What is the effect of reducing grain size on yield strength?

• A. Reducing grain size decreases yield strength
• B. Reducing grain size increases yield strength
• C. Grain size has no effect on yield strength
• D. Increasing grain size increases yield strength

Answer: (B)

Smaller grains strengthen a metal because grain boundaries act as barriers to dislocation motion. With more boundaries per unit volume, dislocations have to keep piling up at boundaries, requiring higher applied stress to continue plastic deformation. This effect is quantified by the Hall-Petch relation, which shows yield strength increasing as grain size decreases (roughly, strength grows with the inverse square root of grain size). So the best answer is that reducing grain size increases yield strength. The other statements contradict this well-established behavior: grain size does affect yield strength, and increasing grain size would typically reduce strength rather than increase it.