Deformation Microstructures : Antarctic Ice from EPICA Dronning Maud Land and Artificial Creep TestIce

Abstract

The primary objective of this thesis is the investigation of microstructures obtained from samples from the EPICA Dronning Maud Land ice core from Antarctica. The goal is to gain understanding of deformation processes an deformation-related recrystallization mechanisms using these structures. The structures are visualized with the new microstructure mapping method using the preferred sublimation along defect regions in the crystal. This method enables observation in high resolution as well as overview over a significant sample volume. In order to provide unambiguous proof of their deformational origin and to offer interpretation and characterization, experimental reproduction of the microstructural features are performed using creep tests.Subgrain boundaries and grain-boundary morphology are identified as the most direct effects of deformation and recrystallization processes, which are still easily observable. They can be used additionally to the conventional parameters (grain size, crystal-orientation distribution) to determine these mechanisms. Different sbugrain-boundary types observed in experimentally deformed samples as well as in natural ice indicate several formation processes.Results obtained from this new and novel data suggest a profound reconsideration of the classical tripartition of recrystallization regimes described in the literature in ice sheets. Instead, dynamic recrystallization in two of its forms (rotation recrystallization and strain-induced migration recrystallization) dominates the microstructure evolution in all depth regions of the EDML ice core.Results of systematic microstructure analysis of creep-test samples demonstrate the correlation of grain-substructure evolution and strain hardening during primary creep. Subgrain boundaries and dislocation walls acting as obstacles for dislocation motion was identified thereby as a main process of strain hardening in ice.The comparison of results from experimentally and naturally deformed ice enables a specification of the well-known difference in flow behaviour for low- and high-stress regimes: in low-stress regimes dislocation-density decreasing processes (recovery and dynamic recrystallization), which restore the properties of the material, play a significantly more important role than in high-stress regimes.