GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51
2nd Revised Pages282 • Chapter 8 / Deformation and Strengthening MechanismsDeformation by Twinning
Under some circumstances limited plastic deformation may occur in BCC and
HCP metals by mechanical twinning. Normally, twinning is important to the de-
gree that accompanying crystallographic reorientations make the slip process more
favorable.Characteristics of Dislocations
Strengthening by Grain Size Reduction
Solid-Solution Strengthening
Strain Hardening
Since the ease with which a metal is capable of plastic deformation is a function of
dislocation mobility, restricting dislocation motion increases hardness and strength.
On the basis of this principle, three different strengthening mechanisms were dis-
cussed. Grain boundaries serve as barriers to dislocation motion; thus refining the
grain size of a polycrystalline material renders it harder and stronger. Solid-solution
strengthening results from lattice strain interactions between impurity atoms and
dislocations. Finally, as a metal is plastically deformed, the dislocation density in-
creases, as does also the extent of repulsive dislocation–dislocation strain field inter-
actions; strain hardening is just the enhancement of strength with increased plastic
deformation.Recovery
Recrystallization
Grain Growth
The microstructural and mechanical characteristics of a plastically deformed metal
specimen may be restored to their predeformed states by an appropriate heat treat-
ment, during which recovery, recrystallization, and grain growth processes are al-
lowed to occur. During recovery there is a reduction in dislocation density and alter-
ations in dislocation configurations. Recrystallization is the formation of a new set of
grains that are strain free; in addition, the material becomes softer and more ductile.
Grain growth is the increase in average grain size of polycrystalline materials, which
proceeds by grain boundary motion.Crystalline Ceramics (Deformation Mechanisms)
Noncrystalline Ceramics (Deformation Mechanisms)
Any plastic deformation of crystalline ceramics is a result of dislocation motion;
the brittleness of these materials is explained, in part, by the limited number of
operable slip systems. The mode of plastic deformation for noncrystalline materials
is by viscous flow; a material’s resistance to deformation is expressed as viscosity.
At room temperature, the viscosities of many noncrystalline ceramics are extremely
high.Deformation of Semicrystalline Polymers
During the elastic deformation of a semicrystalline polymer having a spherulitic
structure that is stressed in tension, the molecules in amorphous regions elongate
in the stress direction. In addition, molecules in crystallites experience bending and
stretching, which causes a slight increase in lamellar thickness.