Rheology of Earth Materials :

Closing the gap between timescales in the laboratory and in the mantle

19 February 2020 - New publication from the LASCO project

In creep conditions where the temperature is high and the applied stress is low, dislocations can move by absorbing or emitting point defects. As a result, part of the dislocation line climbs out of its glide plane, and the initial and climbed dislocation lines are connected by jogs. While text books describe this process qualitatively, quantifying the properties of jogs like their atomic structure and energy remains challenging.

Zhai et al. used simulations at the atomic scale to investigate jogs in magnesium oxide (MgO), along edge dislocations that belong to the slip system ½<110>{110}. Along such dislocations, several types of jogs can form. Elementary jogs occur when the dislocation line climbs by a vector ¼[110], which correspond to one interplanar spacing (but is not a lattice vector). They find that such elementary jogs spread in {111} planes, which is surprizing because these planes do not belong to any favourable slip system of MgO. In addition these jogs carry an electric charge, which can be positive or negative, of magnitude equal to half the charge of a lattice ion. By carefully accounting for spurious elastic and electrostatic interactions, Zhai et al. find that a pair of elementary jogs has a formation energy about 2.47 eV.

Fig. 1. (a) Atomic structure of a pair of elementary jogs. Only atoms that belong to the dislocation are displayed, and the dislocation line is represented as a blue line. (b) Visualization of the charge density around elementary jogs. One jog carries a positive charge (in red), the other one a negative charge (in blue). Overall the jog pair is charge neutral.

When the dislocation climbs by two interplanar spacings (i.e. ½[110]), then super-jogs can form either in [001] plane, or in {111} planes. The latter configuration, which is equivalent to two successive elementary jogs, is the most favourable with an energy comparable to twice the energy of individual elementary jogs. These results are an important first step towards understanding the processes of dislocation climb in MgO.

 

Publication: J. Zhai, P. Hirel, P. Carrez, Atomic-scale properties of jogs along 1/2〈110〉{110} edge dislocations in MgO, Scripta Materialia 181, 66-69 (2020), https://doi.org/10.1016/j.scriptamat.2020.02.013

 

This study was supported by the French government through the Programme Investissement d’Avenir (I-SITE ULNE / ANR- 16-IDEX-0004 ULNE) managed by the Agence Nationale de la Recherche, under the project name LASCO (PI: Philippe Carrez)