Callister - An introduction - 8th edition

7.7 Deformation by Twinning • 211
Slip
planes
Twin
planes
Twin
Figure 7.13 For a single
crystal subjected to a
shear stress , (a)
deformation by slip; (b)
deformation by twinning.
(a)
(b)
respectively, of atoms within the twinned region. As may be noted in this figure,
the displacement magnitude within the twin region (indicated by arrows) is proportional
to the distance from the twin plane. Furthermore, twinning occurs on
a definite crystallographic plane and in a specific direction that depend on crystal
structure. For example, for BCC metals, the twin plane and direction are (112)
and [111], respectively.
Slip and twinning deformations are compared in Figure 7.13 for a single crystal
that is subjected to a shear stress . Slip ledges are shown in Figure 7.13a, the
formation of which was described in Section 7.5; for twinning, the shear deformation
is homogeneous (Figure 7.13b). These two processes differ from each other in
several respects. First, for slip, the crystallographic orientation above and below the
slip plane is the same both before and after the deformation; for twinning, there
will be a reorientation across the twin plane. In addition, slip occurs in distinct atomic
spacing multiples, whereas the atomic displacement for twinning is less than the interatomic
separation.
Mechanical twinning occurs in metals that have BCC and HCP crystal structures,
at low temperatures, and at high rates of loading (shock loading), conditions
under which the slip process is restricted; that is, there are few operable slip systems.
The amount of bulk plastic deformation from twinning is normally small relative
to that resulting from slip. However, the real importance of twinning lies with
the accompanying crystallographic reorientations; twinning may place new slip systems
in orientations that are favorable relative to the stress axis such that the slip
process can now take place.
Mechanisms of Strengthening in Metals
Metallurgical and materials engineers are often called on to design alloys having high
strengths yet some ductility and toughness; ordinarily, ductility is sacrificed when an
alloy is strengthened. Several hardening techniques are at the disposal of an engineer,
and frequently alloy selection depends on the capacity of a material to be tailored
with the mechanical characteristics required for a particular application.
Important to the understanding of strengthening mechanisms is the relation between
dislocation motion and mechanical behavior of metals. Because macroscopic
plastic deformation corresponds to the motion of large numbers of dislocations, the
ability of a metal to plastically deform depends on the ability of dislocations to move.
Because hardness and strength (both yield and tensile) are related to the ease with
which plastic deformation can be made to occur, by reducing the mobility of