Data processing, 3D grain boundary modelling and analysis of in-situ deformation experiments using an automated fabric analyser microscope
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Abstract
The microstructure and fabric of a material influence critically the mechanical properties, such as the tensile strength and ductility. This holds for metals and ceramics, as well as for rocks and ice. Furthermore, microstructure and fabric provide vital information about the deformation and annealing history of polycrystalline solids. Rock and ice thin sections can be scanned in high resolution and the orientation of the crystallographic c-axis determined using an automatic fabric analyser microscope (“Fabric Analyser”). The FAME (Fabric Analyser based Microstructure Evaluation) software, based on the original FAME scripts by Peternell et al. (2014), was developed in MATLAB to analyse the data recorded data. In addition to an improved grain labelling, FAME incorporates a new “testing” gadget to simplify the determination of appropriate grain labelling parameters. FAME introduced a couple of new statistic and plotting tools, including c-axis misorientation maps and a toolbox to export FAME data to the elle (mod-elling software)-supported file format. The data processed by FAME also provided the basis for FAGO (Fabric Analyser Grain boundary recOnstruction), a new and innovative approach for reconstructing grain boundaries in 3D from geological thin sections. Grain boundaries are an important aspect of the microstructure and play a significant role during recrystallization. However, there used to be a lack of non-destructive and easy-to-use computer supported methods to de-termine grain boundary geometries in 3D. The newly developed method basically uses the highest birefringence colour (retardation) at each pixel in the field of view acquired by the 9 different oriented light sources of the Fabric Analyser. Retardation profiles across grain boundaries enable the calculation of grain boundary angle and direction. In combi-nation with the lateral position of the grain boundary, acquired using FAME, the data is used to reconstruct a 3D grain boundary model. The data processing is almost fully automatic by using MATLAB. An important application of FAME is the analysis of in-situ pure shear ice deformation experiments which allow a continuous observation of the microstructure during the pro-gress of deformation using the Fabric Analyser. The experimental modelling of ice pro-vide vital information about the rheological behavior of ice which is necessary to under-stand the movement of glacier. Three ice samples from the Sørsdal Glacier were deformed at -10°C; two at relative fast strain rate (2*10-6 1/s) and one at relative slow rate (1*10-6 1/s) by M. Peternell and C.J.L. Wilson. It was revealed that no steady state was reached in the slow strain experiment, even at 57.6% strain. The concentration of dislocations on large grains in hard glide position seems to lead to cyclic changes in the population of grains in easy glide position and impedes the approach to the steady state in this experi-ment. In contrast to the well-established literature a stabilised mean grain size proved not to be a reliable indicator for the steady state. Instead two new microstructure-based indicators for the steady state were introduced, the “seeding rate” and the “microstructure activity”.