Page 273 - Materials Science and Engineering An Introduction

Summary • 245

Symbol Meaning
f Angle between the tensile axis and the normal to the slip plane for a
single crystal stressed in tension (Figure 7.7)
Material constant
s 0
Yield strength
s y
Processing/Structure/Properties/Performance Summary
An understanding of the strengthening mechanisms for metals necessarily requires some
knowledge concerning (1) the correlation of dislocation motion with plastic deforma-
tion, (2) the characteristics of these defects (i.e., surrounding strain fields and strain-field
interactions), and (3) crystallographic aspects (i.e., the concept of slip systems). The high
hardness (and lack of ductility) of one phase found in steel (martensite, Section 10.7)
is explained by a solid-solution strengthening effect and, in addition, to the presence of
few slip systems. The following concept map represents these relationships.

700

600
Martensite
Brinell hardness number 400 Tempered martensite
500

(tempered at 371°C)
300
200 Fine pearlite

100
0
0 0.2 0.4 0.6 0.8 1.0
Composition (wt% C)
Iron–Carbon Alloys Characteristics Strengthening Mechanical properties of Mechanical properties
(Steels) (Properties) of dislocations mechanisms iron–iron carbide alloys of martensite
(Chapter 7) in metals (Chapter 10) (Chapter 10)
(Chapter 7)
C C 5111681109
Repulsion Slip system
Solid-solution
strengthening

T T 400
Tensile strength (MPa) 300

200
0 10 20 30 40 50
Nickel content (wt%)
Other heat treatments are designed to recrystallize metal alloys that have been strain
hardened, to render them softer and more ductile and to develop more desirable grain
structures. Two such treatments are described in Section 11.7—process annealing and, for
steels, normalizing. The preceding relationships are indicated in the following concept map.