NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 67

boundary, the lattices of the two grains align perfectly, and, in essence, there
is no physical boundary. Coherent strain boundaries have a different spac-
ing of atoms on each side of the boundary, which may be the effect of an
alloying agent. As with the line dislocation, there will be a strain in the
atoms on each side of the boundary due to the difference in the spacing of
the atoms. The semicoherent boundary has a different number of atoms on
each side of the boundary; therefore, not all of the boundaries can match up.
Again, this is similar to line dislocation, with strain occurring on each side
of the boundary. The incoherent boundary has a different orientation of the
crystals on each side of the boundary; thus, the atoms do not match up in a
natural manner.
Grain boundaries have an important effect on the behavior of a material.
Although the bonds across the grain boundary do not have the strength of
the pure crystal structure, the grain boundaries are at a higher energy state
than the atoms away from the grain boundary. As a result, when slip occurs
along one of the slip planes in the crystal, it is blocked from crossing the
grain boundary, and can be diverted to run along the grain boundary. This
increases the length of the slip path, thus requiring more energy to deform
or fracture the material. Therefore, reducing grain size increases the strength
of a material.
The grain structure of metals is affected by plastic strains. Fabrication
methods frequently involve plastic straining to produce a desired shape
(e.g., wire is produced by forcing a metal through successively smaller dies
to reduce the dimension of the metal from the shape produced in the mill to
the desired wire diameter.)
Heat treatments are used to refine grain structure. There are two basic
heat treatment methods: annealing and hardening. Both processes involve
heating the material to a point at which the existing grain boundaries will
break down and re-form upon cooling. The differences between the processes
include the temperature to which the material is heated, the amount of time
it is held at the elevated temperature, and the rate of cooling. In hardening,
rapid cooling is achieved by immersing the material in a liquid. In annealing,
the material is slowly cooled. The slowest rate of cooling is achieved by leav-
ing the material in the furnace and gradually reducing the temperature. This
is an expensive process, since it ties up the furnace. More commonly, the
material is cooled in air. Heat treatments of steel and aluminum are dis-
cussed in Chapters 3 and 4.
The grain size is affected by the rate of cooling. Rapid cooling limits the
time available for grain growth, resulting in small grains, whereas slow cool-
ing results in large grains.

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2.2.4 Alloys

The engineering characteristics of most metallic elements make them unsuit-
able for use in a pure form. In most cases, the properties of the materials can
be significantly improved with the addition of alloying agents. An alloying