Dislocation Interactions in Olivine Revealed by HR‐EBSD

Wallis, David; Hansen, Louis S.; Britton, T. Ben; Wilkinson, Angus J.

doi:10.1002/2017jb014513

Cited by 35 publications

(68 citation statements)

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“…The transverse section was chosen so that most [100] and [001] directions are at low angles to the plane of the map, which is overall the optimal configuration for measuring curvature arising from dislocations with these Burgers vectors (Wallis et al, 2016;Wheeler et al, 2009). Lattice orientation gradients were converted to lower-bound estimates of GND densities following the approach of Wallis et al (2016Wallis et al ( , 2017, who considered six dislocation types. This improved angular resolution lowers the noise floor of associated GND density Geochemistry, Geophysics, Geosystems 10.1002/2017GC007309 estimates, permitting more details of the substructure to be resolved than would be possible using misorientation measurements from conventional EBSD (Jiang et al, 2013;Wallis et al, 2016).…”

Section: Analysis Of Geometrically Necessary Dislocationsmentioning

confidence: 99%

“…This improved angular resolution lowers the noise floor of associated GND density Geochemistry, Geophysics, Geosystems 10.1002/2017GC007309 estimates, permitting more details of the substructure to be resolved than would be possible using misorientation measurements from conventional EBSD (Jiang et al, 2013;Wallis et al, 2016). Lattice orientation gradients were converted to lower-bound estimates of GND densities following the approach of Wallis et al (2016Wallis et al ( , 2017, who considered six dislocation types.…”

Section: Analysis Of Geometrically Necessary Dislocationsmentioning

confidence: 99%

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Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Hansen

Wallis

et al. 2018

Geochem Geophys Geosyst

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To investigate the mechanism that produces the crystallographic preferred orientations (CPO) characteristic of sheared partially molten rocks of mantle composition, we analyzed the microstructures of samples of olivine plus 7% basaltic melt deformed in torsion to shear strains as large as γ= 13.3. Electron backscattered diffraction (EBSD) observations reveal a CPO characterized by a weak a‐c girdle in the shear plane that develops by γ≈ 4. This CPO, which exhibits a slightly stronger alignment of [001] than [100] axes in the shear direction, changes little in both strength and distribution with increasing stress and with increasing strain. Furthermore, it is significantly weaker than the CPO observed for dry, melt‐free olivine aggregates. Orientation maps correlated with grain shape measurements from tangential, radial, and transverse sections indicate that olivine grains are longer along [001] axes than along [100] axes and shortest along [010] axes. This morphology is similar to that of olivine grains in a mafic melt. We conclude that the weak a‐c girdle observed in sheared partially molten rocks reflects contributions from two processes. Due to their shape‐preferred orientation (SPO), grains rotate to align their [001] axes parallel to the flow direction. At the same time, dislocation glide on the (010)[100] slip system rotates [100] axes into the flow direction. The presence of this CPO in partially molten regions of the upper mantle significantly impacts the interpretation of seismic anisotropy and kinematics of flow.

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Section: Analysis Of Geometrically Necessary Dislocationsmentioning

confidence: 99%

Section: Analysis Of Geometrically Necessary Dislocationsmentioning

confidence: 99%

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Hansen

Wallis

et al. 2018

Geochem Geophys Geosyst

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“…Two optimization techniques, L 1 and L 2 , are commonly employed. Each yields a different solution and very few studies (Wallis et al, 2017;Wilkinson and Randman, 2010) have investigated the differences in the results they produce. Here we carry out a systematic comparison of the GND densities predicted by both methods to determine their applicability in different scenarios.…”

Section: Introductionmentioning

confidence: 99%

Consistent determination of geometrically necessary dislocation density from simulations and experiments

Das

Hofmann

Tarleton

2018

International Journal of Plasticity

119

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The use of Nye's dislocation tensor for calculating the density of geometrically necessary dislocations (GND) is widely adopted in the study of plastically deformed materials. The "curl" operation involved in finding the Nye tensor, while conceptually straightforward has been marred with inconsistencies and several different definitions are in use. For the three most common definitions, we show that their consistent application leads to the same result. To eliminate frequently encountered confusion, a summary of expressions for Nye's tensor in terms of elastic and plastic deformation gradient, and for both small and large deformations, is presented. A further question when estimating GND density concerns the optimization technique used to solve the under-determined set of equations linking Nye's tensor and GND density. A systematic comparison of the densities obtained by two widely used techniques, L 1 and L 2 minimisation, shows that both methods yield remarkably similar total GND densities. Thus the mathematically simpler, L 2 , may be preferred over L 1 except when information about the distribution of densities on specific slip systems is required. To illustrate this, we compare experimentally measured lattice distortions beneath nano-indents in pure tungsten, probed using 3D-resolved synchrotron X-ray micro-diffraction, with those predicted by 3D straingradient crystal plasticity finite element calculations. The results are in good agreement and show that the volumetric component of the elastic strain field has a surprisingly small effect on the determined Nye tensor. This is important for experimental techniques, such as micro-beam Laue measurements and HR-EBSD, where only the deviatoric strain component is measured.

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“…These stress heterogeneities are characteristic of back stresses between dislocations piled up along their slip planes (e.g., Guo et al, 2014) and indicate a component of kinematic hardening, in which dislocations interact through their longrange stress fields (Kassner et al, 2013). These long-range interactions may contribute to the organisation of the substructure (Montagnat et al, 2006;Wallis et al, 2017), creating wavelengths up to several tens of micrometres in the misorientation associated with slip bands (Figure 3a, 3d, 3e). The stress distributions in the aggregate are broadly consistent with this model but also contain a contribution from stresses imparted by anisotropic expansion of the grains during decompression.…”

Section: Mechanisms Of Strain Hardening and Recoverymentioning

confidence: 99%

Dislocation interactions during low-temperature plasticity of olivine strengthen the lithospheric mantle

Wallis¹,

Hansen²,

Kumamoto³

et al. 2019

Preprint

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The strength of the lithosphere is typically modelled based on constitutive equations for steady-state flow. However, models of lithospheric flexure reveal differences in lithospheric strength that are difficult to reconcile based on such flow laws. Recent rheological data from low-temperature deformation experiments on olivine suggest that this discrepancy may be largely explained by strain hardening. Details of the mechanical data, specifically the effects of temperature-independent back stresses stored in the samples, indicate that strain hardening in olivine occurs primarily due to long-range elastic interactions between dislocations. These interpretations provided the basis for a new flow law that incorporates hardening by development of back stress. Here, we test this hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation-DIA apparatus at differential stresses of ≤ 4.3 GPa or in a nanoindenter at applied contact stresses of ≥ 10.2 GPa. High-angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10 14 m -2 and intragranular heterogeneities in residual stress on the order of 1 GPa in both sets of samples. Scanning transmission electron micrographs reveal straight dislocations aligned along slip bands and interacting with dislocations of other types that act as obstacles. The stress heterogeneities and accumulations of dislocations along their slip planes are consistent with strain hardening resulting from long-range back-stresses acting between dislocations. These results corroborate the mechanical data in supporting the form of the new flow law for low-temperature plasticity and provide new microstructural criteria for identifying the operation of this deformation mechanism in natural samples. Furthermore, similarities in the structure and stress fields of slip bands formed in single crystals deformed at low temperatures and those formed at high temperatures suggest that similar hardening processes occur in both regimes, providing a new constraint for models of transient creep at high temperatures.

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Dislocation Interactions in Olivine Revealed by HR‐EBSD

Cited by 35 publications

References 54 publications

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Crystallographic Preferred Orientation of Olivine in Sheared Partially Molten Rocks: The Source of the “a‐c Switch”

Consistent determination of geometrically necessary dislocation density from simulations and experiments

Dislocation interactions during low-temperature plasticity of olivine strengthen the lithospheric mantle

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