Page 271 - Materials Science and Engineering An Introduction

Summary • 243

Plastic Deformation • For polycrystalline metals, slip occurs within each grain along those slip systems that
of Polycrystalline are most favorably oriented with the applied stress. Furthermore, during deforma-
Materials tion, grains change shape and extend in those directions in which there is gross plastic
deformation.

Deformation by • Under some circumstances, limited plastic deformation may occur in BCC and HCP
Twinning metals by mechanical twinning. The application of a shear force produces slight
atomic displacements such that on one side of a plane (i.e., a twin boundary), atoms
are located in mirror-image positions of atoms on the other side.

Mechanisms of • The ease with which a metal is capable of plastic deformation is a function of disloca-
Strengthening in tion mobility—that is, restricting dislocation motion leads to increased hardness and
Metals strength.

Strengthening by • Grain boundaries are barriers to dislocation motion for two reasons:
Grain Size Reduction When crossing a grain boundary, a dislocation’s direction of motion must change.
There is a discontinuity of slip planes within the vicinity of a grain boundary.
• A metal that has small grains is stronger than one with large grains because the
former has more grain boundary area and, thus, more barriers to dislocation motion.
• For most metals, yield strength depends on average grain diameter according to the
Hall–Petch equation, Equation 7.7.

Solid-Solution • The strength and hardness of a metal increase with increase of concentration of im-
Strengthening purity atoms that go into solid solution (both substitutional and interstitial).
• Solid-solution strengthening results from lattice strain interactions between impurity
atoms and dislocations; these interactions produce a decrease in dislocation mobility.

Strain Hardening • Strain hardening is the enhancement in strength (and decrease of ductility) of a metal
as it is deformed plastically.
• Degree of plastic deformation may be expressed as percent cold work, which depends
on original and deformed cross-sectional areas as described by Equation 7.8.
• Yield strength, tensile strength, and hardness of a metal increase with increasing
percent cold work (Figures 7.19a and 7.19b); ductility decreases (Figure 7.19c).
• During plastic deformation, dislocation density increases, the average distance be-
tween adjacent dislocations decreases, and—because dislocation–dislocation strain
field interactions, are, on average, repulsive—dislocation mobility becomes more
restricted; thus, the metal becomes harder and stronger.

Recovery • During recovery:
There is some relief of internal strain energy by dislocation motion.
Dislocation density decreases, and dislocations assume low-energy configurations.
Some material properties revert back to their precold-worked values.

Recrystallization • During recrystallization:
A new set of strain-free and equiaxed grains form that have relatively low
dislocation densities.
The metal becomes softer, weaker, and more ductile.