GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51
2nd Revised Pages8.8 Deformation by Twinning • 255Figure 8.11 Alteration of the grain structure of a polycrystalline metal as a result of plastic
deformation. (a) Before deformation the grains are equiaxed. (b) The deformation has
produced elongated grains. 170×. (From W. G. Moffatt, G. W. Pearsall, and J. Wulff,The
Structure and Properties of Materials,Vol. I,Structure,p. 140. Copyright©c1964 by John
Wiley & Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.)each individual grain is constrained, to some degree, in the shape it may assume by
its neighboring grains. The manner in which grains distort as a result of gross plastic
deformation is indicated in Figure 8.11. Before deformation the grains are equiaxed,
or have approximately the same dimension in all directions. For this particular de-
formation, the grains become elongated along the direction in which the specimen
was extended.
Polycrystalline metals are stronger than their single-crystal equivalents, which
means that greater stresses are required to initiate slip and the attendant yielding.
This is, to a large degree, also a result of geometrical constraints that are imposed
on the grains during deformation. Even though a single grain may be favorably
oriented with the applied stress for slip, it cannot deform until the adjacent and less
favorably oriented grains are capable of slip also; this requires a higher applied stress
level.8.8 DEFORMATION BY TWINNING
In addition to slip, plastic deformation in some metallic materials can occur by the
formation of mechanical twins, ortwinning. The concept of a twin was introduced in
Section 5.8; that is, a shear force can produce atomic displacements such that on one
side of a plane (the twin boundary), atoms are located in mirror-image positions of
atoms on the other side. The manner in which this is accomplished is demonstrated
in Figure 8.12. Here, open circles represent atoms that did not move, and dashed and
solid circles represent original and final positions, respectively, of atoms within the
twinned region. As may be noted in this figure, the displacement magnitude within
the twin region (indicated by arrows) is proportional to the distance from the twin
plane. Furthermore, twinning occurs on a definite crystallographic plane and in a
specific direction that depend on crystal structure. For example, for BCC metals, the
twin plane and direction are (112) and [111], respectively.