Dendrites
The most important thing to remember when dealing
with the solidification of metals is that it is not sudden throughout the
entire liquid. As molten pure metal soidifies, crystals start to form and
these crystals grow by the addition of atoms to form a dendrite
which
as can be seen be is a crystal skelton with a backbone from which arms
grow in other directions, fixed by the regularity of the unit cell.
The
growth of a tree , starting from its trunk and growing branches and twigs,
is often used as an analogy to describe dendritic growth.
The dendrites grow outwards until contact
is made with neighbouring growths and this contact surface becomes the
boundary of the crystal or grain.
Click
here to see the dendrites growing.
It should benoted that any impurities in the metal are both pushed
by the growing crystal to the grain boundaries and trapped between the
arms of the dendrites.
