The behaviour of substructures during annealing of deformed single-crystal halite has been investigated by in-situ heating experiments in the SEM. Electron Backscatter Diffraction (EBSD) maps were taken before and after each heating stage, providing information about the crystallographic orientation and misorientation of the sample surface. Two differing samples were used in a number of experiments. Both samples were deformed at a temperature of 450 ºC with GP1a deformed to ~0.180 strain, at a strain rate of ~6*10-6s-1 and TL1 deformed to 0.165 strain, at a strain rate of 6.9*10-6s-1 with a longer cooling phase. The samples were heated to temperatures between 280- 450 ºC with an arbitrary increase in temperature at each heating stage. The length of each heating stage varied from 30 minutes to six hours.
Substructural observations can be divided into two phases of stored energy reduction:
(a) Smoothing of crystallographic variations:
(a1) slight overall decrease in misorientation with variations between some
subgrains (SGs) decreasing
(a2) decrease in the misorientation within individual SGs,
(a3) SGs with a remaining high internal misorientation subdividing into plateaus of
(a4) decrease in misorientation and/or dissipation of many subgrain boundaries
(b) SGB development:
(b1) increase in the misorientation of many SGB segments
(b2) SG coarsening by movement of SGBs
Phase (a) was primarily observed in sample GP1a but similar behaviour took place in TL1 at lower temperatures. Phase (b) was only observed in sample TL1, at heating temperatures above ~400 ºC with occasional marked movement (over distances of ~20µm) apparent.
Results suggest that two distinct annealing phases are operating to achieve energy reduction of the whole system. The first recovery phase (a) focuses on an overall smoothing of the sample, primarily via decrease in internal misorientation of subgrains and the variation between them. During this phase, annihilation of dislocations occurs as dislocations of unlike signs migrate along lattice planes. SGB segments either decrease in misorientation, some dissipating with dislocation annihilation or remain fixed. The differences observed between the samples during (a) may be attributed to a longer cooling time for TL1 after deformation which led to it attaining an advanced stage of recovery. During the movement phase (b) most SGBs are distinct and begin to increase in misorientation as remaining dislocations in the system are added to them, facilitated by increased temperatures. In TL1, phase (a) continues to a minor extent during phase (b) with (a4) taking place to some degree. Stored energy is further reduced by coarsening of SGs via movement of SGB segments.