Herbert M. Miller

Crystallographic Distribution of Internal Interfaces in Spinel Polycrystals

Measurements of the grain boundary character distribution in MgAl2O4 (spinel) as a function of lattice misorientation and boundary plane orientation show that at all misorientations, grain boundaries are most frequently terminated on {111} planes. Boundaries with {111} orientations are observed 2.5 times more frequently than boundaries with {100} orientations. Furthermore, the most common boundary type is the twist boundary formed by a 60° rotation about the [111] axis. {111} planes also dominate the external form of spinel crystals found in natural settings, and this suggests that they are low energy and/or slow growing planes. The mechanisms that might lead to a high population of these planes during solid state crystal growth are discussed.

Distribution of Grain Boundary Planes at Coincident Site Lattice Misorientations

The grain boundary plane distributions in MgO, SrTiO3, MgAl2O4, and Al are compared at lattice misorientations with a coincident site density of greater than or equal to 1/9. In most situations, the most frequently adopted grain boundary orientation is a habit plane of low index and low surface energy that depends on the particular material. Cases where the most common boundary orientation is a plane of high planar coincident site density instead of a characteristic habit plane are rare. In fact, in most cases, the distributions of grain boundary planes at misorientations with high lattice coincidence are not substantially different from the distributions at other, more general misorientations. The results indicate that a model for grain boundary energy and structure based on grain surface relationships is more appropriate than the widely accepted models based on lattice orientation relationships.

Orientation Distribution of Σ3 Grain Boundary Planes in Ni before and after Grain Boundary Engineering

The distribution of grain boundary plane orientations in polycrystalline Ni has been measured before and after grain boundary engineering. The grain boundary engineered microstructure has a relatively higher concentration of Σ3 grain boundaries and, when compared to the initial structure, more of these boundaries have orientations that are inclined by more than 10° from the (111) orientation of the ideal coherent twin. Although the conventionally measured grain size is not affected by the grain boundary engineering process, the average size of the regions containing only Σ3n grain boundaries increases by nearly a factor of two. The observations indicate that the increase in the relative population of Σ3 grain boundaries results both from the preferential elimination of random grain boundaries and the generation of new Σ3 grain boundaries which do not have (111) grain boundary plane orientations.