An interface can be defined as a surface that serves as a common boundary between two phases. Examples include the boundaries between two solids, two immiscible liquids, a solid and a liquid, a solid and a gas, and a liquid and a gas. Interfaces have been studied for decades by scientists of many different disciplines. One reason for this interest is that the atomic structure and the chemical composition at the interface can differ from that of the bulk material on either side of it. Consequently, the properties of the interface can differ greatly from those of either bulk phase, and chemical reactions can occur more readily at the interface than in the bulk.

All the interfaces listed in the previous paragraph are of interest to materials scientists. However, this article will only consider the grain boundary because it has received the most attention by researchers in materials science. Furthermore, we will only consider grain boundaries in metals; nonmetallic systems will be covered in other articles in this issue.

A grain boundary is an interface that exists where two single crystals are joined in such a way that their crystallographic orientations are not completely matched. Thus, any polycrystalline material contains many grain boundaries. They occur wherever the individual grains meet one another and can usually be observed by etching a polished cross section of the surface as shown in Figure 1. Grain boundaries first form in a metal as a result of the multiple nucleation sites that occur during solidification.