Question

Which of the following statements is or 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 CuAl2 precipitates.
• C. GP zones are incoherent with the matrix.
• D. GP zones are coherent with the matrix.

Hint:

GP zones are coherent Cu-rich clusters, while incoherent CuAl2 precipitates appear later. Coherency is the key to precipitation hardening in Al alloys.

Answer 1

The Correct Option is A, D

Step 1: Recall what GP zones are.
GP zones, also called Guinier–Preston zones, form at the initial stage of age hardening in Al-Cu alloys. They are very small, Cu-rich clusters that appear as solute atoms segregate. Step 2: Properties of GP zones.
- They are not CuAl2 precipitates. CuAl2 forms at later stages (theta phase).
- GP zones remain coherent with the aluminum matrix, producing strain fields that strengthen the alloy.
- They are not incoherent. Incoherency occurs at later stages with coarser precipitates. Step 3: Check each option.
(A) Correct, GP zones are Cu-rich clusters.
(B) Incorrect, CuAl2 appears at a later stage.
(C) Incorrect, they are coherent, not incoherent.
(D) Correct, GP zones are coherent with the matrix. Step 4: Conclusion.
Correct statements are (A) and (D). \[ \boxed{\text{Correct: (A) and (D)}} \]