Callister - An introduction - 8th edition

7.10 Strain Hardening • 217
600 24%CW
500
4%CW
0%CW
Figure 7.20 The influence of
cold work on the stress–strain
behavior of a low-carbon
steel; curves are shown for
0%CW, 4%CW, and 24%CW.
Stress (MPa)
400
300
200
100
0
0 0.05 0.1 0.15 0.2 0.25
Strain
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 presence of other dislocations. As the dislocation density increases,
this resistance to dislocation 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 properties
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 passing, for 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
the strain hardening for a given amount of plastic strain.
Concept Check 7.3
When making hardness measurements, what will be the effect of making 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).]