Page 257 - Materials Science and Engineering An Introduction

7.8 Strengthening by Grain Size Reduction • 229

Figure 7.13 For a single crystal subjected to
Twin a shear stress t, (a) deformation by slip;
planes
(b) deformation by twinning.
Slip
planes
Twin

(a) (b)

Mechanisms of Strengthening in Metals

Metallurgical and materials engineers are often called on to design alloys having high
strengths yet some ductility and toughness; typically, 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 be-
tween dislocation motion and mechanical behavior of metals. Because macroscopic
plastic deformation corresponds to the motion of large numbers of dislocations, the abil-
ity of a metal to deform plastically 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 dislocations, the me-
chanical strength may be enhanced—that is, greater mechanical forces are required to
initiate plastic deformation. In contrast, the more unconstrained the dislocation motion,
the greater is the facility with which a metal may deform, and the softer and weaker it
Tutorial Video: becomes. Virtually all strengthening techniques rely on this simple principle: Restricting
Defects in Metals or hindering dislocation motion renders a material harder and stronger.
How do Defects The present discussion is confined to strengthening mechanisms for single-
Affect Metals? phase metals by grain size reduction, solid-solution alloying, and strain hardening.
Deformation and strengthening of multiphase alloys are more complicated, involving
concepts beyond the scope of the present discussion; Chapter 10 and Section 11.9 treat
techniques that are used to strengthen multiphase alloys.

7.8 STRENGTHENING BY GRAIN SIZE REDUCTION
The size of the grains, or average grain diameter, in a polycrystalline metal influences
the mechanical properties. Adjacent grains normally have different crystallographic
orientations and, of course, a common grain boundary, as indicated in Figure 7.14.
During plastic deformation, slip or dislocation motion must take place across this com-
mon boundary—say, from grain A to grain B in Figure 7.14. The grain boundary acts as
a barrier to dislocation motion for two reasons:
1. Because the two grains are of different orientations, a dislocation passing into
grain B must change its direction of motion; this becomes more difficult as the
crystallographic misorientation increases.
2. The atomic disorder within a grain boundary region results in a discontinuity of
slip planes from one grain into the other.
It should be mentioned that, for high-angle grain boundaries, it may not be the case that
dislocations traverse grain boundaries during deformation; rather, dislocations tend to