Page 50 - Microtectonics

38 3 · Deformation Mechanisms
3.6 3.6
Recovery

Any crystal can be imagined to possess a certain amount
of ‘internal strain energy’, which is at its minimum when
the crystal lattice is free of dislocations. If we deform a
crystal and induce dislocations, we increase this internal
strain energy by local changes in the distance between
atoms; the increase in internal energy is proportional to
the increase in total length of dislocations per volume of
crystalline material, also known as the dislocation den-
Fig. 3.19. a Growth twins in plagioclase with steps. b Deformation
twins in plagioclase, with tapering edges nucleated on a high stress sity. Dislocations and dislocation tangles are formed in
site at the edge of the crystal response to imposed differential stress (Figs. 3.13, 3.24).
Other processes tend to shorten, rearrange or destroy the
dislocations. Vacancies can migrate towards dislocation
tangles and straighten the blocked sections, thus anni-
hilating the tangles; bent dislocations can straighten, and
dislocations can be arranged into networks. These proc-
esses can decrease the total dislocation length and hence
the internal strain energy of crystals and will therefore
operate following the thermodynamic principle to mini-
mise total free energy in a system. During deformation,
dislocation generation and annihilation mechanisms will
compete while after deformation stops, dislocation an-
nihilation mechanisms progress towards an equilibrium
situation with the shortest possible length of dislocations

Fig. 3.21a–c. Schematic illustration of the recovery process. a Dislo-
Fig. 3.20. Twin boundary migration recrystallisation in calcite can cations distributed over the crystal give rise to undulose extinction.
sweep whole crystals by migration of twin boundaries. Grain b Recovery causes concentration of dislocations in deformation
boundaries are not affected by this recrystallisation mechanism bands and eventually c in a subgrain boundary (tilt wall)