Free grain boundaries

Nucleation in solids is very similar to nucleation in liquids. Because solids usually contain high-energy defects (like dislocations, grain boundaries and surfaces) new phases usually nucleate heterogeneously homogeneous nucleation, which occurs in defect-free regions, is rare. Figure 7.5 summarises the various ways in which nucleation can take place in a typical polycrystalline solid and Problems 7.2 and 7.3 illustrate how nucleation theory can be applied to a solid-state situation. 

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

By their nature, dislocations cannot end suddenly in the interior of a crystal a dislocation line can only end at a free surface or a grain boundary (or form a closed loop). Where a screw dislocation intersects a free surface there is inevitably a step or ledge in the surface, one atomic layer high, as shown in Fig. 20.30c. Furthermore, the step need not necessarily be straight and will, in fact, almost certainly contain kinks. 

Fig. 20.SS Transmission electronmicrograph showing intergranular and imragranular precipitation and the precipitate-free zone adjacent to the grain boundary in a high-strength precipitation-hardening Al-Zn-Mg alloy (x24 000, courtesy G. Lorimer)...

Here it is assumed that upon fracture of a grain boundary, half of the segregant is left on each free surface, and that the segregant atoms pack at the same density as they would in the pure elemental state. 

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