Which statement best describes precipitation hardening?

• A. It strengthens by increasing porosity.
• B. It relies on grain boundary pinning.
• C. It relies on diffusionless transformations.
• D. It uses finely dispersed precipitates that hinder dislocations; steps include solution treatment, quenching, aging.

Answer: (D)

Precipitation hardening strengthens a metal by introducing finely dispersed precipitates that impede dislocation motion. The process involves dissolving solute to form a single-phase solid solution (solution treatment), then cooling rapidly to trap that solution in a supersaturated state (quenching), and finally aging at an elevated temperature to precipitate small, finely dispersed particles. These precipitates obstruct dislocations as they move, increasing strength and hardness while keeping ductility reasonably intact. The other descriptions refer to different mechanisms: increasing porosity would weaken the material; grain boundary pinning relates to restricting grain growth rather than forming hardening precipitates; diffusionless transformations involve phase changes without diffusion (like martensite), not precipitation.