Page 262 - Materials Science and Engineering An Introduction

234 • Chapter 7 / Dislocations and Strengthening Mechanisms

Figure 7.20 The influence of
cold work on the stress–strain
behavior of a low-carbon steel; 600 24%CW
curves are shown for 0%CW,
4%CW, and 24%CW.
500 4%CW
0%CW
Stress (MPa) 400

300

200

100

0
0 0.05 0.1 0.15 0.2 0.25
Strain

Strain hardening is demonstrated in a stress–strain diagram presented earlier
is plastically deformed to point
(Figure 6.17). Initially, the metal with yield strength s y 0
. The
D. The stress is released, then reapplied with a resultant new yield strength, s y i
.
metal has thus become stronger during the process because s y i is greater than s y 0
The strain-hardening phenomenon is explained on the basis of dislocation–
dislocation strain field interactions similar to those discussed in Section 7.3. The disloca-
tion density in a metal increases with deformation or cold work because of dislocation
multiplication or the formation of new dislocations, as noted previously. Consequently,
the average distance of separation between dislocations decreases—the dislocations are
positioned closer together. On the average, dislocation–dislocation strain interactions
are repulsive. The net result is that the motion of a dislocation is hindered by the pres-
ence of other dislocations. As the dislocation density increases, this resistance to disloca-
tion motion by other dislocations becomes more pronounced. Thus, the imposed stress
necessary to deform a metal increases with increasing cold work.
Strain hardening is often utilized commercially to enhance the mechanical proper-
ties of metals during fabrication procedures. The effects of strain hardening may be
removed by an annealing heat treatment, as discussed in Section 11.7.
In the mathematical expression relating true stress and strain, Equation 6.19, the
parameter n is called the strain-hardening exponent, which is a measure of the ability of
a metal to strain harden; the larger its magnitude, the greater is the strain hardening for
a given amount of plastic strain.

Concept Check 7.3 When making hardness measurements, what will be the effect of mak-
ing an indentation very close to a preexisting indentation? Why?
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
Concept Check 7.4 Would you expect a crystalline ceramic material to strain harden at
room temperature? Why or why not?
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]