Direct observation of phase transitions: in situ diffraction measurements at the crystal scale
Abstract
Phase transitions often display determinate crystallographic orientation relationships between parent and symmetrically degenerate daughter domains. Preferred variant selection and orientation memory under the influence of deviatoric stress are the 'fingerprints' of the transformation mechanism, driving force, and microstructure. For the α-ɛ transition in iron - significant to the structure and anisotropy of the Earth's inner core - these have been studied primarily by texture analysis of polycrystalline powders; the convoluted nature of these data, however, render it essentially impossible to examine variant selection, and even precise orientation relationships themselves. Recently developed X-ray diffraction techniques have made it possible to answer these questions. Results from two measurements are presented. First, heterogeneous variant selection was observed and the specific α/ɛ orientation relationship was determined under quasi-static loading using High Energy Diffraction Microscopy at beamline 1-ID of APS-ANL. Second, the spatial heterogeneity and mutual exclusivity among ɛ variants was observed at the incipient transition using the Laue μ-diffraction technique at beamline 12.3.2. of ALS-LBNL. In both cases, a 60x25μm single crystal of iron was pressurized in a DAC and measured before and after the α-ɛ transition at 13GPa. In the HEDM experiment, spatial resolution is sacrificed for domain-averaged orientation and strain resoltuion, making it possible to quantify the magnitude of the applied deviatoric stress in situ. In the Laue μ-Diffraction technique, the local lattice orientations are mapped with 1μm resolution, displaying marked spatial variation. These pilot experiments open new possibilities for investigating high pressure/high temperature transformations in situ. By mapping both orientations and lattice strains, the nature of the mechanisms and driving forces can be illuminated. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Funding through the Laboratory Directed Research and Development program (10- ERD-053) is gratefully acknowledged.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR23A2388B
- Keywords:
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- 0594 COMPUTATIONAL GEOPHYSICS / Instruments and techniques;
- 3924 MINERAL PHYSICS / High-pressure behavior;
- 4465 NONLINEAR GEOPHYSICS / Phase transitions;
- 5112 PHYSICAL PROPERTIES OF ROCKS / Microstructure