Page 245 - Materials Science and Engineering An Introduction

WHY STUDY Dislocations and Strengthening Mechanisms?

With knowledge of the nature of dislocations and the harden metals and their alloys. Thus, it becomes pos-
role they play in the plastic deformation process, we sible to design and tailor the mechanical properties of
are able to understand the underlying mechanisms materials— for example, the strength or toughness of a
of the techniques that are used to strengthen and metal– matrix composite.

Learning Objectives
After studying this chapter, you should be able to do the following:
1. Describe edge and screw dislocation motion from 6. Describe and explain solid-solution strengthen-
an atomic perspective. ing for substitutional impurity atoms in terms of
2. Describe how plastic deformation occurs by lattice strain interactions with dislocations.
the motion of edge and screw dislocations in 7. Describe and explain the phenomenon of
response to applied shear stresses. strain hardening (or cold working) in terms of
3. Define slip system and cite one example. dislocations and strain field interactions.
4. Describe how the grain structure of a poly- 8. Describe recrystallization in terms of both the
crystalline metal is altered when it is plastically alteration of microstructure and mechanical
deformed. characteristics of the material.
5. Explain how grain boundaries impede dislocation 9. Describe the phenomenon of grain growth from
motion and why a metal having small grains is both macroscopic and atomic perspectives.
stronger than one having large grains.

7.1 INTRODUCTION
Chapter 6 explained that materials may experience two kinds of deformation: elastic
and plastic. Plastic deformation is permanent, and strength and hardness are measures
of a material’s resistance to this deformation. On a microscopic scale, plastic defor-
mation corresponds to the net movement of large numbers of atoms in response to
an applied stress. During this process, interatomic bonds must be ruptured and then
re-formed. In crystalline solids, plastic deformation most often involves the motion of
dislocations, linear crystalline defects that were introduced in Section 4.5. This chapter
discusses the characteristics of dislocations and their involvement in plastic defor-
mation. Twinning, another process by which some metals deform plastically, is also
treated. In addition, and probably most important, several techniques are presented
for strengthening single-phase metals, the mechanisms of which are described in terms
of dislocations. Finally, the latter sections of this chapter are concerned with recovery
and recrystallization—processes that occur in plastically deformed metals, normally at
elevated temperatures—and, in addition, grain growth.

Dislocations and Plastic Deformation

Early materials studies led to the computation of the theoretical strengths of perfect
crystals, which were many times greater than those actually measured. During the 1930s
it was theorized that this discrepancy in mechanical strengths could be explained by a
type of linear crystalline defect that has come to be known as a dislocation. Not until the
1950s, however, was the existence of such dislocation defects established by direct ob-
servation with the electron microscope. Since then, a theory of dislocations has evolved
that explains many of the physical and mechanical phenomena in metals [as well as
crystalline ceramics (Section 12.10)].

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