Which statement best defines a grain in metals?

Which statement best defines a grain in metals?

• A. A crystal whose growth was impeded by contact with another grain or a boundary surface
• B. A region of uniform chemical composition
• C. An amorphous region with no crystalline order
• D. A single crystal spanning the entire sample

Answer: (A)

Grains are crystallites with a specific lattice orientation that grow until they meet other crystals. When growing crystals encounter each other, their progress is blocked by the boundary with the neighboring crystal, so each region keeps its own orientation. This is why a grain is defined as a crystal whose growth was impeded by contact with another grain or a boundary surface. The boundary between grains marks the interface where orientations change. The other descriptions don’t capture this idea. A region of uniform chemical composition describes composition, not structure or orientation. An amorphous region has no crystalline order at all, so it isn’t a grain. A single crystal spanning the entire sample would lack grain boundaries, which contradicts the usual polycrystalline structure that grains describe.

Grains are crystallites with a specific lattice orientation that grow until they meet other crystals. When growing crystals encounter each other, their progress is blocked by the boundary with the neighboring crystal, so each region keeps its own orientation. This is why a grain is defined as a crystal whose growth was impeded by contact with another grain or a boundary surface. The boundary between grains marks the interface where orientations change.

The other descriptions don’t capture this idea. A region of uniform chemical composition describes composition, not structure or orientation. An amorphous region has no crystalline order at all, so it isn’t a grain. A single crystal spanning the entire sample would lack grain boundaries, which contradicts the usual polycrystalline structure that grains describe.