Transitions and Phase Equilibria Among Grain Boundary Structures

The characteristics of equilibrium first-order phase changes of the grain boundaries themselves are such that it leads to a definition of grain boundary phases in which smoothly curving boundaries are of the same phase. Different grain boundary phases coexist at facet edges and corners. The phase rule, phase diagrams, and some phase change mechanisms are developed. For a wide variety of problems orientation of the normal is shown to be analogous to composition in ordinary three-component systems. The role of symmetry in modifying the phase rule and in sectioning phase diagrams is explored. Reports of boundary phase changes are re-examined critically.