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Under certain conditions, the structure can recrystallize during deformation giving rise to dynamic recrystallization. In principle, this form of recrystallization can also occur during cold deformation, but in practice, this is only exceptionally observed, e.g. in very pure metals. In this section, dynamic recrystallization is classified as either discontinuous dynamic recrystallization (Derby [1987, 1991]) or as one of two other types. The latter are geometric dynamic recrystallization

(McQueen et al. [19]) and dynamic recrystallization through progressive subgrain rotation (Gardner and Grimes [1979]), and both involve strain-induced phenomenon with limited or no movements of high-angle boundaries. Following usual

practice, these other types have been included here as part of the present section on dynamic recrystallization.

5.3.5.1 Discontinuous dynamic recrystallization. Figure 5.7 shows typical flow curves during cold and hot deformation. During hot deformation, the shape of the

flow curve can be ‘restricted’, or work hardening rates counterbalanced, by

dynamic recovery or by dynamic recrystallization (i.e. discontinuous dynamic

recrystallization). Dynamic recovery is typical of high-SFE metals (e.g. aluminium, low-carbon ferritic steel, etc.), where the flow stress saturates after an initial period of work hardening. This saturation value depends on temperature, strain rate and composition. On the other hand, as shown in Figure 5.7, a broad peak (or multiple peaks) typically accompany dynamic recrystallization.

Figure 5.8 illustrates schematically the microstructure developments during dynamic recovery and dynamic recrystallization. During dynamic recovery, the

original grains get increasingly strained, but the sub-boundaries remain more or

less equiaxed. This implies that the substructure is ‘dynamic’ and re-adapts continuously to the increasing strain. In low-SFE metals (e.g. austenitic stainless steel,

copper, etc.), the process of recovery is slower and this, in turn, may allow sufficient stored energy build-up. At a critical strain, and correspondingly at a

value/variation in driving force, dynamically recrystallized grains appear at the

original grain boundaries – resulting in the so-called ‘necklace structure’. With further deformation, more and more potential nuclei are activated and new recrystallized grains appear. At the same time, the grains, which had already recrystallized in a previous stage, are deformed again. After a certain amount of strain,

saturation/equilibrium sets in11(see Figure 5.8b). Typically equilibrium is reached between the hardening due to dislocation accumulation and the softening due to dynamic recrystallization. At this stage, the flow curve reaches a plateau and the

microstructure consist of a dynamic mixture of grains with various dislocation densities. It is important, at this stage, to bring out further the structural developments and structure–property correlation accompanying dynamic recovery and dynamic

recrystallization respectively. Both the subgrain size (dsubgrain, from dynamic recovery) and grain size (Drex, dynamic recrystallization) are increasing functions of

temperature and of inverse strain rate. Both follow a Hall–Petch-type (Eq. (4.3))

where _ is flow stress and _1, k1, n1, _2, k2 and n2 are constants. Corresponding to dynamic recovery (Eq. (5.16)), _1 has a low value and n1 is close to 1, while k1

depends on alloy composition (being higher at higher solute content). For dynamic recrystallization (Eq. (5.17)), n2 typically falls within 0.5–0.8.

Unlike static and dynamic recovery, recrystallization includes another classification – a sort of grey area between dynamic and static: meta-dynamic recrystallization. In this situation, the recrystallized nuclei form or nucleate dynamically, during hot deformation, but growth takes place during subsequent static annealing.