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 the Question:
The question asks which of the given options can simultaneously increase both the tensile strength (resistance to deformation at lower temperatures) and the creep resistance (resistance to deformation at high temperatures under a constant load) of a metal.
Step 2: Detailed Explanation:
Let's analyze the effect of each option on mechanical properties:

• (A) Increase in the grain size: A larger grain size generally decreases tensile strength at room temperature (contrary to the Hall-Petch relationship). However, at high temperatures where creep is dominated by grain boundary sliding, larger grains can improve creep resistance. Since it decreases tensile strength, this option is incorrect.
  
• (B) Decrease in the grain size: According to the Hall-Petch relationship, decreasing the grain size increases the tensile strength by increasing the number of grain boundaries that impede dislocation motion. This also improves creep resistance at lower temperatures. However, at very high temperatures, a fine grain structure can lead to faster creep due to increased grain boundary sliding. Therefore, it does not reliably enhance both properties under all conditions.
  
• (C) Addition of dispersoids: Dispersoids are fine, stable, and non-coherent particles (e.g., oxides, carbides) distributed within the metal matrix. This process is known as dispersion strengthening. These particles act as very effective obstacles to dislocation motion at both room temperature and elevated temperatures. This significantly increases both tensile strength and creep resistance, as the dislocations must climb over or bypass these particles, a process that requires high energy.
  
• (D) Annealing: This is a heat treatment process that typically involves heating a metal to a specific temperature and then cooling it slowly. The primary purpose of annealing is to relieve internal stresses, increase ductility and toughness, and produce a specific microstructure. It generally results in a softer material with lower tensile strength and hardness. Thus, it is the opposite of what is required.
  

Step 3: Final Answer:
The addition of dispersoids is the most effective method among the choices for simultaneously enhancing both tensile strength and high-temperature creep resistance.