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This paper describes a methodology for characterizing the orientation and position of grains of an orthorhombic polycrystalline material at high pressure in a diamond anvil cell. The applicability and resolution of the method are validated by simulations and tested on an experimental data set collected on MgSiO3 post-perovskite at 135 GPa. In the simulations, ∼95% of the grains can be indexed successfully with ∼80% of the peaks assigned. The best theoretical average resolutions in grain orientation and position are 0.02° and 1.4 µm, respectively. The indexing of experimental data leads to 159 grains of post-perovskite with 30% of the diffraction peaks assigned with a 0.2-0.4° resolution in grain orientation. The resolution in grain location is not sufficient for in situ analysis of spatial relationships at high pressure. The grain orientations are well resolved and sufficient for following processes such as plastic deformation or phase transformation. The paper also explores the effect of the indexing parameters and of experimental constraints such as rotation range and step on the validity of the results, setting a basis for optimized experiments. [ABSTRACT FROM AUTHOR]

Microstructures govern the mechanical properties of materials and change dramatically during phase transformations. A detailed understanding of microstructures at different stages of a transformation is important for the design of new materials and for constraining geophysical processes. However, experimental studies of transformation microstructures at the grain scale have been mostly based on ex situ observations of quenched products, which are difficult to correlate with bulk sample properties and transformation kinetics. Here, it is shown how multi-grain crystallography on polycrystalline samples, combined with a resistively heated diamond anvil cell, can be applied to investigate the microstructural properties of a material undergoing a phase transition in situ at high pressure and high temperature. This approach allows the extraction of the crystallographic parameters and orientations of several hundreds of grains inside a transforming sample. Important bulk information on grain size distributions and orientation relations between the parent and the newly formed phase at the different stages of the transformation can be monitored. These data can be used to elucidate transformation mechanisms ( e.g. coherent versus incoherent growth), growth rates and orientation-dependent growth of individual grains. The methodology is demonstrated on the α-γ phase transitions in hydrous Mg2SiO4·H2O up to 22 GPa and 940 K. This transformation most likely occurs in the most abundant mineral of the Earth's upper mantle (Mg0.8Fe0.2SiO4) in deep cold subducted slabs and plays an important role in their subduction behaviour. [ABSTRACT FROM AUTHOR]

Multifit/ Polydefix is an open source IDL software package for the efficient processing of diffraction data obtained in deformation apparatuses at synchrotron beamlines. Multifit allows users to decompose two-dimensional diffraction images into azimuthal slices, fit peak positions, shapes and intensities, and propagate the results to other azimuths and images. Polydefix is for analysis of deformation experiments. Starting from output files created in Multifit or other packages, it will extract elastic lattice strains, evaluate sample pressure and differential stress, and prepare input files for further texture analysis. The Multifit/ Polydefix package is designed to make the tedious data analysis of synchrotron-based plasticity, rheology or other time-dependent experiments very straightforward and accessible to a wider community. [ABSTRACT FROM AUTHOR]

The investigation of materials plastic properties at high pressure is a fast-growing field, owing to the coupling of high-pressure deformation apparatuses with X-ray synchrotron radiation. In such devices, materials strain and strain rate are measured by time-resolved radiography, while differential stress is deduced from the elastic response of the d spacing of the crystallographic planes as measured by X-ray diffraction. Here a new protocol is presented, which allows the in situ measurement of stress and texture development in aggregates deformed at high pressure for experiments carried out with the recently installed ten-element energy-dispersive detector at the X17B2 beamline of the National Synchrotron Light Source (Brookhaven National Laboratory, Upton, NY, USA). Cycling deformation of a forsterite specimen was carried out at a pressure of ∼6 GPa and a temperature of ∼1373 K, using a deformation-DIA apparatus. Diffraction peak energies are analysed in terms of differential stress and principal stress direction, while the intensities of peaks obtained at different azimuths are analysed in terms of lattice preferred orientation (LPO). The development and evolution of a marked LPO, with the (010) plane perpendicular to the compression axis, is observed in situ during the run and is confirmed by electron backscatter diffraction measurements on the run product. [ABSTRACT FROM AUTHOR]