Page 55 - Mechanical Behavior of Materials

56 Chapter 2 Structure and Deformation in Materials

Figure 2.19 Shear deformation occurring in an incremental manner due to dislocation
motion. (Adapted from [Van Vlack 89] p. 265, c 1989 by Addison-Wesley Publishing Co., Inc.,
by permission of Pearson Education, Inc., Upper Saddle River, NJ, and by permission of the
Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI.)

2.5.1 Plastic Deformation by Dislocation Motion
Single crystals of pure metals that are macroscopic in size and which contain only a few dislocations
are observed to yield in shear at very low stresses. For example, for iron and other BCC metals, this
occurs around τ o = G/3000, that is, about τ o = 30 MPa. For FCC and HCP metals, even lower
values are obtained around τ o = G/100,000, or typically τ o = 0.5 MPa. Thus, shear strengths for
imperfect crystals of pure metals can be lower than the theoretical value for a perfect crystal of
τ b = G/10 by at least a factor of 300 and sometimes by as much as a factor of 10,000.
This large discrepancy can be explained by the fact that plastic deformation occurs by motion of
dislocations under the influence of a shear stress, as illustrated in Fig. 2.19. As a dislocation moves
through the crystal, plastic deformation is, in effect, proceeding one atom at a time, rather than
occurring simultaneously over an entire plane, as implied by Fig. 2.18. This incremental process can
occur much more easily than simultaneous breaking of all the bonds, as assumed in the theoretical
shear strength calculation for a perfect crystal.
The deformation resulting from dislocation motion proceeds for edge and screw dislocations,
as illustrated in Fig. 2.20 and Fig. 2.21, respectively. The plane in which the dislocation line moves
is called the slip plane, and where the slip plane intersects a free surface, a slip step is formed. Since
dislocations in real crystals are usually curved and thus have both edge and screw character, plastic
deformation actually occurs by a combination of the two types of dislocation motion.
Plastic deformation is often concentrated in bands called slip bands. These are regions where
the slip planes of numerous dislocations are concentrated; hence, they are regions of intense plastic
shear deformation separated by regions of little shear. Where slip bands intersect a free surface,
steps are formed as a result of the combined slip steps of numerous dislocations. (See Fig. 2.22.)