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When a metal cools from its liquid state, part of the nucleation happens from pre-existing clusters (a group of unit cells of the crystal lattice) already present in the liquid.

In the case of a phase change from vapour directly to solid, on rapid cooling, is it possible for Aluminium (or some other metal) to exhibit a different crystal structure or worse, be completely amorphous? As I understand it, Aluminum atoms won't have the time to arrange themselves in FCC right?

When BMGs (Bulk Metallic Glasses) are formed by rapid solidification, there is no long range order, only short range order, which is because this is already present at the liquid phase. But in this scenario of rapid gas-to-solid transformation, would there be any order, long or short?

What sort of a microstructure would you get on bubbling Aluminium vapors through a large tank of Helium? Would it be a glassy microstructure and/or would you get nano-powders and/or a new crystal structure formation because of the excess helium which would be present in the Aluminium matrix?

Brandon Enright
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Ash
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1 Answers1

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I cant imagine what bubbling metal vapour through any liquid will do.I dont think anybody does that to create piece of solid metal.The technique used for super fast cooling is called Melt Spinning where melted metal is slowly flowing down on to spinning copper wheel.

That wheel is internaly cooled by either flowing water or nitrogen.Liquid helium should be possible too,that would give it the maximum rapid cooling performance possible.I think cooling rate of around ten million kelvin per second would be possible for thin ribbon in tens of micron thickness range.

The higher the cooling rate,the smaller the grains.I expect any kind of extreme rapid cooling to produce either amorphous,nanocrystaline or combined structure,that is nano grains dispersed in amorphous metal. Its definately going to be different than aluminum produced by normal technique.

The smaller grains will produce grain boundary strenghtening,the smaller the grains,the stronger the metal,this keeps increasing till the average grain size is around 10 nm,beyond that it stops improving or worse,it starts declining,its called Hall - Petch limit. RSP is company that specialize in nanocrystaline aluminum produced by melt spinning.

I do not think that either the rapid cooling,or excess helium would cause it to create new crystal structure,I think its just going to change the grain sizs,down to the extreme low end that is amorphous state.

wav scientist
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