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Galvanized surface with
macroscopic crystalline
features. Twin boundaries
are visible as striations
within each
crystallite, most
prominently in the
bottom-left and top-right.
A twin boundary occurs
when two
crystals of the same type
intergrow, so that only a slight
misorientation exists between
them. It is a highly symmetrical
interface, often with one crystal
the mirror image of the other;
also, atoms are shared by the two
crystals at regular intervals.
This is also a much lower-energy
interface than the
grain boundaries that form
when crystals of arbitrary
orientation grow together.
Twin boundaries are partly
responsible for
shock hardening and for many
of the changes that occur in
cold work of metals with
limited
slip systems or at very low
temperatures. They also occur due
to
martensitic transformations:
the motion of twin boundaries is
responsible for the pseudoelastic
and shape-memory behavior of
nitinol, and their presence is
partly responsible for the
hardness due to
quenching of
steel.
