In solid solution strengthening, which condition enables substitutional solid solution strengthening?

• A. Solute atoms replace host atoms with a size difference of 15% or less.
• B. Solute atoms occupy interstitial positions.
• C. The alloy must form a new crystal structure.
• D. The impurities must be larger than host atoms by more than 20%.

Answer: (A)

Substitutional solid solution strengthening happens when solute atoms take the place of host atoms in the crystal lattice and create local distortions that hinder dislocation motion, making the material stronger. This works best when the two types of atoms are similar in size so the lattice can accommodate the substitution with only modest distortion. A common guideline is a size difference of about 15% or less. That keeps the crystal structure intact while still introducing the strains needed to impede dislocations.

If atoms occupy interstitial sites instead, you’d get interstitial solid solution strengthening, which is a different mechanism. Requiring a new crystal structure isn’t necessary for substitutional strengthening, and a large size disparity (larger than about 20%) would make substitution unlikely due to high lattice strain and limited solubility, often leading to phase separation or other structures.