Undissociated dislocation

Z)D(undissoc) diffusivity along an undissociated dislocation core (i.e., a cylinder, or a pipe of diameter, <5)... 

The diffusivity in free surfaces is larger than that in general grain boundaries, which is about the same as that in undissociated dislocations. Furthermore, the diffusivity in undissociated dislocations is greater than that in dissociated dislocations, which is greater than that in the crystal 5... 

Since it appears that two types of dislocations are nucleated in two very different temperature and stress regimes, it is of interest to derive the transition stress at which the nature of the nucleated dislocations change from dissociated to perfect. For that purpose, it is necessary to compare deformation tests performed in very different experimental conditions. In particular, the deformation conditions that produce deformation microstructures containing undissociated dislocations are such... 

The microstructure obtained after room temperature deformation is complex. Nevertheless, several features can be evidenced. In some areas of the thin foil, the microstructure is not different from what is observed in an initially dislocation-free sample. It consists of undissociated dislocations with a/2<110> Burgers vectors elongated along the <123> direction. However, the more frequently observed... 

Fig. 15. Snapshots taken during an in situ anneal of undissociated dislocations up to 685 °C in the TEM.

This experiment reveals no evidence of a massive transformation of preexisting perfect dislocations. It rather shows nucleation events of glide dislocations from lateral crack surfaces, where undissociated dislocations are nncleated at room temperature and under high stresses. 

At temperatures above 500°C, dislocation climb becomes apparent, and extensive cross slip of undissociated screw dislocations from 1012 to (0001) takes place. 

The dislocations expected in the M0S2 structure are formally analogous to those that can exist in graphite—see the list of four distinct kinds in Section IV,C,l,a. As before the glissile basal-plane dislocations are by far the most widely occurring (176, 177) but are of least significance in the oxidation process. About lO undissociated basal-plane dislocations occur in cleaved sections of naturally occurring molybdenite... 

Climb dissociation may be more important in a ceramic than in an fee metal because in fee metals the glide plane is also the plane with by far the lowest SFE. A point to remember is that the word dissociation refers to the final configuration it does not tell you that the perfect dislocation ever had a compact (undissociated) core. 

Deformation substructure in Si indented at low temperature Asaoka et al. [66] recently revisited the indentation of silicon with the aim of deforming it plastically below room temperature. These authors indented silicon at 77 K and showed that it can be deformed plastically. TEM observations of the microstructure showed dislocations aligned along the <110> and <112> directions. Weak-beam dark field showed these dislocations were perfect ones and had a/2<110> Burgers vectors. A HREM observation was also performed on a dislocation seen edge-on, which was shown to have an undissociated core. The exact location of this core, in a glide plane or a shuffle plane, could not be determined. 

Since two types of dislocations exist in sihcon, namely dissociated and undissociated ones, it is of interest to determine whether they are nucleated in separate temperature and stress domains, or could result from a transformation from one form to the other upon switching from one stress and temperature domain to the other one. 

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