Which mechanism strengthens a metal by increasing grain boundary area that hinders dislocation motion?

• A. Grain size reduction increases grain boundary area, hindering dislocation motion.
• B. Increasing the temperature to promote dislocation motion.
• C. Reducing the number of grain boundaries to allow easier slip.
• D. Introducing more impurities to create lattice strain.

Answer: (A)

Grain refinement strengthens a metal by increasing the grain boundary area, which hinders dislocation motion. Dislocations have to move from one grain to another, and grain boundaries act as barriers to their glide, causing them to pile up and require higher stress to continue plastic deformation. When grains are finer, there are more boundaries per unit volume, so dislocations encounter obstacles more often and the material appears stronger. This strengthening is quantified by the Hall-Petch relation, showing yield strength rises as grain size decreases. The other ideas don’t match this mechanism: simply raising temperature promotes dislocation motion and softening; reducing the number of grain boundaries would make slip easier; introducing impurities strengthens by lattice strain (solid-solution or precipitate strengthening) through a different path, not by increasing boundary area.