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50 Chapter 1 The Structure of Metals

l.6 Plastic Deformation of Polycrystalline Metals
When a polycrystalline metal with uniform eqniaxed grains (grains having equal
dimensions in all directions) is subjected to plastic deformation at room tempera-
ture (a process known as cold working), the grains become deformed and elongat-
ed, as shown schematically in Fig. 1.11. Deformation may be carried out, for
example, by compressing the metal piece, as is done in a forging operation to make
a turbine disk (Chapter 14) or by subjecting it to tension, as is done in stretch form-
ing of sheet metal to make an automobile body (Chapter 16). The deformation
within each grain takes place by the mechanisms described in Section 1.4 for a
single crystal.
During plastic deformation, the grain boundaries remain intact and mass conti-
nuity is maintained. The deformed metal exhibits higher strength, because of the
entanglement of dislocations with grain boundaries and with each other. The increase
in strength depends on the degree of deformation (strain) to which the
metal is subjected; the higher the deformation, the stronger the metal be-
comes. The strength is higher for metals with smaller grains, because
> * * 1 @5315 they have a larger grain-boundary surface area per unit volume of metal
=,=,= zfzfzw and hence more entanglement of dislocations.

Anisotropy (Texture). Note in Fig. 1.11b that, as a result of plastic
deformation, the grains have elongated in one direction and contract-
ed in the other. Consequently, this piece of metal has become
<a> <b>
anisotropic, and thus its properties in the vertical direction are differ-
ent from those in the horizontal direction. The degree of anisotropy
FIGURE l.ll Plastic deformation of depends on the temperature at which deformation takes places and on
idealized (equiaxed) grains in a specimen
subjected to compression (such as occurs in how uniformly the metal is deformed. Note from the direction of the
the forging or rolling of metals): (a) before crack in Fig. 1.12, for example, that the ductility of the cold-rolled
deformation; and (b) after deformation. sheet in the transverse direction is lower than that in its rolling direc-
Note the alignment of grain boundaries tion. (See also Section 16.5.)
along a horizontal direction; this effect is Anisotropy influences both mechanical and physical properties
known as preferred orientation. of metals, described in Chapter 3. For example, sheet steel for electri-
cal transformers is rolled in such a way that the resulting deformation
1 .,.. imparts anisotropic magnetic properties to
Top Vlew the sheet. This operation reduces magnetic-
B0llllf‘Q hysteresis losses and thus improves the effi-
Crack dlrecllon ,,,__ Ciency of transformers. (See also amorphous
"= alloys, Section 6.14.) There are two general
i
I V ” VQ types of anisotropy in metals: preferred ori-

“r"'
Sheet entation and mechanical fibering.
`>'i "'i ` ‘ ‘i " ' S Preferred Orientation. Also called crystal-
Side View
lographic anisotropy, preferred orientation
(a) (b) can be best described by referring to
Fig. 1.5a. When a single-crystal piece of metal
is subjected to tension, the sliding blocks ro-
FIGURE |.I2 (a) Schematic illustration of a crack in sheet metal that tate toward the direction of the tensile force;
has been subjected to bulging (caused, for example, by pushing a steel as a result, slip planes and slip bands tend to
ball against the sheet). Note the orientation of the crack with respect
align themselves with the general direction of
to the rolling direction of the sheet; this sheet is anisotropic. (b)
Aluminum sheet with a crack (vertical dark line at the center) deformation. Similarly, for a polycrystalline
developed in a bulge test; the rolling direction of the sheet was metal, with grains in random orientations, all
vertical. Courtesy: ].S. Kallend, Illinois Institute of Technology. slip directions tend to align themselves with