Modeling dislocation glide and lattice friction in Mg₂SiO₄wadsleyite in conditions of the Earth’s transition zone

Abstract

Thermally activated dislocation glide in Mg 2 SiO 4 wadsleyite at 15 GPa has been modeled to investigate its potential contribution to plastic deformation of wadsleyite in the Earth's transition zone. Modeling is based on a multiphysics approach that allows calculating the constitutive equations associated with single slip for a wide range of temperatures and strain rates typical for the laboratory and the Earth's mantle. The model is based on the core structures of the rate limiting ½<111>{101} and [100](010) dissociated screw dislocations. After quantifying their lattice friction, glide is modeled through an elastic interaction model that allows calculating the critical configurations that trigger elementary displacements of dissociated dislocations. The constitutive relations corresponding to glide are then deduced with Orowan's equation to describe the average intracrystalline plasticity. The high stresses predicted by the model are found to be in good agreement with experimental data on plastic deformation of wadsleyite at high-pressure conditions. Moreover, it is found that even at appropriate mantle strain rates, glide of dislocations remain difficult with critical resolved shear stress (CRSS) values typically larger than 100 MPa. This implies the inefficiency of dislocation glide to the overall plastic deformation of Mg 2 SiO 4 wadsleyite under transition zone conditions.

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

“…Regarding the critical kink-pair geometry, the critical height h associated with uncorrelated nucleation is found to be ~0.5a′ at τ c and decreases monotonically with increasing stress following the same trend as in Koizumi et al [18], Ritterbex et al [33], and Ritterbex et al [34]. The variation in critical kink-pair width w 1 and w 2 (Figure 1(a)) as a function of resolved shear stress is presented in Figure 3(b).…”

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

“…For the high-pressure phases existing in the deeper mantle (wadsleyite, ringwoodite, and bridgmanite), experiments available so far do not provide information on dislocation mobility. However, recent modeling based on atomic-scale computations has successfully yielded dislocation glide velocity ( v g ) models for wadsleyite ( 18 ), ringwoodite ( 19 ), and bridgmanite ( 20 ), which are able to reproduce the rare experimental data available well. This shows that the high stress levels observed experimentally ( 1 – 3 ) result from the very high lattice friction exhibited by wadsleyite, ringwoodite, and bridgmanite at high pressure.…”

Low viscosity and high attenuation in MgSiO3 post-perovskite inferred from atomic-scale calculations

“…Our results on dislocation glide in Mg-ppv, including certain unexpected results, shed new light on the rheology of high-pressure mantle phases. Indeed, all recent studies, either experimental or theoretical, of wadsleyite 14 40 41 42 , ringwoodite 15 43 44 , periclase 16 45 and bridgmanite 17 46 consistently show that pressure in the transition zone and lower mantle range leads to a significant increase of lattice friction, which inhibits dislocation glide as a strain-producing mechanism. In particular, these results have important implications regarding the (non-)formation of seismic anisotropy from the deformation of the above-mentioned phases.…”

“…Multiscale numerical modelling represents an alternative that is currently able to describe plasticity by dislocations of high-pressure minerals. Recent applications to wadsleyite 14 , ringwoodite 15 , periclase 16 and bridgmanite 17 have shown their ability to reproduce laboratory experiment data. The main implication of these studies (both numerical and experimental) is that pressure has a strong effect on the lattice friction opposed to dislocation glide.…”