Recovery (metallurgy)

The physical processes that fall under the designations of recovery, recrystallization and grain growth are often difficult to distinguish in a precise manner. Doherty et al. (1998) stated:

"The authors have agreed that ... recovery can be defined as all annealing processes occurring in deformed materials that occur without the migration of a high-angle grain boundary"

Thus the process can be differentiated from recrystallization and grain growth as both feature extensive movement of high-angle grain boundaries.

If recovery occurs during deformation (a situation that is common in high-temperature processing) then it is referred to as 'dynamic' while recovery that occurs after processing is termed 'static'. The principal difference is that during dynamic recovery, stored energy continues to be introduced even as it is decreased by the recovery process - resulting in a form of dynamic equilibrium.

Fig 1. The annihilation and reorganisation of an array of edge dislocations in a crystal lattice
Fig 2. Animation of the annihilation and reorganisation of edge dislocations in a crystal lattice

A heavily deformed metal contains a huge number of dislocations predominantly caught up in 'tangles' or 'forests'. Dislocation motion is relatively difficult in a metal with a low stacking fault energy and so the dislocation distribution after deformation is largely random. In contrast, metals with moderate to high stacking fault energy, e.g. aluminum, tend to form a cellular structure where the cell walls consist of rough tangles of dislocations. The interiors of the cells have a correspondingly reduced dislocation density.

As mentioned above, the deformed structure is often a 3-D cellular structure with walls consisting of dislocation tangles. As recovery proceeds these cell walls will undergo a transition towards a genuine subgrain structure. This occurs through a gradual elimination of extraneous dislocations and the rearrangement of the remaining dislocations into low-angle grain boundaries.

Sub-grain formation is followed by subgrain coarsening where the average size increases while the number of subgrains decreases. This reduces the total area of grain boundary and hence the stored energy in the material. Subgrain coarsen shares many features with grain growth.

If the sub-structure can be approximated to an array of spherical subgrains of radius R and boundary energy γs; the stored energy is uniform; and the force on the boundary is evenly distributed, the driving pressure P is given by:

Since γs is dependent on the boundary misorientation of the surrounding subgrains, the driving pressure generally does not remain constant throughout coarsening.