Page 68 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 1.5 Grams and Gra|n Boundanes

I.4.2 Work Hardening (Strain Hardening)
Although the presence of a dislocation lowers the shear stress required to cause slip,
dislocations can:

l. Become entangled and interfere with each other, and
2. Be impeded by barriers, such as grain boundaries, impurities, and inclusions in
the material.

The increased shear stress required to overcome entanglements and impedi-
ments results in an increase in the overall strength and the hardness of the metal and
is known as work hardening or strain hardening. The greater the deformation, the
greater is the number of entanglements and hence the higher the increase in the
metal’s strength. Work hardening is used extensively for strengthening in metal-
working processes at ambient temperatures. Typical examples are producing sheet
metal for automobile bodies and aircraft fuselages by cold rolling (Chapter 13), pro-
ducing the head of a bolt by forging (Chapter 14), and strengthening wire by reduc-
ing its cross section by drawing it through a die (Chapter 15).

|.5 Grains and Grain Boundaries

When a mass of molten metal begins to solidify, crystals begin to form independently
of each other at various locations within the liquid mass; they have random and
unrelated orientations (Fig. 1.10). Each of these crystals then grows into a crystalline
structure, or grain. Each grain consists of either a single crystal (for pure metals) or a
polycrystalline aggregate (for alloys).
The number and size of the grains developed in a unit volume of the metal
depends on the rate at which nucleation (the initial stage of crystal formation) takes
place. The median size of the grains developed depends on the number of different
sites at which individual crystals begin to form (note that there are seven in
Fig. 1.10a) and the rate at which these crystals grow. If the nucleation rate is high,
the number of grains in a unit volume of metal will be large, and thus grain size will
be small. Conversely, if the rate of growth of the crystals is high (compared with
their nucleation rate), there will be fewer grains per unit volume, and thus grain
size will be larger. Generally, rapid cooling produces smaller grains, whereas slow
cooling produces larger grains.

sfiase

5 §

H b (C) (Ci)

FIGURE l.|0 Schematic illustration of the stages during the solidification of molten metal;
each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten
metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of
crystals as solidification continues. (d) Solidified metal, showing individual grains and grain
boundaries; note the different angles at which neighboring grains meet each other.