Dislocations in MgO

FIGURE 12.18 Forming a pair of kinks and a pair of jogs and how this leads to helical dislocations in MgO. 

Fig. 17. Map of the interaction between dislocations in MgO as function of angles that define the orientation relative to the intersection of sUp planes. Gray shades display interaction force (white attraction, black repulsion), (a) Lomer lock, (b) Hirth lock [443].

Figure 5.4 Atomistic structure of a screw-edge dislocation in MgO. The atoms comprising the core, which protrude out of the surface, are shown as dark (oxygen) and light (magnesium) spheres. Reproduced from Sayle with permission from the Royal Society of Chemistry.

Vacancies and impurity atoms may also attract to form complexes. One expects large substitutional atoms and interstitial impurity atoms to be attracted to vacancies because of reduction in total strain energy similar to the attraction of impurity atoms to dislocations to form atmospheres. Segregation about dislocations may reach the stage where a distinct second phase forms decorating the dislocation. Dislocations in MgO decorated by MgFe204 precipitates are shown in Figure 22. 

Fig. 22. Magnesioferrite precipitates decorating dislocations in MgO-1.35 cat. 7 Fe single crystal was aged 64 hr at 900°C (50 x). From Dr. G. W. Groves, Northwestern University, Evanston, Illinois.

Gragert and Meyer (Fig. 6.2.1) and Boyarskaya (Fig. 6.2.2) by observation of surface deformations induced by indentation with a tungsten carbide ball and by scratch. The observations were carried out using secondary electron beam and in cathodoluminescence. They demonstrated on MgO and LiF crystals the occurrence of cracks around the impression of the ball similar to those induced by a Vickers indenter, and also the occurrence of a concentration of screw and edge dislocations in the area of the cracks. 

Benia HM, Myrach P, Gonchar A, Risse T, Nilius M, Freund HJ. Electron trapping in misfit dislocations of MgO thin films. Phys Rev B. 2010 81 241415. 

K. S. Kim and R. J. Clifton, Dislocation Motion in MgO Crystals Under Plate Impact, Journal of Material Science, 19, 1428-1438 (1984). 

Mitchell states that dislocation dissociation is rare in oxides. The only two cases in which it has been clearly observed (AI2O3 and MgAl204,) involve dissociation by climb, rather than glide, in situations where the point defects are probably helping the dissociation process. Therefore, it is of interest to study such cases as additional examples in which climb is involved, as one of the mechanisms in the recovery process. Figure 3.71 illustrates dislocation dissociation by climb in MgO-3.5 AI2O3 spinel. 

At this point, it is constmctive to observe the dislocation structure in MgO polycrystalline ceramics, as revealed by TEM. In Fig. 6.36, a typical dislocation structure is shown for creep deformation, not unlike that found in metals, with the presence of subgrains, in which a 3D dislocation network may be seen. Note that the long dislocation segments seldom run in a straight line from one node to another, but are bowed out, often in only one plane, though, sometimes, as seen in Fig. 6.36a,... 

Examples of dislocation structures after fatigue deformation appear in a series of microstructures (Figs. 7.55-7.59) obtained by TEM. Subramanian [27] claims that these dislocation substructures are similar to that of unidirectionaUy-stressed MgO. The micrographs presented below are of single-crystal magnesia which underwent a large number of cycles (in the millions) of low strain amplitude. The maximum strain was about 0.1 % per cycle. The characteristics of the dislocation structure in MgO, having a rock-salt structure, is as follows ... 

Since MgO has a structure similar to that of NaCl crystal, its slip systems are also of the type 110 (110). There are six such planes in a cubic structure. In a cubic structure, four of the six 110 slip-plane projections onto the (001) slip planes leave traces along the (100) direction and lie at 45° to the surface these are known as 45° slip planes . Two have traces along the (110) direction, lie at 90° to the surface and are referred to as 90° slip planes . The shortest Burgers vector for a perfect dislocation in the NaCl structure is a/2 (110). This is the operative slip direction. There is only one (110) slip direction in each 110 slip plane. Therefore, there can be only one kind of mobile dislocation in each slip plane. The secondary slip systems are of the type 001 (110). Since the primary and secondary slip planes have a common Burgers vector, this is the cross-slip plane. No two primary slip planes have the same Burgers vector. 

In ceramics, such a mechanism is in complete contradiction to the aforementioned dislocation mechanism leading to failure. Other ductile ceramics may involve dislocations in their plastic deformation, for example MgO single... 

C. S. Morgan Oak Ridge National Laboratory) Does the fact that you do not see very many dislocations in the MgO specimens examined by electron transmission microscopy mean that they are not formed during crystal growth or could it be that they form but move out ... 

---