The conditions for structural transitions at the core of a grain boundary separating two crystals was investigated with a diffuse interface model that incorporates disorder and crystal orientation. The model predicted that limited structural disorder near the grain boundary core could be favorable below the melting point. This disordered material was a precursor to a liquid phase and therefore the model represented grain boundary pre-melting. This model was shown to be isomorphic to Cahn’s critical point wetting theory and predicted first- and higher-order structural grain boundary transitions. A graphical construction predicted the equilibrium grain boundary core disorder, the grain boundary energy density, and the relative stability of multiple grain boundary “complexions.” The graphical construction permits qualitative inference of the effect of model properties, such as empirical homogeneous free energy density and assumed gradient energy coefficients, on properties. A quantitative criterion was derived which determines whether a first-order grain boundary transition will occur. In those systems where first-order transition did occur, they were limited to intermediate grain-boundary misorientations and to a limited range of temperatures below the melting point. Larger misorientations lead to continuously increasing disorder up to the melting point at which the disorder matches a liquid state. Smaller misorientation continuously disorder but were not completely disordered at the melting point. Characteristic grain boundary widths and energies were calculated as was the width’s divergence behavior at the melting point. Grain boundary phase diagrams were produced. The relations between the model’s predictions and atomistic simulations and with experimental observations were examined.

Diffuse Interface Model for Structural Transitions of Grain Boundaries. M.Tang, W.C.Carter, R.M.Cannon: Physical Review B, 2006, 73[2], 024102 (14pp)