Properties of Metals
The reduction of area is reported as additional information (to the percent elongation) on the
deformational characteristics of the material. The two are used as indicators of ductility, the
ability of a material to be elongated in tension. Because the elongation is not uniform over the
entire gage length and is greatest at the center of the neck, the percent elongation is not an
absolute measure of ductility. (Because of this, the gage length must always be stated when the
percent elongation is reported.) The reduction of area, being measured at the minimum diameter
of the neck, is a better indicator of ductility.
Ductility is more commonly defined as the ability of a material to deform easily upon the
application of a tensile force, or as the ability of a material to withstand plastic deformation
without rupture. Ductility may also be thought of in terms of bendability and crushability.
Ductile materials show large deformation before fracture. The lack of ductility is often termed
brittleness. Usually, if two materials have the same strength and hardness, the one that has the
higher ductility is more desirable. The ductility of many metals can change if conditions are
altered. An increase in temperature will increase ductility. A decrease in temperature will cause
a decrease in ductility and a change from ductile to brittle behavior. Irradiation will also decrease
ductility, as discussed in Module 5.
Cold-working also tends to make metals less ductile. Cold-working is performed in a temperature
region and over a time interval to obtain plastic deformation, but not relieving the strain
hardening. Minor additions of impurities to metals, either deliberate or unintentional, can have
a marked effect on the change from ductile to brittle behavior. The heating of a cold-worked
metal to or above the temperature at which metal atoms return to their equilibrium positions will
increase the ductility of that metal. This process is called annealing.
Ductility is desirable in the high temperature and
Figure 7 Malleable Deformation of a Cylinder
Under Uniform Axial Compression
high pressure applications in reactor plants because
of the added stresses on the metals. High ductility
in these applications helps prevent brittle fracture,
which is discussed in Module 4.
Where ductility is the ability of a material to
deform easily upon the application of a tensile
force, malleability is the ability of a metal to
exhibit large deformation or plastic response when
being subjected to compressive force. Uniform
compressive force causes deformation in the
manner shown in Figure 7. The material contracts
axially with the force and expands laterally. Restraint due to friction at the contact faces induces
axial tension on the outside. Tensile forces operate around the circumference with the lateral
expansion or increasing girth. Plastic flow at the center of the material also induces tension.