Dislocation reactions, plastic anisotropy and forest strengthening in MgO at high temperature

Highlights

• We model reactions between dislocations in MgO using DD simulation.

• The role of forest reactions on plastic deformation is studied at high temperature.

• Strengthening coefficients are calculated.

• Massive DD simulation provides information on frequent experimental observations.

Abstract

The collective properties of dislocations in MgO are investigated in the high temperature regime and at constant strain rate with 3D Dislocation Dynamics simulations. Intersections between slip systems 1/2〈1 1 0〉{1 1 0} and 1/2〈1 1 0〉{1 0 0} allow essentially two types of junction reactions. These junctions are energetically stable and are expected to promote strong forest strengthening at high temperature. Large-scale DD simulations show that MgO strain hardening at high temperature may be dominated by forest reactions. Important parameters for dislocation density based modeling of MgO plasticity are finally calculated and verified to be consistent with experimental observations.

Introduction

MgO is a model ceramic with NaCl-structure intensively studied in the past decades for its potential uses as a refractory material (T_f > 3000 K at ambient pressure). Alloyed to a small amount of iron (∼10%), MgO is also the second most abundant phase of the Earth’s lower mantle after the magnesium-silicate perovskite phase MgSiO₃. Therefore, the study of MgO plastic properties is of great interest in both Geophysics and Materials Sciences.

At ambient pressure, plastic strain in MgO single crystal is expected to be governed either by lattice friction, impurities strengthening or dislocation–dislocation elastic interactions depending on the investigated temperature range (Barthel, 1984, Haasen et al., 1985, Sato and Sumino, 1980).

At low temperature, a large lattice friction is measured on the two experimentally observed slip systems 1/2〈1 1 0〉{1 1 0} and 1/2〈1 1 0〉{1 0 0}, respectively. Dislocations in the 1/2〈1 1 0〉{1 1 0} slip system are found to glide at the lowest stress level (Barthel, 1984). In both slip systems, the kink-pairs nucleation process is the glide controlling rate mechanism and plastic deformation is governed by the slow motion of long screw dislocations (Appel and Wielke, 1985). At intermediate temperatures, lattice friction is less important and dislocations interactions with solute elements start to affect the MgO flow stress (Appel et al., 1984, Gorum et al., 1960, Srinivasan and Stoebe, 1974). The MgO mechanical properties in this intermediate temperature regime was recently overviewed by Messerschmidt (2010). Lastly, at temperatures higher than the athermal transition temperature T_a (respectively about 600 K and 1500 K for the 1/2〈1 1 0〉{1 1 0} and the 1/2〈1 1 0〉{1 0 0} slip systems), new features are expected to influence MgO plastic properties. Deformation microstructures are now made of curved dislocation segments and small debris (Clauer and Wilcox, 1976, Haasen et al., 1986). Whatever the active slip system and the impurity concentration, MgO flow stress is low about 10 MPa in this temperature regime (Amodeo et al., 2011, Copley and Pask, 1965, Hulse et al., 1963).

Very few quantitative analysis exist on the elementary mechanisms controlling MgO plasticity in the high temperature regime (Copley and Pask, 1965, Day and Stokes, 1964). This matter of fact is problematic as MgO has gained in recent years renewed interest to investigate small scale plasticity (Dong et al., 2010, Gaillard et al., 2006, Howie et al., 2012, Korte and Clegg, 2011, Korte et al., 2011) and in geophysics (Amodeo et al., 2012, Cordier et al., 2012, Girard et al., 2012, Merkel et al., 2002). For instance, Cordier and collaborators (2012) have recently shown that the mechanical properties of MgO in the athermal regime (i.e. for T > T_a) are of primary importance to understand the rheology of the Earth’s lower mantle. For these reasons, it is expected that Dislocation Dynamics (DD) simulations dedicated to MgO plasticity can provide new inputs and hopefully can improve our understanding of the Earth’s mantle flow mechanisms.

Three-dimensional DD simulations are today acknowledged as a unique tool to investigate collective properties of dislocations and to access crystal plasticity at the microstructure level (i.e. at the mesoscopic scale). DD simulations have shown interest in lots of different studies, but mainly in the case of metal plasticity (Bulatov et al., 2006, Devincre et al., 2008, Dimiduk et al., 2006, Madec et al., 2003). In a recent study, it was shown that MgO yield stress can be precisely evaluated with DD simulations for MgO single crystals and large grains polycrystals (Amodeo et al., 2011). Here, we present new calculations performed in the athermal regime to examine the contribution of forest reactions to MgO plastic flow at ambient pressure and in laboratory conditions of strain rate.

The effect of forest interactions (i.e. dislocation–dislocation contact reactions) is explored in two steps. First, we systematically investigate the reactions occurring between two intersecting dislocations depending on their slip systems and their relative orientations. Then, large-scale DD simulations are performed to quantify the contribution of forest interactions to plastic strain depending on the active slip systems. Finally, we calculate physical parameters, which are essential ingredients of crystal plasticity models at large scale.