Which statement best describes the purpose of strengthening mechanisms?

• A. To maximize dislocation motion
• B. To create barriers to dislocation motion
• C. To reduce electrical conductivity
• D. To increase grain size

Answer: (B)

Strengthening mechanisms raise the resistance to plastic deformation by hindering dislocation motion. In metals, plastic deformation primarily occurs when dislocations glide on slip planes. If you introduce obstacles—such as grain boundaries, second-phase particles, solute atoms, or a higher dislocation density from prior deformation—these dislocations must overcome additional barriers. Each obstacle raises the stress needed to move dislocations, so the material becomes stronger and harder.

That’s why describing strengthening as creating barriers to dislocation motion is the best fit. Other options miss the core idea: maximizing dislocation motion would promote deformation and soften the material; reducing electrical conductivity isn’t the aim of strengthening (even if some strengthening methods affect conductance, the goal is mechanical stability); increasing grain size generally lowers strength (smaller grains typically strengthen the material via more grain boundary barriers).