Originally posted by cucumber
do dendrites (those tree like shapes formed when a metal solidifies) only occur in alloys, or in any metal, pure or not? can they be explained in terms that an A-level student could grasp (i mean, why does the metal not crystalise in the same way salts do? why do they form tree structures?)
Dendritic growth does not only happen in metals. A similar case would be for snow, which forms when very pure water is cooled below freezing and falls through the atmosphere. Upon hitting another drop or dust, it rapidly crystallises, forming the fractal (if extended by a few more generations) snowflake structure. There are many similar analogies in nature, from the fractal geometry of tree branches, to snowflakes and dendritic growth in metals.
You get dendrites when a molten metal undercools significantly and is not disturbed (e.g. stirring) during the process. In this context, undercooling is when the liquid metal's temperature is below its freezing point. You can also see the reverse of this in other pure liquids - if you get some distilled water and put it in a clean Pyrex beaker. Put the beaker in the microwave and leave the thing on high for a few minutes. If you left it long enough, and drop some salt in, it will suddenly 'boil' and (be careful!) foam over the top. You can get undercooling if the metal is very pure (no solidification throughout), cooled quickly or cooled in microgravity without touching the container's walls, for example.
At the base of the dendrite, you will have an impurity which started the solidification process. With less impurities, you will end up with less dendrites. As the dendrite grows into the undercooled liquid, the solidification process releases the latent heat of fusion. The shape of the dendrite is such that it maximises its surface area for dissipating this heat to the undercooled liquid. The shape of the dendrite is also governed by how quick the solidification time is; a rapid solidification time will yield dendrites with tightly packed secondary branches (or arms). You get quick solidification times if you undercool the liquid significantly, and this gives you a tougher, stronger and more ductile metal.
The dendrites formed in alloys and "pure" metals would differ. Assuming the absence of container walls (or a
very clean, smooth wall surface), you would be able to undercool a pure metal much more than an alloy. Because of the purity, you would not be able to get many dendrites, but when you do, they will be big. For alloys, it will consist of many, smaller ones.
By "not crystallising the same way salts do", I am assuming you are talking about a supersaturated aqueous salt solution being cooled, and crystals forming in it. The short answer is that the salt crystals do not adhere well to the container's walls. In addition, in dendritic growth, heat dissipation is the dominant factor. In any analogy with an aqueous supersaturated salt solution, there would also be issues of solubility. E.g. as a crystal forms is would release heat which increases the temperature of the water around it and increases its solubility...
and is there an easy way to explain why impurities tend to collect between these dendrites? is it just that they don't fit into the crystal structure well?
Between a crystalline solid that contains impurities and one that does not, generally, a pure solid is at a lower thermodynamic state than an impure one. This makes it more likely to form.
