MadSci Network: Physics

Re: Is aluminium crystalline or polycrystalline as a) a solid b) a liquid?

Date: Thu Jan 20 06:30:45 2005
Posted By: Andy Goddard, Staff, Teaching and Learning Resources, Strathclyde University
Area of science: Physics
ID: 1104355091.Ph

Hi there!

A liquid metal is a jumbled soup of metal atoms whose individual kinetic
energies exceeds the bond energy that the atoms use to link up when in a
solid state. As liquid metals cool (i.e. as atoms lose their kinetic
energy) atoms within a liquid can latch onto neighbouring atoms and start
to form crystalline lattices. These are networks of atoms arranged in a
regular, ordered sequence, whose layout depends on the number of bonds that
each individual metal atom can form.

It is at least theoretically possible to cool a tank of any liquid metal to
form one single crystal. However, in practical terms, usually many smaller
crystals start to form throughout the liquid. Each of these crystals grow
as cooling spreads, in different orientations, eventually giving rise to a
polycrystalline ("many crystal") structure. 

Naturally the junction between crystals is typically weaker than the
crystals themselves, so care has to be taken when casting metals (that is,
turning them from liquid to solid) in order to produce a solid metal with
the desired properties for a particular use.

All metals are crystalline as solids, in that they are made of crystals,
but generally all metals are polyscrystalline when solid (made of more than
one crystal). Liquid metals can form perfectly crystalline solids, but in
practical terms they rarely do so.

I hope this helps to clear up your confusion.

Andy Goddard

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