Characterization by Scanning Precession Electron Diffraction of an Aggregate of Bridgmanite and Ferropericlase Deformed at HP‐HT

Abstract

Scanning precession electron diffraction is an emerging promising technique for mapping phases and crystal orientations with short acquisition times (10–20 ms/pixel) in a transmission electron microscope similarly to the Electron Backscattered Diffraction (EBSD) or Transmission Kikuchi Diffraction (TKD) techniques in a scanning electron microscope. In this study, we apply this technique to the characterization of deformation microstructures in an aggregate of bridgmanite and ferropericlase deformed at 27 GPa and 2,130 K. Such a sample is challenging for microstructural characterization for two reasons: (i) the bridgmanite is very unstable under electron irradiation, (ii) under high stress conditions, the dislocation density is so large that standard characterization by diffraction contrast are limited, or impossible. Here we show that detailed analysis of intracrystalline misorientations sheds some light on the deformation mechanisms of both phases. In bridgmanite, deformation is accommodated by localized, amorphous, shear deformation lamellae whereas ferropericlase undergoes large strains leading to grain elongation in response to intense dislocation activity with no evidence for recrystallization. Plastic strain in ferropericlase can be semiquantitatively assessed by following kernel average misorientation distributions.

Application of Scanning Precession Electron Diffraction in the Transmission Electron Microscope to the Characterization of Deformation in Wadsleyite and Ringwoodite

“…As previously shown with bridgmanite and ferropericlase [14], we present here a new demonstration that SPED is well adapted to the study of plastic deformation of mantle phases deformed at high-pressure and high-temperature. This is made possible thanks to the short acquisition time at each probe location which preserves beam-sensitive phases.…”

“…We propose that the KAM represents a proxy of plastic strain which can be used to compare strains in different grains. In a previous study performed on an aggregate of bridgmanite and ferropericlase [14], we avoided making such comparisons, since the two phases were undergoing very different deformation mechanisms (dense shear lamellae for bridgmanite, and homogeneous dislocation glide for ferropericlase). Here, wadsleyite and ringwoodite both deform by dislocation glide, and exhibit very comparable dislocation microstructures.…”

“…This effect has been known and used for a long time, since undulatory extinction represents a common criterion for intragranular plasticity without recovery in optical microscopy. With the advent of automated orientation mapping by electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM), the measurement of intragranular misorientation has been used as a proxy for plastic strain [11,12], with applications for severely deformed materials [13,14]. Such measurements are now possible in the TEM with a very high spatial resolution, and we have demonstrated recently the capability of the technique in providing information about deformation mechanisms in a bridgmanite and ferropericlase aggregate deformed at high-pressure and high-temperature [14].…”

Systematic theoretical study of [001] symmetric tilt grain boundaries in MgO from 0 to 120 GPa

“…In polycrystalline aggregates, the properties of grain boundaries (GBs) are critical to the understanding of the mechanical behaviour. In the laboratory, deformation of aggregates of bridgmanite and ferropericlase has shown that the former, which is the weaker, localises the strain leading to grain elongation and fragmentation (Girard et al 2015;Nzogang 2018), i.e., in an increase of the GB fraction. In a high-temperature regime, GBs are expected to play a key role in grain growth, in strain production by GB migration (Sun et al 2017), or in controlling point defects' concentrations by acting as sources or sinks for vacancies (Karki et al 2015).…”

Microstructural Evidence for Grain Boundary Migration and Dynamic Recrystallization in Experimentally Deformed Forsterite Aggregates

“…Here, we present an alternative approach based on scanning precession electron diffraction (SPED), which allows the orientation maps to be recorded with high resolution (6 nm in the present case). This technique has already been used by us to characterize highly deformed specimens from high-pressure experiments [30,31]. In those studies, we had shown that the microdiffraction spot patterns were very robust against the defect microstructure and that high-quality orientation maps could be obtained on deformed samples.…”