As an example of the grain structure that
develops in a casting, Fig. 10.2a shows a cross
section of a box-shaped mold. At the mold walls,
which are at ambient temperature or typically are
much cooler than the molten metal, the metal
cools rapidly and produces a solidified skin, or
shell, of fine equiaxed grains. The grains generally
grow in a direction opposite to that of the heat
transfer out through the mold. Those grains that
have favorable orientation grow preferentially
and are called columnar grains (Fig. 10.3). Those
grains that have substantially different orientations
are blocked from further growth. As the
driving force of the heat transfer is reduced away
from the mold walls, the grains become equiaxed
and coarse. This process of grain development is
known as homogenous nucleation, meaning that
the grains (crystals) grow upon themselves, starting
at the mold wall.