Experimental evaluation of the Peierls stresses in a variety of crystals and their relation to the crystal structure

This review paper focuses on dislocations and plastic deformation in magnesium oxide crystals. MgO is an archetype ionic ceramic with refractory properties which is of interest in several fields of applications such as ceramic materials fabrication, nano-scale engineering and Earth sciences. In its bulk single crystal shape, MgO can deform up to few percent plastic strain due to dislocation plasticity processes that strongly depend on external parameters such as pressure, temperature, strain rate, or crystal size. This review describes how a combined approach of macro-mechanical tests, multi-scale modeling, nano-mechanical tests, and high pressure experiments and simulations have progressively helped to improve our understanding of MgO mechanical behavior and elementary dislocation-based processes under stress.

https://hal.archives-ouvertes.fr/hal-01807690/document

Dislocations and Plastic Deformation in MgO Crystals: A Review

We present here the first numerical modelling of dislocations in MgSiO3 post-perovskite at 120 GPa. The dislocation core structures and properties are calculated through the Peierls–Nabarro model using the generalized stacking fault (GSF) results as a starting model. The GSFs are determined from first-principle calculations using the VASP code. Dislocation properties such as planar core spreading and Peierls stresses are determined for the following slip systems: [100](010), [100](001), [100](011), [001](010), [001](110), [001](100), [010](100), [010](001), ½[110](001) and ½[110](110). Our results confirm that the MgSiO3 post-perovskite is a very anisotropic phase with a plasticity dominated by dislocation glide in the (010) plane.

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Peierls–Nabarro model for dislocations in MgSiO3 post-perovskite calculated at 120 GPa from first principles

We present in this study modelling of dislocation core properties in MgO as a function of pressure from ambient conditions to 100 GPa. The calculations are based on the Peierls–Nabarro model and we use first-principles calculations of the generalized stacking fault as input parameters. Our results confirm that {1 1 0} is the dominant slip system in MgO at ambient pressure and is easier than {1 0 0} as the pressure increases. Nevertheless, the {1 1 0} slip hardens with increasing pressure leading to a decrease in the difference in the Peierls stresses between {1 1 0} and {1 0 0}.

http://iopscience.iop.org/article/10.1088/0965-0393/17/3/035010/pdf

Peierls?Nabarro modelling of dislocations in MgO from ambient pressure to 100?GPa

A hierarchical multi-scale model was used to study the effect of high pressure on the critical shear stresses of MgO. The two main slip systems, ½⟨110⟩{110} and ½⟨110⟩{100}, were considered. Based on a generalised Peierls–Nabarro model, it is shown that the core structure of ½⟨110⟩ screw dislocations is strongly sensitive to pressure. Mostly planar and spread in {110} at ambient pressure, the core of screw dislocations tends to spread in {100} with increasing pressure. Subsequently, an inversion of the easiest slip systems is observed between 30 and 60 GPa. At high pressure, the plasticity of MgO single crystals is expected to be controlled by ½⟨110⟩{100} slip systems, except at high temperature where both slip systems become active. Pressure is also found to increase the critical resolved shear stresses and to shift the athermal temperature toward higher temperatures. Under high pressure, MgO is thus characterised by a significant lattice friction on both slip systems.

/pdf/modelling-the-effect-of-pressure-on-the-critical-shear-2l4irzvpkb.pdf

Modelling the effect of pressure on the critical shear stress of MgO single crystals

Within the framework of the Peierls-Nabarro model we present an analytical model for several of the quantities characterizing the mobility of a dislocation line, which covers the whole range of possible values of dislocation width. These quantities include the first-order Peierls stress σ1p (the minimum stress required to move a straight segment of dislocation), the kink profile. the kink pair activation energy H kp and the second-order Peierls stress σ2p (the minimum stress required to move a kink in the dislocation line). These quantities are expressed in terms of fundamental properties of the material, and in particular the relevant generalized stacking-fault surface segment.

J. Zhou

Papers

The Peierls-Nabarro model and the mobility of the dislocation line