Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-08 GTBL042-Callister-v3 October 4, 2007 11:51

2nd Revised Pages

256 • Chapter 8 / Deformation and Strengthening Mechanisms

Polished surface

Twin plane Twin plane





(a) (b)

Figure 8.12 Schematic diagram showing how twinning results from an applied shear stress

τ.In(b), open circles represent atoms that did not change position; dashed and solid circles

represent original and final atom positions, respectively. (From G. E. Dieter,Mechanical

Metallurgy,3rd edition. Copyright©c1986 by McGraw-Hill Book Company, New York.

Reproduced with permission of McGraw-Hill Book Company.)

Slip and twinning deformations are compared in Figure 8.13 for a single crys-

tal that is subjected to a shear stressτ. Slip ledges are shown in Figure 8.13a, the

formation of which was described in Section 8.6; for twinning, the shear deforma-

tion is homogeneous (Figure 8.13b). These two processes differ from each other in

several respects. First, for slip, the crystallographic orientation above and below the

slip plane is the same both before and after the deformation; for twinning, there will

be a reorientation across the twin plane. In addition, slip occurs in distinct atomic

spacing multiples, whereas the atomic displacement for twinning is less than the

interatomic separation.

Mechanical twinning occurs in metals that have BCC and HCP crystal struc-

tures, at low temperatures, and at high rates of loading (shock loading), conditions

under which the slip process is restricted; that is, there are few operable slip systems.

The amount of bulk plastic deformation from twinning is normally small relative

to that resulting from slip. However, the real importance of twinning lies with the

accompanying crystallographic reorientations; twinning may place new slip systems

in orientations that are favorable relative to the stress axis so that the slip process

can now take place.

(a) (b)

 

 

Slip

planes

Twin

Twin

planes

Figure 8.13 For a single

crystal subjected to a shear

stressτ,(a) deformation by

slip; (b) deformation by

twinning.