M5 Recrystallization and Grain Growth

Topic

When this metal undergoes cold plastic deformation, it accumulates a dislocation density—as described by another model in this series—leaving it in a state of high stored energy; subjecting the metal in this state to annealing (a heat treatment maintained at a sufficiently high temperature) triggers recrystallization: the nucleation and growth of entirely new, strain-free grains that progressively replace the deformed, dislocation-rich grains resulting from the cold work. If the metal enters the annealing stage with greater accumulated plastic deformation—and thus more available stored energy—recrystallization nucleates more easily and proceeds more rapidly during the process.

As these new, strain-free grains nucleate and grow at the expense of the remaining deformed grains, the recrystallization process follows the Avrami equation—which, in other transformations within this series, describes how the transformed fraction evolves over time. The process follows a curve that advances slowly at first (when few new grains have nucleated), accelerates as the new grains grow and others appear, and slows down again toward the end, when little deformed material remains to be recrystallized. According to the Avrami equation, a higher annealing temperature causes recrystallization to proceed more rapidly for a given elapsed time.

Once recrystallization is complete and no deformed grains remain to be replaced, continued annealing no longer produces new grains but instead causes the existing recrystallized grains to grow; this is known as normal grain growth, a process that follows a parabolic time law with a rate constant that depends on temperature according to an Arrhenius relationship. The fact that this annealing process continues for an extended period after recrystallization is complete—in accordance with that same parabolic law—causes the metal's grain size to keep increasing, albeit at a progressively slower rate.

Consequently, this model—which begins with cold working and recrystallization—relies on two other models within this set that independently established components combined here: the model describing the metal's grain size (based on the initial grain size from which recrystallization starts) and the model describing the metal's atomic diffusion (governed by the same Arrhenius dependency that controls both this recrystallization and the subsequent normal grain growth). The fact that this same Avrami equation describes the nucleation and growth of strain-free grains in this context places it within the same family of models already used to describe other analogous transformations: glass vitrification and devitrification, and grain growth during ceramic sintering.

ID:4437