Partial dislocations composition

Although the process of twinning is physically distinct from that of slip—where there is no rotation of the lattice—it is often convenient 16, 17) to regard the creation of a twin as being the result of the alignment of partial dislocations. Often, twin boundaries occur in pairs within a crystal, so that reference is frequently made to twin bands or twin lamellae, which are the regions bounded by the pair of twin composition planes. Such a situation prevails in the case of graphite (see Fig. 13 and later). 

The core of PS and DG dislocations can be transformed from one to the other through several elementary mechanisms. The two basic mechanisms that allow dislocations moving over one atomic distance to switch from one set to the other are cross slip and climb (Fig. 32). Some mechanisms that can be involved in such transformations are similar to those proposed in the frame of composite models of dislocation core structures, in which a dissociated dislocation can move from glide set to shuffle set in its dissociated form (see, e.g., [1]). However, in composite models, the transformation mechanisms are relevant to the movement of partial dislocations from glide to shuffle positions and a constriction of the parent dislocations is not required. In the present case, the transformation mechanism concerns the change from perfect to dissociated dislocations (as well as the reverse transformation), and a different mechanism can also be involved, namely cross slip [57]. 

For crystalline-crystalline interfaces we further discriminate between homophase and heterophase interfaces. At a homophase interface, composition and lattice type are identical on both sides, only the relative orientation of the lattices differ. At a heterophase interface two phases with different composition or/and Bravias lattice structure meet. Heterophase interfaces are further classified according to the degree of atomic matching. If the atomic lattice is continuous across the interface, we talk about a fully coherent interface. At a semicoherent interface, the lattices only partially fit. This is compensated for by periodic dislocations. At an incoherent interface there is no matching of lattice structure across the interface. 

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