Substructure dynamics incorporate all features occurring on a subgrain-scale. The substructure governs the rheology of a rock, which in turn determines how it will respond to different processes during tectonic changes. This project details an in-depth study of substructural dynamics during post-deformational annealing, using single-crystal halite as an analogue for silicate materials. The study combines three different techniques; in-situ annealing experiments conducted inside the scanning electron microscope and coupled with electron backscatter diffraction, 3D X-ray diffraction coupled with in-situ heating conducted at the European Radiation Synchrotron Facility and numerical simulation using the microstructural modelling platform Elle. The main outcome of the project is a significantly refined model for recovery at annealing temperatures below that of deformation preceding annealing. Behaviour is highly dependent on the temperature of annealing, particularly related to the activation temperature of climb and is also strongly reliant on short versus long range dislocation effects. Subgrain boundaries were categorised with regard to their behaviour during annealing, orientation and morphology and it was found that different types of boundaries have different behaviour and must be treated as such. Numerical simulation of the recovery process supported these findings, with much of the subgrain boundary behaviour reproduced with small variation to the mobilities on different rotation axes and increase of the size of the calculation area to imitate long-range dislocation effects. Dislocations were found to remain independent to much higher misorientation angles than previously thought, with simulation results indicating that change in boundary response occurs at ~7º for halite. Comparison of 2D experiments to 3D indicated that general boundary behaviour was similar within the volume and was not significantly influenced by effects from the free surface. Boundary migration, however, occurred more extensively in the 3D experiment. This difference is interpreted to be related to boundary drag on thermal grooves on the 2D experimental surface. While relative boundary mobilities will be similar, absolute values must therefore be treated with some care when using a 2D analysis.

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 280-470 ºC. 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 ºC) some low angle boundaries (LABs), i.e. with 1º-15º misorientation, increase in misorientation angle, while others decrease. In regime II (~300 ºC) all LABs undergo a decrease in misorientation angle. Regime III (>300 ºC) 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.

In-situ 3D X-ray diffraction (3DXRD) annealing experiments were conducted at the ID-11 beamline at the European Synchrotron Radiation Facility in Grenoble. This allowed us to non-destructively document and subsequently analyse the development of substructures during heating, without the influence of surface effects. A sample of deformed single crystal halite was heated to between 260-400 ºC. Before and after heating a volume of 500 by 500 by 300 mm was mapped using a planar beam, which was translated over the sample volume at intervals of 5-10 µm in the vertical dimension. In the following we present partially reconstructed orientation maps over one layer before and after heating for 240min at 260 ºC. Additional small syn-heating “maps” over a constrained sample rotation of 12-30º. The purpose of this was to illuminate a few reflections from 1 or 2 subgrains and follow their evolution during heating.

Preliminary results show that significant changes occurred within the sample volume, for which, surface effects can be excluded. Results show a number of processes, including: i) change in subgrain boundary misorientation angle and ii) subgrain subdivision into areas of similar lattice orientation with new subgrain boundary formation. These results demonstrate that 3DXRD coupled with in-situ heating is a successful non-destructive technique for examining real-time post-deformational annealing in strongly deformed crystalline materials with complicated microstructures.

Recognising post-deformational annealing is key to interpreting rheological adjustments after deformation. The focus of this study is to use coupled in-situ experimental techniques with numerical simulation to increase understanding of substructure dynamics in a geological material, and as a consequence recognise microstructures formed during post-deformational annealing.

In-situ annealing experiments have been conducted in the Scanning Electron Microscope, using Electron Backscatter Diffraction (EBSD) to collect information about the crystallographic orientation of the surface. A single crystal of halite, pre-deformed under uniaxial compression at temperatures of ~450 ºC with a strain rate of 6.9x10^-6s^-1, and to a final strain of 0.165, was examined. Different temperature time-paths were investigated with temperatures between 280-470 ºC and durations of heating between 30 min and 6 h. EBSD maps were taken before, during and after heating. Behaviour during annealing was found to be temperature dependent and could be divided into three main phases of development. Subgrain boundaries could be divided into five categories based on behaviour during annealing, morphology and orientation. Observations could be directly linked to the variable mobility of two groups of dislocations introduced during deformation, as well as temperature control on dislocation glide and climb. We infer that the dislocation budget throughout annealing changed significantly, with respect to both ratio of dislocation types, as well as their location in the substructure. We thus suggest that by investigating the dislocation budget of a system with known deformation geometry the temperature of annealing can potentially be established.

In conjunction with experimentation, development of a numerical model for the processes occurring during annealing has also been undertaken. The experiments were directly compared to the simulation in order to improve the model. A first attempt at modelling the behaviour in the experiments applied a phase field method of gradient reduction. While the model did reproduce some of the results from the experiments, including the subdivision of subgrains into areas of like orientation, it did not realistically replicate other features. To combat the limitations presented by this method a new model is being developed, focusing on new techniques in dislocation density and burgers vector calculation. Information from the experimental results will be used in the development of the code. Once this model is able to reproduce behaviour observed in the experiments it can be used to model the substructure dynamics at a variety of conditions and within other materials.

The understanding of substructural behaviour during post-deformational annealing is key to interpreting rheological adjustments during tectonic change, and the processes which cause them. The focus of this study is to use in-situ experimental techniques to increase understanding of substructure dynamics in geological materials. 2D in-situ annealing experiments have been conducted in the scanning electron microscope, using electron backscatter diffraction (EBSD) to collect information about the crystallographic orientation of the surface. A single crystal of halite, pre-deformed under uniaxial compression at temperatures of ~450 ºC with a strain rate of 6.9x10^-6s^-1, and to a final strain of 0.165, was examined. Different temperature time-paths were investigated with temperatures between 280-470 ºC and durations of heating between 30 min and 6 h. EBSD maps were taken before, during and after heating. Behaviour during annealing was found to be temperature dependent and could be divided into three main phases of development. Subgrain boundaries could be divided into five categories based on behaviour during annealing, morphology and orientation. Annealing behaviour could be directly related to preferential activation of one set of slip systems due to the chosen aspect ratio of the crystal.

While the 2D experiments provided valuable information, it is impossible to rule out the potential influence surface effects may have on annealing behaviour. In order to verify the results of these experiments, a 3D X-ray diffraction experiment was conducted at the synchrotron in Grenoble, France. The experiment followed a similar heating procedure as that for the 2D experiments and was performed on the same sample. This newly developed technique allows non-destructive internal examination of the crystal. Data was collected before, during and after each heating stage. During heating crystallographic information was collected within a limited rotation threshold (12-30º) in order to illuminate one or two subgrains and allow us to follow their progress. Comparison of the shape and strength of intensity spots has allowed us to draw some early conclusions from the data without a full crystallographic analysis. Preliminary results suggest that similar processes may be occurring as those observed in the 2D experiments, including spots becoming more distinct as well as some spots rotating away from the bulk of the subgrain indicating some subdivision and potential polygonisation. We can thus suggest that some of the behaviour exhibited in the 3D experiment is similar to that from the 2D experiment. Full crystallographic analysis of large maps taken after heating will allow us to examine the behaviour of the substructure in more detail and potentially rule out surface effects from the 2D experiments.

1. Introduction and methods

Static in-situ annealing of deformed single-crystal halite shows that three distinct temperature dependent stages of dislocation rearrangement result in an overall decrease in the crystallographic variation of the sample.

Substructure dynamics have been investigated in “real-time” by in-situ heating experiments conducted in the SEM. Electron Backscatter Diffraction (EBSD) maps were taken before, during and after each heating stage, to collect detailed information about the crystallographic orientation and misorientation of the sample substructure. Samples were pre-deformed under uniaxial compression at a temperature of ~450 ºC to strains of 0.165, at a strain rate of 6.9*10-6s-1. Samples were then annealed within an SEM in several heating stages at temperatures between 280-470 ºC, with an arbitrary increase in temperature at each heating stage (see fig. 1b). The length of each heating stage varied from 30 minutes to six hours. The setup of the heating stage and an orientation contrast image of the areas of the crystal analysed are shown in fig. 1.

2. Results

Behaviour during annealing of halite can be divided into three distinct phases based on the low-angle boundary (LAB <15º) behaviour and overall changes in the substructure. LABs were divided into five categories based on their morphology, orientation and behaviour. Fig 2 shows a schematic of the boundary types and a table detailing their important features.

Characteristic behaviour of the annealing phases (see fig. 3):

Annealing phase one 280-300 ºC

a) Type 1 and 2 LABs: increase in the misorientation

b) Type 3 and 4 LABs: decrease in the misorientation

c) Type 5: boundary movement with an average velocity of 0.085µm/min

d) subdivision of some subgrains into plateaus of like orientation

Annealing phase two ~300 ºC

a) All LAB types: decrease in the misorientation

b) Type 5: significant decrease in boundary velocity to an average of 0.032µm/min

c) continued subdivision of subgrains and formation of new LABs at plateau borders

Annealing phase three >300 ºC

a) All LAB types: increase in misorientation of remaining LABs

b) Type 5: increase in boundary movement to an average velocity of 0.169µm/min

c) no new plateau formation occurs

3. Substructural evolution during annealing

We suggest that annealing behaviour is both temperature dependent and varies according to which boundary is examined. Fig. 4 shows a diagram of inferred behaviour occurring during annealing. At lower temperatures (T<300 ºC) annihilation of dislocations in the subgrain interior and at the boundary site for LABs aligned with the harder slip system occurs, resulting in a decrease in misorientation. Concurrently, dislocations are added into the boundaries in preferred alignment, which then increase in misorientation. In areas where there are no dislocations of opposite sign, annihilation is not possible and dislocations of like sign begin to align. Consequently, next to these aligned dislocations plateaus of like orientation form. Phase two (T ~300 ºC) marks a switch in behaviour for type 1 and 2 LABs, which begin to decrease as dislocations annihilate at the boundaries. As temperature increases the length scale on which dislocations are attracted to boundaries extends and previously trapped dislocations begin to move. New tilt boundaries form at borders of plateau regions as more dislocations are added. At T>300 ºC changes are dominated by LAB development, where remaining dislocations move towards boundaries as the range of attraction increases further. The cumulation of these processes results in an overall decrease in the crystallographic variation of the sample and thus a significant decrease in the stored energy of the system.

The understanding of substructural behaviour during post-deformational annealing is key to interpreting rheological adjustments during tectonic change, and the processes which cause them. The focus of this study is to use coupled in-situ experimental techniques with numerical simulation to increase understanding of substructure dynamics in geological materials. 2D in-situ annealing experiments have been conducted in the Scanning Electron Microscope, using Electron Backscatter Diffraction (EBSD) to collect information about the crystallographic orientation of the surface. A single crystal of halite, pre-deformed under uniaxial compression at temperatures of ~450 ºC with a strain rate of 6.9x10-6s-1, and to a final strain of 0.165, was examined. Different temperature time-paths were investigated with temperatures between 280-470 ºC and durations of heating between 30 min and 6 h. EBSD maps were taken before, during and after heating. Behaviour during annealing was found to be temperature dependent and could be divided into three main phases of development. Subgrain boundaries could be divided into five categories based on behaviour during annealing, morphology and orientation.

While the 2D experiments provided valuable information, it is impossible to rule out the potential influence surface effects may have on annealing behaviour. In order to verify the results of these experiments, a 3D X-ray diffraction experiment was conducted at the synchrotron in Grenoble, France. The experiment followed a similar heating procedure as that for the 2D experiments and was performed on the same sample. This newly developed technique allows non-destructive internal examination of the crystal. Preliminary results suggest that similar processes may be occurring as those observed in the 2D experiments. Full analysis and comparison to the 2D results should determine whether behaviour is truly accurate, or if the experiments show some surface effects.

Development of a numerical model for the processes occurring during annealing has also been undertaken. The experiments were directly compared to the simulation in order to improve the model. A first attempt at modelling the behaviour in the 2D experiments applied a phase field method of gradient reduction. While the model did reproduce some of the results from the 2D experiments, including plateau-building it did not realistically replicate other features. To combat the limitations presented by this method a new model is being developed focusing on new techniques in dislocation density and burgers vector calculation. Information about dominant rotation axes will be directly input along with Euler angle orientation. This model should be able to reproduce behaviour observed in the experiments more accurately.

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

like misorientation

(a4) decrease in misorientation and/or dissipation of many subgrain boundaries

(SGBs <15º)

(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.