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_Journal of Structural Geology 32 (2010) 982-996_ _Contents lists available at ScienceDirect_ _Journal of Structural Geology_ _journal homepage: www.elsevier.com/locate/jsg_ _Post-deformational annealing at the subgrain scale: Temperature dependent behaviour revealed by in-situ heating experiments on deformed single crystal halite_ _V.E. Borthwick*, S. Piazolo_ _Department of Geology and Geochemistry, Stockholm University, Svante Arrhenius v?g 8C, Stockholm 10691, Sweden_ _article info_ _Article history: Received 5 November 2009; Received in revised form 3 June 2010; Accepted 16 June 2010; Available online 7 July 2010_ _Keywords: Halite Annealing EBSD Substructure In-situ_ _abstract_ _The dynamics of substructures, which encompass all structures present at the subgrain scale, were investigated by static, in-situ annealing experiments. Deformed, single crystal halite was annealed inside a scanning electron microscope at temperatures between 280 and 470 x14C. Electron backscatter diffraction maps provided detailed information about crystallographic orientation changes. Three temperature dependent regimes were distinguished based on boundary misorientation changes. In regime I (280-300 x14C) some low angle boundaries (LABs), i.e., with 1-15 x14C misorientation, increase in misorientation angle while others decrease. In regime II (>300 x14C) all LABs undergo a decrease in misorientation angle. Regime III (>300 x14C) is defined by enhancement of the subgrain structure as remaining LABs increase and some undergo a rotation axis change. Throughout regimes I and II, new LABs develop, subdividing subgrains. LABs could be divided into four categories based on annealing behaviour, orientation and morphology. We suggest that these observations can be directly related to the mobility and activation temperature of climb of two dislocation groups introduced during deformation. Therefore, with in-depth investigation of a substructure with known deformation geometry, we can infer ratios of dislocation types and their post-deformation and post-annealing location. These can potentially be used to estimate the post-deformational annealing temperature in crystalline materials._ _? 2010 Elsevier Ltd. All rights reserved._ _1. Introduction_ _Interpretation of microscale behaviour is key to developing a greater understanding of tectonic processes. Examination of the microstructure of a rock can give insight into the processes that occurred during its deformation history and the conditions under which these processes took place. The inherent problem in microstructural interpretation is that we are viewing the “frozen-in” final microstructure, a result of the accumulation of a sequence of processes. In particular, post-deformational annealing can drastically change the microstructure by growth of new strain-free grains and reorganisation of grain boundaries (e.g., Heilbronner and Tullis, 2002). However, at the same time, these changes have the potential to provide evidence of the time-temperature path of the rock, as well as its rheological evolution._ _An important part of post-deformational annealing is substructural rearrangement. Deformation is accommodated by the storage of defects in the crystal lattice. Strain energy is stored as_ _* Corresponding author. Fax: ?46 (0) 8 6747897. E-mail address: verity.borthwick@geo.su.se (V.E. Borthwick)._ _0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.06.006_ _point defects, dislocations and dislocation arrays (subgrain boundaries or grain boundaries) (Passchier and Trouw, 2005). The driving force for post-deformational annealing is the reduction of the stored energy of the system (Urai et al., 1986; Drury and Urai, 1990; Baker, 2000). This recovery is driven by the interaction of dislocations via their long-range stress fields and occurs through two main processes, annihilation of dislocations of opposite signs and polygonisation, where dislocations align to form low energy arrays (LABs) (Gottstein, 2004; Humphreys and Hatherly, 2004). As dislocations align, their areas of distortion overlap so that, with increasing misorientation the energy per dislocation decreases (Hull and Bacon, 2001). These processes are facilitated by dislocation climb, which is thermally activated and can limit mobility at lower temperatures. In particular, climb is important for dislocations in a boundary to rearrange to decrease spacing (Hull and Bacon, 2001). Once a polygonised substructure is attained, the stored energy can be further lowered by a coarsening of the substructure to reduce total boundary area (Humphreys and Hatherly, 2004). In particular, symmetrical tilt boundaries can move by glide of the edge dislocations that comprise the boundary (Humphreys and Hatherly, 2004). Mobility in this case is high, and_ _V.E. Borthwick, S. Piazolo Journal of Structural Geology 32 (2010) 982-996_ _983_ _boundary migration can occur even at low temperatures (Parker and Washburn, 1952). It is important to note that the energies of LABs are strongly dependent on both misorientation and the boundary plane; therefore changes in the misorientation and/or boundary plane may also result in energy reduction even if the total grain boundary length increases (Piazolo et al., 2004)._ _LABs have, in general, not received as much attention as high angle grain boundaries in geological materials. Previous experiments carried out on polycrystalline halite (Bestmann et al., 2005; Piazolo et al., 2006) indicated that substructural elements did not behave exactly as predicted by the above outlined classical theory, where LABs are generally expected to undergo a stable increase in misorientation once formed, progressing on to subgrain growth to reduce the total boundary length (Humphreys and Hatherly, 2004). Bestmann et al. (2005) and Piazolo et al. (2006) observed LABs both increasing and decreasing in misorientation, rearranging within grains as well as in some cases dissipating completely._ _Recently developed techniques in the field of microstructural analysis enable us to investigate substructural behaviour in more detail. Electron backscatter diffraction (EBSD) (Prior et al., 1999 and references therein) allows us to fully characterise misorientation axes and angles between grains and subgrains. There have been a number of studies of microstructures taken from various stages of annealing in both geological materials (e.g., calcite) (Barnhoorn et al., 2005) and metals (Ferry and Humphreys, 1996, 2006; Huang and Humphreys, 2000, 2001; Huang et al., 2000). In-situ heating within the scanning electron microscope (SEM) (Le Gall et al., 1999) and analysis with EBSD are thus an essential addition, providing a powerful tool for “real-time” microanalysis of structural changes during annealing (Humphreys, 2001; Seward et al., 2002; Piazolo et al., 2005). At present a handful of studies have been carried out using this method, on materials including titanium (Seward et al., 2004), aluminium (Huang and Humphreys, 1999; Piazolo et al., 2005; Kirch et al., 2008) rocksalt (Bestmann et al., 2005; Piazolo et al., 2006), copper (Mirpuri et al., 2006; Field et al., 2007) and AleMn alloys (Lens et al., 2005)._ _NaCl was chosen as the experimental material for this study. Halite plays a significant role in fold-and-thrust belts, delta tectonics, basin evolution and hydrocarbon accumulation, as well as being a possible medium for storage of nuclear waste (Franssen, 1993 and references therein; Rempe, 2007; Schl?der and Urai, 2007). The development of subgrain-scale microstructures in halite occurs at experimentally attainable conditions (>20 MPa and >200 x14C) (Senseny et al., 1992) and is similar to that occurring at higher temperatures and pressures in silicates, making it a good analogue material (Guillope and Poirier, 1979; Drury and Urai, 1990). Due to its ionic-bonded, cubic crystal structure, NaCl provides a simple starting point for studying these complex processes._ _In this contribution, we aim to provide a comprehensive characterisation of the substructural dynamics of a crystalline geological material during post-deformational annealing. On the basis of this information we attempt to recognise key features occurring at different annealing temperatures, which may be used for interpreting post-deformational annealing conditions in natural samples. In particular, by recognising different types of LABs and their respective behaviour during annealing we aim to improve our understanding of substructural development._ _2. Methods_ _the possibility of rapid high angle grain boundary migration removing the substructure (Piazolo et al., 2006) and the high purity meant that boundary pinning by impurities would be less likely to occur (Smith, 1948). Rectangular samples with a size of w7 ? 10 ? 15 mm were cleaved along {100} faces. These particular edge ratios were chosen to limit dislocation glide to two sets of perpendicular planes (Fig. 1a) (Davidge and Pratt, 1964). Due to the asymmetry of the sample one set of glide planes will be activated more easily. With this chosen geometry the sample is more relevant to geological materials with low symmetry crystallography, which tend to have a well-developed dominant slip system with a number of subsidiary slip systems._ Ключевые слова: random colour, piazolo journal, increase, material, tilt geometry, geophysical, huang, increase misorientation, surface, humphreys huang, background error, aligned parallel, temperature regime, activated, crystallographic orientation, boundary selected, type labs, dislocation annihilation, contrast, wheeler, humphreys, post, image, analysis area, society, subgrain growth, lab site, post-deformational annealing, experimental process, in-situ experiment, annealing experiment, dislocation density, sample studied, acta materialia, behaviour, natural sample, wa, lab type, grain boundary, trimby, substructure, journal structural, post-deformational, peripheral zone, substructural, gottstein, status, deformation temperature, axis, type based, electron, dynamic recrystallization, experiment, subgrain subdivision, sample, lab movement, halite, elsevier, high, decrease, temperature window, angle, sample surface, ebsd data, pixel, climb, process, nal microstructure, lensoid shape, dislocation, labs decrease, science, deformed rock, drury, lab, occurs, subgrain, edge dislocation, crystallographic variation, annealing behaviour, change, regime, iii, compression axis, piazolo, deformation, critical boundary, rotation, grain, subgrain structure, tectonophysics, signicant change, misorientation decrease, deformation geometry, geometry, grain growth, error, occurring, metallurgical society, stored energy, urai, rotation axis, heating, kuwahara lter, dislocation align, interpretation, dependent, observed, signi?cant, slip, journal, orientation spread, driving force, average misorientation, in-situ, ax, journal structural geology, rate, trace, small, mm, diffraction, view annealing, subgrain interior, boundary analysis, study, misorientations, boundary, heating regime, temperature, huang humphreys, equal area, data, boundary site, prior, number, elongate band, deformation history, point, mobility, structural geology, tilt boundary, annealing regime, misorientation analysis, area, rst point, result, occur, method, tilt wall, slight increase, structural, geological material, materials, misorientation axis, boundary selection, single, heating step, length, discussion, rock, secondary slip, materialia, deformed, microstructures, misorientation increase, scanning, regime iii, franssen, ii, qmis, misorientation, geology, ltering technique, tilt, senseny, movement, stage, noise reduction, pennock, annealing temperature, type, behaviour occurs, trace analysis, acta, average, misorientation angle, boundary trace, annealing, reduction, borthwick piazolo, labs, energy, characteristic behaviour, dislocation addition, room temperature, natural setting, recrystallization, ebsd, crystal, engineering, borthwick, subgrains, dislocation climb, energy reduction, higher, heating stage, substructural behaviour, map, analysis, misorientation distribution, orientation, non-indexed point