bicrystals

A bicrystal is a sample consisting of two crystalline grains that meet at a single, planar grain boundary. The two grains have distinct crystallographic orientations, defined by a misorientation angle and the orientation of the boundary plane. The boundary may be classified as tilt, twist, or a mixed boundary. In tilt boundaries the rotation axis lies in the boundary plane; in twist boundaries the axis is perpendicular to the boundary plane. The boundary energy and mobility depend on the misorientation and boundary character.

Bicrystal samples are prepared by starting with two single-crystal pieces of the same material, orienting them with the desired misorientation, and joining them by diffusion bonding, brazing, or directional solidification. The joint is then annealed to form a clean, straight boundary. Orientation and structure are typically characterized by electron backscatter diffraction and transmission electron microscopy.

Bicrystals serve as simplified models for studying grain-boundary phenomena in metals, ceramics, and semiconductors. They enable controlled measurements of grain-boundary energy as a function of misorientation (gamma plots), boundary migration under capillarity, and diffusion or segregation of impurities along the boundary. Observables include boundary mobility, step structure, and dislocation content at the boundary.

Limitations include that a bicrystal represents a single, ideal boundary and may not capture the complexity of random polycrystals with many interacting boundaries. Nevertheless, bicrystal studies provide fundamental insight into the structure and properties of grain boundaries and help calibrate models of polycrystalline behavior.