Pierre Hirel

Evidence of two plastic regimes controlled by dislocation nucleation in silicon nanostructures

We performed molecular dynamics simulations of silicon nanostructures submitted to various stresses and temperatures. For a given stress orientation, a transition in the onset of silicon plasticity is revealed depending on the temperature and stress magnitude. At high temperature and low stress, partial dislocation loops are nucleated in the {111} glide set planes. But at low temperature and very high stress, perfect dislocation loops are formed in the other set of {111} planes called shuffle. This result confirmed by three different classical potentials suggests that plasticity in silicon nanostructures could be controlled by dislocation nucleation.

Effects of temperature and surface step on the incipient plasticity in strained aluminium studied by atomistic simulations

Atomistic simulations using an EAM potential are carried out to investigate the first stages of plasticity in aluminium slabs, in particular the effect of both temperature and step geometry on the nucleation of dislocations from surface steps. Temperature is shown to significantly reduce the elastic limit, and to activate the nucleation of dislocation half-loops. Twinning occurs by successive nucleations in adjacent glide planes. The presence of a kinked step is shown to have no influence on the nucleation mechanisms.

Effects of Sublattice Symmetry and Frustration on Ionic Transport in Garnet Solid Electrolytes

We use rigorous group-theoretic techniques and molecular dynamics to investigate the connection between structural symmetry and ionic conductivity in the garnet family of solid Li-ion electrolytes. We identify new ordered phases and order-disorder phase transitions that are relevant for conductivity optimization. Ionic transport in this materials family is controlled by the frustration of the Li sublattice caused by incommensurability with the host structure at noninteger Li concentrations, while ordered phases explain regions of sharply lower conductivity. Disorder is therefore predicted to be optimal for ionic transport in this and other conductor families with strong Li interaction.

Pure climb creep mechanism drives flow in Earth’s lower mantle

Climb creep provides an efficient deformation mechanism for bridgmanite under lower mantle conditions. At high pressure prevailing in the lower mantle, lattice friction opposed to dislocation glide becomes very high, as reported in recent experimental and theoretical studies. We examine the consequences of this high resistance to plastic shear exhibited by ringwoodite and bridgmanite on creep mechanisms under mantle conditions. To evaluate the consequences of this effect, we model dislocation creep by dislocation dynamics. The calculation yields to an original dominant creep behavior for lower mantle silicates where strain is produced by dislocation climb, which is very different from what can be activated under high stresses under laboratory conditions. This mechanism, named pure climb creep, is grain-size–insensitive and produces no crystal preferred orientation. In comparison to the previous considered diffusion creep mechanism, it is also a more efficient strain-producing mechanism for grain sizes larger than ca. 0.1 mm. The specificities of pure climb creep well match the seismic anisotropy observed of Earth’s lower mantle.

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

The properties of [001] symmetric tilt grain boundaries (STGBs) in magnesium oxide are investigated systematically with atomic-scale simulations. Their formation energies, atomic structures, and excess volumes are computed. STGBs are found to prefer a symmetric configuration, except for tilt angles larger than 67.4°, where the decomposition of the STGBs into ½[110] dislocations is preferred. Then, the effects of pressure are investigated from 30 up to 120 GPa, a pressure range relevant to the Earth’s lower mantle. Pressure is found to change the atomic configuration of all investigated GBs, sometimes several times as pressure increases. We also find that these changes are irreversible, the GBs retaining their high-pressure configuration even after pressure is released. Implications for the deformation of ferropericlase in the conditions of the Earth’s lower mantle are discussed.

On low temperature glide of dissociated 〈1 1 0〉 dislocations in strontium titanate

Abstract An elastic interaction model is presented to quantify low temperature plasticity of SrTiO3 via glide of dissociated 〈1 1 0〉{1 1 0} screw dislocations. Because 〈1 1 0〉 dislocations are dissociated, their glide, controlled by the kink-pair mechanism at T < 1050 K, involves the formation of kink-pairs on partial dislocations, either simultaneously or sequentially. Our model yields results in good quantitative agreement with the observed non-monotonic mechanical behaviour of SrTiO3. This agreement allows to explain the experimental results in terms of a (progressive) change in 〈1 1 0〉{1 1 0} glide mechanism, from simultaneous nucleation of two kink-pairs along both partials at low stress, towards nucleation of single kink-pairs on individual partials if resolved shear stress exceeds a critical value of 95 MPa. High resolved shear stress allows thus for the activation of extra nucleation mechanisms on dissociated dislocations impossible to occur under the sole action of thermal activation. We suggest that stress condition in conjunction with core dissociation is key to the origin of non-monotonic plastic behaviour of SrTiO3 at low temperatures.