NATURE OF MATERIALS - Materials for Civil and Construction Engineers - page 66

the material is the size and arrangement of the grains in the material. The
grain structure (microscopic structure) of the material should be distin-
guished from the atomic structure. Figure 2.12, for example, is an optical
photomicrograph of a low-carbon steel. This photomicrograph was obtained
using a scanning electron microscope with a magnification of 500 times.
Note that this microscopic scale is much different than the atomic scale of
Figure 2.9, which has a magnification in the order of 10,000,000 times.
In metals manufacturing, the material is heated to a liquid, impurities
are removed from the stock material, and alloying agents are added. An alloy
is simply the addition of a second element to a metal. As the material cools
from the liquid state, crystals form. Under normal cooling conditions, mul-
tiple nuclei will form, producing multiple crystals. As these crystals grow,
they will eventually contact each other, forming boundaries. For a given
material, the size of the grains depends primarily on the rate of cooling.
Under rapid cooling, multiple nuclei are formed, resulting in small grains
with extensive boundaries.
There are four types of grain boundaries: coherent, coherent strain,
semicoherent, and incoherent, as shown in Figure 2.13. At the coherent

FIGURE 2.12 Optical pho-
tomicrograph of low-carbon
steel (magnification: 500x).

Grain
boundaries

Grain
boundaries

(a) (b) (c) (d)

FIGURE 2.13 Types of grain boundaries: (a) coherent, (b) coherent strain,
(c) semi-coherent, and (d) incoherent.