What is a grain boundary in a polycrystalline solid?

• A. The interface between grains with different crystallographic orientations.
• B. A region where atoms are densely packed in a perfect lattice.
• C. A void or vacancy in the crystal structure.
• D. A dislocation line within a single grain.

Answer: (A)

The concept being tested is that a grain boundary is the interface where neighboring crystals in a polycrystal meet and their lattice orientations misalign. When grains with different crystallographic directions touch, the atoms at the boundary cannot line up in a single perfect lattice, so the arrangement becomes more disordered and the boundary region carries higher energy. This misorientation and disordered structure influence how the material deforms and diffuses, often acting as barriers to dislocation motion or as preferred diffusion paths depending on the situation.

This boundary is not a region of perfect packing inside a single grain, nor a void, nor a dislocation line within a grain. Those describe other types of defects or features entirely.