Question

Which of the following statements is/are CORRECT with respect to the initial stage of GP zone formation in a precipitation-hardenable Al - 4.5 wt.% Cu alloy?

• A. GP zones are Cu-rich clusters.
• B. GP zones are CuAl\(_2\) precipitates.
• C. GP zones are incoherent with the matrix.
• D. GP zones are coherent with the matrix.

Hint:

In the precipitation-hardening process, GP zones are the early stages of precipitation that consist of Cu-rich clusters and are coherent with the matrix, contributing to the strength of the alloy.

Answer 1

The Correct Option is A, D

The GP zones (Guinier-Preston zones) are formed during the initial stages of precipitation hardening in alloys like Al-Cu. Let's break down each option to understand the correct answer.
Step 1: Analysis of each option - Option (A): "GP zones are Cu-rich clusters." - Correct: In the initial stage of precipitation hardening, GP zones are small clusters that are rich in copper atoms. These clusters form before the actual CuAl\(_2\) precipitates are formed. The Cu-rich clusters are not yet ordered in a specific crystal structure, making them an intermediate phase in the precipitation process.
- Option (B): "GP zones are CuAl\(_2\) precipitates." - Incorrect: CuAl\(_2\) precipitates are a later stage of precipitation in Al-Cu alloys, after the formation of GP zones. GP zones are not CuAl\(_2\) precipitates, but rather the clusters that form before the actual precipitate phase.
- Option (C): "GP zones are incoherent with the matrix." - Incorrect: GP zones are coherent with the matrix, meaning they are aligned with the crystal lattice of the aluminum matrix. This coherency allows for the strengthening effect of GP zones, as they hinder dislocation motion.
- Option (D): "GP zones are coherent with the matrix." - Correct: GP zones are indeed coherent with the matrix. This coherency is crucial because it helps to impede dislocation movement, which is responsible for the strengthening of the alloy. The coherency between the matrix and GP zones contributes to the alloy's overall hardness and strength.
Step 2: Conclusion The correct statements are Option A and Option D. The GP zones are Cu-rich clusters and are coherent with the matrix. Hence, the correct answer is Option A and D.