Faceting and Wetting Transitions of Anisotropic Interfaces and Grain Boundaries

A three‐dimensional construction is presented that illus‐trates conditions under which anisotropic interfaces will be fully wetted, partially wetted, or not wetted by a second phase. Recent experimental observations on the equili‐brated morphologies of solid or fluid “wetting” phases along anisotropic interfaces and grain boundaries reveal features that are predicted—and, in some cases, required—by the construction. Theory distinguishes between cases where surfaces are smoothly curved and where there are facets, edges, and corners. In the latter case, the conven‐tional comparison of the surface energy of the original sur‐face with the sum of the surface energy of the two surfaces of the wetting layer leads to erroneous predictions. The correct predictions are obtained by comparing the Wulff shape of the original surface with a carefully defined “sum” of Wulff shapes of the surfaces of the wetting layer. Where orientations that are wetted join with those that are not, an abrupt change of orientation usually is present. Faceting on two hierarchical levels can occur. Microscopic morphology changes along macroscopically curved surfaces follow well‐defined rules that are predicted by the theory. The anal‐ogy between the thermodynamics of surface faceting and phase transformations allows the well‐known concepts of phase equilibria to be used to understand the predicted structures.