Callister - An introduction - 8th edition

216 • Chapter 7 / Dislocations and Strengthening Mechanisms
140
800
700
1040 Steel
120
100
900
800
1040 Steel
120
Yield strength (MPa)
600
500
400
Brass
80
60
Yield strength (ksi)
Tensile strength (MPa)
700
600
500
Brass
100
80
Tensile strength (ksi)
300
Copper
40
400
60
Copper
200
300
20
40
100
0 10 20 30 40 50 60 70
200
0 10 20 30 40 50 60 70
Percent cold work
Percent cold work
(a)
(b)
70
60
50
Ductility (%EL)
40
30
Brass
20
10
1040 Steel
Copper
0
0 10 20 30 40 50 60 70
Percent cold work
(c)
Figure 7.19 For 1040 steel, brass, and copper, (a) the increase
in yield strength, (b) the increase in tensile strength, and
(c) the decrease in ductility (%EL) with percent cold work.
[Adapted from Metals Handbook: Properties and Selection:
Irons and Steels, Vol. 1, 9th edition, B. Bardes (Editor),
American Society for Metals, 1978, p. 226; and Metals
Handbook: Properties and Selection: Nonferrous Alloys and
Pure Metals, Vol. 2, 9th edition, H. Baker (Managing Editor),
American Society for Metals, 1979, pp. 276 and 327.]
point D. The stress is released, then reapplied with a resultant new yield strength,
s yi
. The metal has thus become stronger during the process because s yi
is greater
than s y0
.
The strain-hardening phenomenon is explained on the basis of dislocation–
dislocation strain field interactions similar to those discussed in Section 7.3. The
dislocation density in a metal increases with deformation or cold work, because of