Question

Both creep resistance and tensile strength of a metal can be enhanced by

• A. increase in the grain size
• B. decrease in the grain size
• C. addition of dispersoids
• D. annealing

Hint:

Remember that strengthening mechanisms that hinder dislocation motion (like precipitation and dispersion strengthening) are effective at both low and high temperatures, thus improving both tensile strength and creep resistance.

Answer 1

The Correct Option is C

Step 1: Understanding Strengthening Mechanisms:
Tensile strength is improved by restricting dislocation motion at normal temperatures, whereas creep resistance requires resistance to deformation at high temperatures.
Step 2: Role of Dispersoids:
Dispersoids are fine, stable, non-coarsening particles uniformly distributed within the metal matrix. These particles act as strong obstacles to dislocation movement.
Step 3: Effect on Tensile Strength and Creep Resistance:

• They increase tensile strength by impeding dislocation glide (Orowan strengthening mechanism).
• They enhance creep resistance by preventing dislocation climb and reducing grain boundary movement at elevated temperatures.

Step 4: Analysis of Other Options:
Increasing grain size improves creep resistance but reduces tensile strength. Decreasing grain size increases tensile strength but may reduce creep resistance due to grain boundary sliding. Annealing generally softens the material.
Final Conclusion:
The addition of dispersoids improves both tensile strength and creep resistance simultaneously.