Alexandre Mussi

Characterization of dislocation interactions in olivine using electron tomography

We have investigated by electron tomography, in a transmission electronic microscope, the interactions between dislocations in olivine single crystals and polycrystals deformed in axial compression at T < 1000 °C (T < 0.5Tm). Dislocations are mostly of the [0 0 1] type, except in the polycrystal where [1 0 0] and [0 0 1] dislocations have been activated. A few 〈1 0 1〉 junctions have been found and characterized. Many collinear interactions have been identified either involving direct interactions between crossing dislocations of opposite Burgers vectors or indirect interactions between dislocations gliding in parallel planes and sessile dislocation loops. We suggest that collinear interaction, already identified as the primary source of strain hardening in FCC metals, is the main dislocation interaction mechanism in olivine deformed at temperatures below 1000 °C.

TEM study of defects generated in 4H-SiC by microindentations on the prismatic plane

The deformation microstructure of single crystals of 4H-SiC resulting from microindentations on a prismatic surface was investigated by TEM. Indentations were performed at 400 and 675°C, i.e. below the brittle to ductile transition temperature of 4H-SiC (temperature close to 1100°C). TEM analysis reveals dissociated dislocations as well as extended stacking faults in the basal plane. In addition, perfect edge dislocations are observed on prismatic planes. From the observations, it is assumed that perfect dislocations are nucleated in the prismatic plane and cross-slip on the basal one where they dissociate.

Transmission electron microscopy of dislocations in cementite deformed at high pressure and high temperature

Abstract Polycrystalline aggregates of cementite (Fe3C) and (Fe,Ni)3C have been synthesised at 10 GPa and 1250 °C in the multianvil apparatus. Further, deformation of the carbides by stress relaxation has been carried out at temperature of 1250 °C and for 8 h at the same pressure. Dislocations have been characterised by transmission electron microscopy. They are of the [1 0 0] and [0 0 1] type, [1 0 0] being the most frequent. [1 0 0] dislocations are dissociated and glide in the (0 1 0) plane. [0 0 1] dislocations glide in (1 0 0) and (0 1 0). Given the plastic anisotropy of cementite, the morphology of the lamellae in pearlitic steels appears to have a major role in the strengthening role played by this phase, since activation of easy slip systems is geometrically inhibited in most cases.

Crystal defects in dense hydrous magnesium silicate phase A deformed at high pressure: characterization by transmission electron microscopy

Dense hydrous magnesium silicate (DHMS) Phase A has been deformed at 11 GPa, 700 and 400 °C in the multianvil apparatus. Transmission electron microscopy (TEM) characterizations, using the weak-beam dark-field (WBDF) technique, have shown that at 700 °C, two different types of perfect dislocations are activated with ⅓〈2110〉 and ⅓〈2113〉 Burgers vectors. The ⅓〈2110〉 dislocations glide in the basal plane, but for ⅓〈2113〉 dislocations, no glide plane could be identified. At 400 °C, dissociation of ⅓〈2110〉 dislocations is observed in the basal plane. Furthermore, prismatic and pyramidal glide planes were identified, with dissociations in pyramidal planes, at this temperature. Also a growth reticular merohedral twin has been observed; the twin law results from a rotation of 180° around the [1540] axis.

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.

Hardening mechanisms in olivine single crystal deformed at 1090 °C: an electron tomography study

Abstract The dislocation microstructures in a single crystal of olivine deformed experimentally in uniaxial compression at 1090 °C and under a confining pressure of 300 MPa, have been investigated by transmission electron tomography in order to better understand deformation mechanisms at the microscale relevant for lithospheric mantle deformations. Investigation by electron tomography reveals microstructures, which are more complex than previously described, composed of and dislocations commonly exhibiting 3D configurations. Numerous mechanisms such as climb, cross-slip, double cross-slip as well as interactions like junction formations and collinear annihilations are the source of this complexity. The diversity observed advocates for microscale deformation of olivine significantly less simple than classic dislocation creep reported in metals or ice close to melting temperature. Deciphering mechanism of hardening in olivine at temperatures where ionic diffusion is slow and is then expected to play very little role is crucial to better understand and thus model deformation at larger scale and at temperatures (900–1100 °C) highly relevant for the lithospheric mantle.

Transmission electron microscopy characterization of the dislocations and slip systems of the dense hydrous magnesium silicate superhydrous B

Superhydrous B (shy B) is a dense hydrous magnesium silicate (DHMS) phase, for which experimental studies have indicated a large stability field along cold subduction geotherms in the Earth’s transition zone and top region of the lower mantle. The rheological properties of shy B may therefore be relevant to the rheology and possibly the deep seismic characteristics of subduction zones. Samples of shy B have been synthesized and deformed in a multianvil apparatus at 20 GPa and at temperatures 1000–1100 °C. The resulting dislocations have been characterized by transmission electron microscopy (TEM) with the weak-beam dark-field (WBDF) and the large-angle convergent-beam electron diffraction (LACBED) techniques. We emphasize the role of (010) plane in the plasticity of phase shy B. We report that [100] and [001] glide in (010), and also climb of [100]. We also observe 〈101〉 glide in (010) and {111} planes under high stress conditions.

Effect of electronic doping on the plasticity of homoepitaxial 4H-SiC single crystals

Instrumented micro-indentations have been performed at room temperature on 4H-SiC homoepitaxial single crystals with different doping. For these experiments, it appears that the pop-in event occurs at the same level of load for intrinsic and n-type SiC and at a higher load level for p-type. Correlation of the pop-in event with dislocation nucleation indicates that doping acts on dislocation nucleation and that p-type doping plays a hardening role on the plastic behaviour of 4H-SiC. This result is confirmed by the conventional measurement of imprint size using scanning electron microscopy.

Dislocation Activity in 4H-SiC in the Brittle Domain

Results of deformation experiments on 4H-SiC single crystals below the usual brittle to ductile transition temperature are reported and discussed in comparison of previous literature data. Si-core and C-core partials are evidenced in the basal plane, and perfect dislocations are also observed on other crystallographic planes. These results could indicate that dislocation activity under high stress is more complex than expected.