Metals are crystalline substances. Figure 2.1 shows a section of a metal. The
term
grain
is used to describe the crystals within the metal. A grain is merely a
crystal without its geometrical shape and flat faces because its growth was
impeded by contact with other crystals. Within a grain the arrangement of
particles is just as regular and repetitive as within a crystal with smooth faces. A
simple model of a metal with its grains is given by the raft of bubbles on the
surface of a liquid (Figure 2.2). The bubbles pack together in an orderly and
repetitive manner but if 'growth' is started at
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