Page 61 - Mechanical Behavior of Materials

62 Chapter 2 Structure and Deformation in Materials

In metals, the defects that lower the strength are primarily dislocations. These move under
the influence of applied stresses and cause yielding behavior. In large single crystals containing
a few dislocations, yielding occurs at very low stresses that are lower than the theoretical value by a
factor of 300 or more. Strengths are increased above this value if there are obstacles to dislocation
motion, such as grain boundaries, hard second-phase particles, alloying elements, and dislocation
entanglements. The resulting strength for engineering metals in bulk form may be as high as one-
tenth of the theoretical value of E/10, that is, around E/100.
Materials are also subject to time-dependent deformation called creep. Such deformation is
especially likely at temperatures approaching melting. Physical mechanisms vary with material and
temperature. Examples include diffusion of vacancies in metals and ceramics and sliding of chain
molecules in polymers.
The necessarily brief treatment given in this chapter on structure and deformation in materials
represents only a minimal introduction to the topic. More detail is given in a number of excellent
books, a few of which are listed as references at the end of this chapter.

NEW TERMS AND SYMBOLS

body-centered cubic (BCC) structure melting temperature, T m
close-packed planes, directions metallic bond
covalent bond polycrystalline material
diamond cubic structure screw dislocation
edge dislocation secondary (hydrogen) bond
face-centered cubic (FCC) structure slip plane
glass transition temperature, T g slip step
grain boundary substitutional impurity
hexagonal close-packed (HCP) structure theoretical cohesive strength, σ b ≈ E/10
interstitial theoretical shear strength, τ b ≈ G/10
ionic bond unit cell
lattice plane; lattice site vacancy

REFERENCES

CALLISTER, W. D., Jr., and D. G. RETHWISCH. 2010. Materials Science and Engineering: An Introduction,
8th ed., John Wiley, Hoboken, NJ.
COURTNEY, T. H. 2000. Mechanical Behavior of Materials, McGraw-Hill, New York.
DAVIS, J. R., ed. 1998. Metals Handbook: Desk Edition, 2d ed., ASM International, Materials Park, OH.
HAYDEN,H.W., W. G. MOFFATT,and J. WULFF. 1965. The Structure and Properties of Materials, Vol. III:
Mechanical Behavior, John Wiley, New York.
HOSFORD, W. H. 2010. Mechanical Behavior of Materials, 2nd ed., Cambridge University Press, New York.
KELLY,A., andN.H. MACMILLAN. 1986. Strong Solids, 3d ed., Clarendon Press, Oxford, UK.