Introduction

An interface can experience a number of different types of force, including chemical forces (e.g., due to compositional differences across the interface), curvature forces (due to interface curvature) and mechanical forces (Sutton and Balluffi, 2006; Asaro and Lubarda, 2006). In view of the focus of this book, I consider only mechanical forces.

In general, an interface experiences a mechanical force when it lies between two adjoining regions containing different elastic fields and therefore different strain energy densities and elastic displacement fields. In such cases movement of the interface, in which one region grows at the expense of the other, can produce a decrease in the overall energy of the body and thereby give rise to a force on the interface, expressed by Eq. (5.38). Such a force can occur under a variety of circumstances. For example, during the recrystallization of a plastically deformed crystalline body, relatively strain-free crystals form and then grow into the surrounding plastically deformed and dislocated matrix. Here, the reduction in energy that occurs as the strain-free crystals grow at the expense of the dislocated matrix produces outward forces on the interfaces bounding the strain-free crystals. In other situations, elastic fields that differ across interfaces, and therefore generate a mechanical interface force, often occur in polycrystalline materials in the form of compatibility stresses, arising as a result of elastic anisotropy, anisotropic thermal expansion, and differing modes of plastic deformation in the crystals adjoining the interfaces (e.g., Sutton and Balluffi, 2006).