Which statement about grain boundaries is true?

Which statement about grain boundaries is true?

• A. The atoms at grain boundaries have perfect crystalline structure
• B. They increase the resistance of the metal to deformation
• C. They are disordered and reduce strength
• D. They do not affect mechanical properties

Answer: (B)

Grain boundaries are interfaces between crystals that are oriented differently. They disrupt the regular lattice and act as barriers to dislocation motion, which is how metals normally deform plastically. Because these boundaries impede dislocations, the material requires higher stress to continue deforming, increasing its strength and resistance to deformation. That’s why finer-grained materials, with more boundaries, tend to be stronger. The other statements are not correct: grain boundaries are not perfect crystalline regions; they are disordered compared to the crystal lattice, and they do affect mechanical properties (they don’t simply leave them unchanged).

Grain boundaries are interfaces between crystals that are oriented differently. They disrupt the regular lattice and act as barriers to dislocation motion, which is how metals normally deform plastically. Because these boundaries impede dislocations, the material requires higher stress to continue deforming, increasing its strength and resistance to deformation. That’s why finer-grained materials, with more boundaries, tend to be stronger. The other statements are not correct: grain boundaries are not perfect crystalline regions; they are disordered compared to the crystal lattice, and they do affect mechanical properties (they don’t simply leave them unchanged).