Question

For Al – 4.5 wt% Cu alloy, the correct sequence of precipitation during age hardening at room temperature is:

• A. GP zone \( \rightarrow \) \( \theta'' \) \( \rightarrow \) \( \theta' \) \( \rightarrow \) \( \theta \)
• B. GP zone \( \rightarrow \) \( \theta' \) \( \rightarrow \) \( \theta'' \) \( \rightarrow \) \( \theta \)
• C. GP zone \( \rightarrow \) \( \theta'' \) \( \rightarrow \) \( \theta' \) \( \rightarrow \) \( \theta \)
• D. GP zone \( \rightarrow \) \( \theta' \) \( \rightarrow \) \( \theta'' \) \( \rightarrow \) \( \theta \)

Hint:

In age hardening, the sequence of precipitates starts from the GP zones and progresses through the \( \theta'' \) and \( \theta' \) phases before reaching the final stable \( \theta \) phase.

Answer 1

The Correct Option is A

In the age hardening process for Al-4.5 wt% Cu alloy, the sequence of precipitation occurs as the alloy is aged at room temperature. The correct order of precipitation stages is as follows:
1. GP zone: The initial stage where clusters of solute atoms form, called the Guinier-Preston (GP) zones. These zones are not yet distinct precipitates, but the atomic clusters have some short-range ordering.
2. \( \theta'' \): The GP zones evolve into the \( \theta'' \) phase, which is a more ordered precipitate.
3. \( \theta' \): As the aging continues, the \( \theta'' \) phase transforms into the \( \theta' \) phase, which is more stable and larger in size.
4. \( \theta \): Finally, the \( \theta' \) phase coarsens and forms the stable \( \theta \) phase, which is a solid solution of copper in aluminum.
Thus, the correct sequence of precipitation is:
\[ {GP zone} \rightarrow \theta'' \rightarrow \theta' \rightarrow \theta \] Therefore, the correct answer is option (A).