Which mechanism strengthens metal by introducing impurity atoms that create lattice distortion?

• A. Strain hardening
• B. Solid solution alloying
• C. Grain boundary strengthening
• D. Precipitation hardening

Answer: (B)

Solid solution strengthening occurs when impurity atoms are introduced into a metal and distort the lattice, creating stress fields that hinder dislocation motion. When atoms of different size replace the host atoms (substitutional) or fit into interstitial sites (interstitial), the local lattice is strained. Dislocations moving through this distorted lattice must interact with many solute atoms, raising the energy and stress required for them to glide. The result is a stronger material, with greater yield strength that typically increases with solute concentration and the size difference between solute and solvent atoms.

Other strengthening methods work differently: strain hardening strengthens by increasing dislocation density through plastic deformation; grain boundary strengthening relies on obstacles at grain boundaries to dislocation motion; and precipitation hardening uses hard second-phase particles to block dislocations. The impurity-induced lattice distortion is the hallmark of solid solution strengthening.