Nucleation module.|
|Back to Nucleation Science Overview|
|1. Homogeneous nucleation|
|3. Heterogeneous nucleation|
|4. Size and density of precipitates|
|5. Precipitation and structures|
TTT Diagrams: 1. The Basic Idea
The Al - Cu System
Transmission Electron Microscopes
Phenomenological Modelling of Diffusion
Size and Density of Precipitates
The Second Law
Precipitation and Structures
© H. Föll (Iron, Steel and Swords script)