Polycrystalline material

A solid made of many small crystals, or grains, joined at interfaces called grain boundaries. The grains may differ in orientation, composition, or both.

Crystal: A solid whose atoms, ions, or molecules form an ordered structure that repeats through space. Each grain is a crystal with its own orientation and extent.
Dislocation: A line defect in a crystal lattice that enables plastic deformation through its motion. Grain boundaries impede or redirect dislocation motion across grains.
Recrystallization (metallurgy): The formation of new, relatively strain-free grains in a deformed material during heating. It resets grain structure after plastic working.
Single crystal: A crystalline solid whose lattice orientation is continuous throughout the specimen, without internal grain boundaries. It lacks the interfaces that distinguish a polycrystalline material.
Crystallographic orientation: The alignment of a crystal’s lattice axes relative to a chosen reference frame. Neighboring grains commonly have different lattice alignments.
Hall–Petch relationship: An empirical relationship in which yield strength often increases as average grain size decreases. It links grain refinement to resistance against plastic deformation.
Sintering: The densification of a powder compact by heating, typically below its melting point. It joins crystalline particles into a polycrystalline solid.
Amorphous solid: A solid that lacks long-range periodic atomic order. It has no crystalline grains or crystallographic grain boundaries.
Grain boundary: An interface separating two crystalline regions with different orientations or structures. These interfaces separate grains and influence their interactions.
Diffusion: The net movement of atoms or molecules driven by concentration gradients or random thermal motion. Grain boundaries can provide faster diffusion paths than crystal interiors.

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