3.2 Phase Formation and Identification
3.2.3 Recrystallization, Secondary Recrystallization, and
at above a critical temperature. It is a nucleation and growth process after the metal has been cold worked. During this kinetic process, a metal goes through a subtle microstructure evolution, which can refine coarse grains and release residual stress from prior strain. This metallurgical phenom- enon is called recrystallization. A minimum critical amount of cold work is required to recrystallize metals within a reasonable time period. This required minimum cold work varies with the type of deformation, that is, tension, compression, torsion, rolling, etc. For instance, torsion can promote the recrystallization process at a relatively small amount of deformation. A metal subject to a larger amount of deformation usually recrystallizes faster than a metal that is less deformed.
FIgurE 3.3
Microstructure of a tungsten wire with elongated grain morphology.
The kinetic process of recrystallization can be quantitatively described by Equation 3.2. In the experimental study of recrystallization, the time is usu- ally measured at 50% completion of recrystallization. However, the time, τ, in Equation 3.2 is not restricted to represent 50% completion only; it might repre- sent the time of full recrystallization. The recrystallization activation energy is a collective energy barrier that needs to be overcome during this process.
1τ =Ce−Q RT (3.2)
where τ = time (usually for half recrystallization), C = empirical constant, Q = activation energy for recrystallization, R = universal gas constant
≈ 2 cal/(mol K), and T = absolute temperature.
The temperature at which a cold-deformed metal can be completely recrys- tallized in a finite period of time, usually 1 hour, is defined as the recrystal- lization temperature. In engineering practice it is usually acceptable to define a specific recrystallization temperature for a metal or alloy with the under- standing that it is only a term for convenience. This temperature is a function of the amount of deformation, and strictly speaking, it is not an intrinsic mate- rial property. However, recrystallization is a kinetic process with a very large activation energy, Q. As a result, the recrystallization process is very sensitive to the annealing temperature that affects the recrystallization rate exponen- tially, as shown in Equation 3.2. A small change in temperature could signifi- cantly shorten or delay the recrystallization process. It therefore appears that each metal has a low limit for the recrystallization temperature, below which the recrystallization process will stall to a virtual stop. It is also worth noting that though the recrystallization temperature is practically fixed for a pure metal, it can be significantly raised up to several hundred degrees by a very small amount of impurity, as little as 0.01 atomic percent.
One of the benefits of recrystallization is grain refinement, which is depen- dent on the ratio of nucleus number to the grain growth rate. The higher this ratio, the finer the final grain will be because more nuclei grow slowly and compete with one another for the limited space. The smaller the grains before cold work, the greater the rate of nucleation will be and the smaller the sub- sequent recrystallized grain will emerge for a given degree of deformation.
Occasionally, isolated coarse grains can be accidentally introduced during recrystallization. This is a result of inhomogeneous deformation through- out the alloy matrix. If a metallic object is deformed unevenly, a region con- taining a critical amount of cold work might exist between the worked and unworked areas. Annealing in this case can lead to a localized, very coarse grain due to recrystallization.
As discussed earlier, recrystallization is a kinetic process of nucleation and growth. It depends on alloy composition, impurity content, annealing time, prior grain size, and the complexity of deformation that initiates it. However,
the growth of newly recrystallized grains can be inhibited due to the interfer- ence of inclusions or other crystalline defects. A secondary recrystallization might occur when the annealing temperature of primary recrystallization is raised in this circumstance. In contrast to the primary recrystallization, the driving force for secondary recrystallization is surface tension, instead of strain energy. The surface-tension-induced grain boundary moves toward the curvature center. As a result, small grains with their grain boundaries concave inward will be coalesced into the neighboring large grains that have grain boundaries concave outward. This is a grain coalescence process to satisfy surface tension considerations. A grain from secondary recrystalliza- tion usually has a large grain size and multiple concave outward sides, as shown in Figure 3.4 (Reed-Hill, 1973).
Recovery is another metallurgical phenomenon observed in cold worked metals. However, it is a process very different from recrystallization. In iso- thermal annealing, the recovery process rate always decreases with time as the driving force, stored strain energy, gradually dissipates. On the other hand, recrystallization occurs by a nucleation and growth process; it starts very slowly and gradually builds up to the maximum reaction rate. It then finishes slowly until the entire matrix is recrystallized. The rate profile of recrystallization is like a bell curve, while the profile of recovery resembles only the second half of this curve, starting from the peak and sliding down.
The analysis of recrystallization, secondary recrystallization, or recovery can provide a lot of information relative to the thermal treatment the mate- rial has experienced in reverse engineering.
FIgurE 3.4
Schematic of secondary recrystallization.