Semicoherent interface

Normally an interface will have misfit in two dimensions, which will result in two, non-parallel sets of misfit dislocations. A semicoherent interface may be considered to be somewhat analogous to the low-angle grain boundaries described in Chapter 4. 

The interfacial energy of a semicoherent interface will contain two contributions, namely a chemical term associated with the bonding across the interface and a second term arising from the energies of the misfit dislocations  

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Stress builds up at a coherent interface between two phases, a and / , which have a slight lattice mismatch. For a sufficiently large misfit (or a large enough interfacial area), misfit dislocations (= localized stresses) become energetically more favorable than the coherency stress whereby a semicoherent interface will form. The lattice plane matching will be almost perfect except in the immediate neighborhood of the misfit dislocation. Usually, misfits exist in more than one dimension. Sets (/) of nonparallel misfit dislocations occur at distances... 

Heterophase Interfaces. In certain cases, sharp heterophase interfaces are able to move in military fashion by the glissile motion of line defects possessing dislocation character. Interfaces of this type occur in martensitic displacive transformations, which are described in Chapter 24. The interface between the parent phase and the newly formed martensitic phase is a semicoherent interface that has no long-range stress field. The array of interfacial dislocations can move in glissile fashion and shuffle atoms across the interface. This advancing interface will transform... 

Figure 19.8 Interfacial structure for (a) coherent and (b) semicoherent interfaces...

It is often useful to describe the dislocation content of coherent and semicoherent interfaces in terms of another framework which employs coherency dislocations and anticoherency dislocations. The basic idea is illustrated in Fig. B.8, which shows the same two boundaries shown previously in Fig. B.76 and c. The coherency dislocations possess a stress field equivalent to the long-range coherency stresses associated with the coherent interface. They are not real dislocations in the... 

Finally, incoherent interfaces can be regarded as the limiting case of semicoherent interfaces for which the density of dislocations is so great that their cores overlap and that essentially all of the coherence characteristic of the reference structure has been destroyed. The cores of incoherent interfaces are therefore continuous slabs of bad material, and consequently the interfaces lack long-range order. 

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. 

In a solution, nanoparticles interact with each other in a number of ways. Widely separated nanoparticles may be brought into contact by Brownian motion. As they approach each other, electrostatic, van der Waals forces, and hydrogen bonding, in addition to Brownian motion, can cause two nanoparticles to rotate with respect to each other, and collide. Evidently, under some conditions, the collisions that result in fusion are those that involve two nanoparticles in appropriate orientations to form a coherent (or semicoherent) interface. Particle rotation in the absence of a fluid has been modeled computationally (Zhu and Averback 1996), implying that oriented assembly-based crystal growth can also occur in dry systems. 

If the misfit is too large it will be energetically favorable for the interface to contain misfit dislocations to take up some of the misfit. This will result in a semicoherent interface, which is shown schematically in Figure 5.1(b). Most of the misfit in one dimension can be accommodated if the dislocations are separated by a distance D given by... 

Pumphrey, P.H. andEdington, J.W. (1974) The structure of the semicoherent interface between grain boundary nucleated MjjCg and austenitic stainless steel. Acta Metall, 22(1), 8994. 

A. Vattre, T. Jourdan, H. Ding, M.C. Marinica, M.J. Demkowicz, Non-random walk diffusion enhances the sink strength of semicoherent interfaces, Nat. Commun. 7 (2016) 10424. 

The semicoherent interface between different phases also consists of regions of good fit separated by a dislocation grid and the interfacial energy may be computed on this basis. Such dislocations are called van der Merwe dislocations. ... 

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