Interfaces in Polycrystals

Abstract

This chapter is part 2 of the three-part series on interfaces. Crystalline solids usually consist of a large number of randomly oriented grains separated by grain boundaries (GBs). Each grain is a single crystal and contains many of the defects already described.

A GB is defined as the surface between any two grains that have the same crystal structure and composition.

Many GBs, but not all, can be modeled as arrays of dislocations.

The dislocation model can be misleading; the GBs that may be most important in a ceramic may be the ones that do not appear to contain dislocations. So the warning is, we may sometimes concentrate on a particular type of GB just because we can understand that type of GB. Unless we fully understand GBs in ceramics, we will never have a full understanding of what happens during ceramic processing or why ceramics have certain mechanical properties, conductivity (thermal or electrical), etc. GBs become even more important as fine-grained nanostructured ceramics become more available. We also need to understand the junctions formed when three or four grains join: these are known as triple junctions (TJs) and quadruple junctions (QJs), which we might regard as new line and point defects, respectively. We conclude with a discussion of properties, not because they are unimportant (nor because of the bias of the authors). We want to understand GBs so as to explain some properties and to predict others.