GrainBoundary

Grain boundary is the interface between two crystallites, or grains, in a polycrystalline material where the orientation of the crystal lattice changes. It forms during solidification, recrystallization, or grain growth when adjacent grains meet but are not coherently aligned.

Grain boundaries are characterized by their misorientation angle and the boundary plane. Low-angle boundaries are composed of dislocation arrays and have relatively low energy, whereas high-angle boundaries have greater structural disorder and higher energy. Some boundaries are special, such as Coincidence Site Lattice (CSL) boundaries with specific misorientation relationships; coherent twins (Sigma 3) are common in many metals.

Properties: grain boundaries affect mechanical, thermal, and chemical behavior. They act as barriers to dislocation motion, contributing to strength via the Hall-Petch effect; but they can also be sites for diffusion, corrosion, and creep. Boundary energy and mobility influence grain growth and recrystallization. Segregation of impurities to boundaries can modify properties.

Processing and control: grain size and boundary character can be engineered through thermomechanical processing and annealing. Grain boundary engineering aims to increase fraction of low-energy or highly coherent boundaries to improve corrosion resistance and mechanical performance.

Characterization: techniques include electron microscopy, particularly EBSD to map grain orientation and boundary character; TEM for atomic structure; and computational methods such as atomistic simulations to study boundary structures and energies.