Which strengthening mechanism relies on reducing grain size to hinder dislocation motion?

• A. Solid-solution strengthening
• B. Precipitation hardening
• C. Grain-boundary strengthening
• D. Work hardening

Answer: (C)

Grain-boundary strengthening hinges on how dislocations move through a polycrystal. Dislocations glide within grains, but when they reach a grain boundary, the misalignment between neighboring grains acts as a barrier that can reflect, absorb, or pin the dislocations. This raises the stress required to move dislocations, so the material becomes stronger and harder. Reducing the grain size increases the number of boundaries per unit volume, meaning dislocations encounter barriers more often and the strength goes up. The quantitative idea is the Hall-Petch relation, which shows strength growing as grain size decreases (sigma_y = sigma_0 + k_y d^(-1/2)). This is different from solid-solution strengthening (lattice distortions from solute atoms), precipitation hardening (barriers from dispersed particles), or work hardening (increased dislocation density from plastic deformation).