Interfacial Defects

External Surfaces:

Surface atoms have have unsatisfied atomic bonds, and higher energies than the bulk atoms ⇒ Surface energy, γ (J/m2)

• Surface areas tend to minimize (e.g. liquid drop)

• Solid surfaces can “reconstruct” to satisfy atomic bonds at surfaces.

Grain Boundaries:

Polycrystalline material comprised of many small crystals or grains. The grains have different crystallographic orientation. There exist atomic mismatch within the regions where grains meet. These regions are called grain boundaries. Surfaces and interfaces are reactive and impurities tend to segregate there. Since energy is associated with interfaces, grains tend to grow in size at the expense of smaller grains to minimize energy. This occurs by diffusion (Chapter 5), which is accelerated at high temperatures.

High and Low Angle Grain Boundaries:

Depending on misalignments of atomic planes between adjacent grains we can distinguish between the low and high angle grain boundaries.