Page 264 - Materials Science and Engineering An Introduction

236 • Chapter 7 / Dislocations and Strengthening Mechanisms

of dislocations, and dislocation configurations (similar to that shown in Figure 4.9) are
produced having low strain energies. In addition, physical properties such as electrical
and thermal conductivities recover to their precold-worked states.

7.12 RECRYSTALLIZATION
Even after recovery is complete, the grains are still in a relatively high strain energy
recrystallization state. Recrystallization is the formation of a new set of strain-free and equiaxed grains
(i.e., having approximately equal dimensions in all directions) that have low dislocation
densities and are characteristic of the precold-worked condition. The driving force to
Tutorial Video: produce this new grain structure is the difference in internal energy between the strained
Annealing and unstrained material. The new grains form as very small nuclei and grow until they
What’s the Difference completely consume the parent material, processes that involve short-range diffusion.
between Recovery and Several stages in the recrystallization process are represented in Figures 7.21a to 7.21d;
Recrystallization? in these photomicrographs, the small speckled grains are those that have recrystallized.
Thus, recrystallization of cold-worked metals may be used to refine the grain structure.
Also, during recrystallization, the mechanical properties that were changed as a result
of cold working are restored to their precold-worked values—that is, the metal becomes
softer and weaker, yet more ductile. Some heat treatments are designed to allow recrystal-
lization to occur with these modifications in the mechanical characteristics (Section 11.7).
The extent of recrystallization depends on both time and temperature. The degree
(or fraction) of recrystallization increases with time, as may be noted in the photomicro-
graphs shown in Figures 7.21a to 7.21d. The explicit time dependence of recrystallization
is addressed in more detail near the end of Section 10.3.
The influence of temperature is demonstrated in Figure 7.22, which plots tensile
strength and ductility (at room temperature) of a brass alloy as a function of the tem-
perature and for a constant heat treatment time of 1 h. The grain structures found at the
various stages of the process are also presented schematically.
The recrystallization behavior of a particular metal alloy is sometimes specified in
recrystallization terms of a recrystallization temperature, the temperature at which recrystallization just
temperature reaches completion in 1 h. Thus, the recrystallization temperature for the brass alloy
of Figure 7.22 is about 450 C (850 F). Typically, it is between one-third and one-half
of the absolute melting temperature of a metal or alloy and depends on several fac-
tors, including the amount of prior cold work and the purity of the alloy. Increasing
the percent cold work enhances the rate of recrystallization, with the result that the
recrystallization temperature is lowered, and approaches a constant or limiting value
at high deformations; this effect is shown in Figure 7.23. Furthermore, it is this limiting
or minimum recrystallization temperature that is normally specified in the literature.
There exists some critical degree of cold work below which recrystallization cannot be
made to occur, as shown in the figure; typically, this is between 2% and 20% cold work.
Recrystallization proceeds more rapidly in pure metals than in alloys. During re-
crystallization, grain-boundary motion occurs as the new grain nuclei form and then
grow. It is believed that impurity atoms preferentially segregate at and interact with
these recrystallized grain boundaries so as to diminish their (i.e., grain boundary)
mobilities; this results in a decrease of the recrystallization rate and raises the recrystalli-
zation temperature, sometimes quite substantially. For pure metals, the recrystallization
temperature is normally 0.4T , where T m is the absolute melting temperature; for some
m
commercial alloys it may run as high as 0.7T m . Recrystallization and melting tempera-
tures for a number of metals and alloys are listed in Table 7.2.
It should be noted that because recrystallization rate depends on several variables,
as discussed previously, there is some arbitrariness to recrystallization temperatures
cited in the literature. Furthermore, some degree of recrystallization may occur for an
alloy that is heat treated at temperatures below its recrystallization temperature.