RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

Apr 26, 2016 Dislocation locking in the mantle Results

New article published in Scripta Materialia about the climb dissociation of <110> dislocations in SrTiO3 and MgSiO3 perovskites

Strontium titanate SrTiO3 is a cubic perovskite with a very peculiar mechanical behavior. At low temperature it is ductile thanks to the mobility of dislocations with a <110> Burgers vector, however above 1050 K it becomes brittle, meaning that these dislocations cannot glide anymore. It was suspected that the <110> dislocations change their core structure under the effect of temperature, however there was no direct evidence for this phenomenon. Moreover, if dislocations lose their mobility in SrTiO3, one can wonder if it also happens in other compounds with the perovskite structure. In particular, bridgmanite MgSiO3 is an important phase of the Earth's lower mantle, and it is crucial to determine if the <110> slip system is active at high temperature, or if it is also inhibited.

We used atomic-scale simulations to investigate the effect of temperature on <110> dislocations of pure edge character. The simulations demonstrate for the first time the mechanism of climb dissociation, as illustrated in Fig. 1. First, the dislocation reduces its width, closing the stacking fault and bringing the two partial dislocations closer. Then, one partial dislocation moves on top of the other, thus achieving dissociation in the climb plane.

 

Fig. 1 - The change of core structure of the <110> edge dislocation in SrTiO3, from glide-dissociated into climb-dissociated configuration. First the dislocation reduces its width, then it dissociates by climb.

We determined the activation energy for this mechanism by means of the Nudged Elastic Band method. The activation energy is about 0.88 eV/Å in SrTiO3. The application of this mechanism to bridgmanite MgSiO3 shows that the activation energy is much lower, about 0.042 eV/Å. The reason is that in MgSiO3 the edge dislocation has a very compact core structure, therefore it can readily dissociate by climb, contrary to SrTiO3 where the dissociation distance has first to be reduced.

As a result, in perovskite materials the behavior of <110> edge dislocations strongly depends on the conditions of temperature T and strain rate ε̇, as illustrated in Fig. 2. Such a dislocation can cross one of two energy barriers: the barrier for glide (i.e. the Peierls barrier), or the barrier for climb dissociation. In conditions of high stress and low temperature, the applied stress lowers significantly the energy barrier for glide, and temperature is too low to permit climb dissociation, hence dislocation glide is favored. On the contrary, when the applied stress is small and temperature is high, climb dissociation is the favored mechanism. In intermediate situations, some edge dislocations may dissociate by climb and become sessile, while others may still be able to glide.

 

Fig. 2 - Schematic illustration of the bevahior of <110> edge dislocations in perovskites, as function of temperature and strain rate.

 

Reference:

"From glissile to sessile: Effect of temperature on <110> dislocations in perovskite materials"

P. Hirel, P. Carrez, P. Cordier, Scripta Mater. 120 (2016) 67-70.