Small-angle boundaries

The example in Fig. 13.4 is an extension of the model for the motion of a small-angle boundary by the glide and climb of interfacial dislocations (Fig. 13.3). Figure 13.4 presents an expanded view of the internal surfaces of the two crystals that face each other across a large-angle grain boundary. Crystal dislocations have... 

Figure 13.10 shows evidence for small-angle boundaries in Au acting as efficient sinks for supersaturated vacancies under a large driving force. Here, the supersaturated vacancies have collapsed in the form of vacancy precipitates in the region of... 

Fig. 8-35 Lang topograph of a thin crystal of lithium fluoride. Dark lines are dislocations, some randomly arranged and some nearly parallel. The latter form the band running diagonally across the photograph these dislocations, which intersect the top and bottom surfaces of the crystal, constitute a small angle boundary between two subgrains. Mo KiXi radiation, 200 reflection. (Courtesy of A. R. Lang. [8.26.])...

A small-angle boundary (or a subgrain, a low angle, or a polygoniza-tion boundary— the terms are synonymous) is composed of planar arrays of dislocations formed by the union of two crystals which were... 

Energy of Dislocation Arrays Small-Angle Boundaries)... 

Read has shown 21) that an equation of the form of (16) gives the variation of energy with angle of misorientation for all kinds of small-angle boundaries. 

So far as the question of truly solid-state, rather than surface, reactions is concerned, it is apparent from the equation for the free energy AG of the formation of the product nucleus within the host that the increase in strain energy, Ae, arising from unit increase in volume of the nucleus will be smaller at dislocations and small-angle boundaries than elsewhere in the lattice. Thus, AG is given by... 

Other reactions, such as the formation of graphitic carbon from gas-phase species (acetylene and methane) are catalyzed by certain metals and Presland and Walker (138a) have recently shown that the arrays of dislocations constituting small-angle boundaries in platinum play an important, but as yet incompletely understood, role in the production of thin films of near ideal graphite at catalyst temperatures less than 1000°C. 

Thomas and Roscoe (54, 62) have shown that all the possible kinds of small-angle boundaries, discussed in Section II,B, may be detected in graphite by preferential oxidation at dislocation cores. 

There is strong evidence that emergent dislocations (isolated and in small-angle boundaries) and surface steps only are the preferred sites of decomposition (see Fig. 60), but some of these sites tend to become readily poisoned. 

Fig. 3.66 HREM image of 29 = 5.0° symmetric tilt small angle boundary. Periodical dislocations can be seen along the boundary.

Small-angle boundaries are ones across which the difference in orientation is small. They are also called subgrain boundaries. There are two general classes of small-angle houndaries tilt boundaries, which comprise an array of edge dislocations, and twist boundaries, which comprise two or more arrays of screw dislocations. (The reader who is unfamiliar with the line defects known as dislocations will find a clear and readable description in Reference [6].)... 

---