Karine Gouriet

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.

Dislocation modelling in Ti2AlN MAX phase based on the Peierls–Nabarro model

In this study, we determined the core structure and the Peierls stress of dislocations in Ti2AlN MAX phase. We use a generalized Peierls–Nabarro model, called Peierls–Nabarro–Galerkin (PNG), coupled with first principles calculations of generalized stacking fault (GSF). The GSF calculations show that dislocation glide in the basal plane will occur preferentially between M (here Ti) and A (here Al) planes. Additionally, the results of PNG calculations demonstrate that whatever the dislocation character, dislocations are dissociated in the basal plane, with a dissociation distance below the experimental resolution of transmission electron microscopy observations. Finally, the Peierls stress calculations show that the edge and screw characters are the easiest characters to glide in the basal plane.

Dislocation emission from a crack under mixed mode loading studied by molecular statics

The dislocation emission surface in (k I, k II, k III) space is calculated by means of atomistic simulations for the {111}⟨110⟩ crack in Al. For each relevant combination of loading mode, the precise nature of the dislocations and of the emission process are determined. When appropriate, the analytic formulas proposed by Rice are used by calculating the unstable stacking energy including the effect of the mixed mode loading. Quantitative agreement with the full atomistic calculation is found in the case where dislocations glide in the crack plane. This clearly identifies when and how ab initio data can be introduced in the calculation.

Mechanisms of nanoparticle formation by short laser pulses

Numerical modeling is performed to study cluster formation by laser ablation. The developed model allows us to compare the relative contribution of the two channels of the cluster production by laser ablation: (i) direct cluster ejection upon the laser-material interaction, and (ii) collisional sticking, evaporation and coalescence during the ablation plume expansion. Both of these mechanisms are found to affect the final cluster size distribution. Plume cluster composition is correlated with plume dynamics. The results of the calculations demonstrate that cluster precursors are formed during material ablation through both thermal and mechanical target decomposition processes. Then, clusters react in collisions within the plume. In vacuum, rapid plume expansion and cooling take place leading to the overall decrease in the reaction rates. In the presence of a gas, additional collisions with background gas species affect the cluster size distribution. Growth of larger clusters can be observed at this stage. Calculation results explain several recent experimental observations.

Formation of nanoparticles by short and ultra-short laser pulses

The main objective of this study is to explain the experimental observations. To simulate material ablation, plume formation and its evolution, we developed a combined molecular dynamics (MD) and direct simulation Monte Carlo (DSMC) computational study of laser ablation plume evolution. The first process of the material ablation is described by the MD method. The expansion of the ejected plume is modelled by the DSMC method. To better understand the formation and the evolution of nanoparticles present in the plume, we first used separate MD simulations to analyse the evolution of a cluster in the presence of background gas with different properties (density, temperature). In particular, we examine evaporation and growth reactions of a cluster with different size and initial temperature. As a result of MD calculations, we determinate the influence of the background gas parameters on the nanoparticles. The reactions rates such as evaporation/condensation, which are obtained by MD simulations, are directly transferred to the DSMC part of our combined model. Finally, several calculations performed by using MD-DSMC model demonstrate both plume dynamics and longer-time cluster evolution. Calculations results are compared with experimental findings.