Question

Which one of the following options is correct?
In Al - 4 wt.% Cu alloy during isothermal aging, the correct precipitation sequence is:

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

Hint:

Precipitation in Al-Cu alloys always begins with the easiest, fully coherent structure (GP zone) and ends with the stable, incoherent \(\theta\) phase, passing through \(\theta''\) then \(\theta'\).

Answer 1

The Correct Option is D

Step 1: Recall the alloy system.
Al - 4 wt.% Cu is the classic age-hardening (precipitation-hardening) aluminium alloy. When it is solution treated, quenched, and then aged at a moderate temperature, it does not form the equilibrium \(\theta\) phase (\(CuAl_2\)) directly. Instead it passes through a sequence of intermediate, metastable structures on the way there.

Step 2: Recall why GP zones form first.
Right after quenching, the alloy is supersaturated with copper and also holds a large excess of vacancies. Copper atoms cluster on \(\{100\}\) planes of the aluminium matrix with the help of these vacancies, forming very small, fully coherent copper rich clusters called Guinier-Preston (GP) zones. This is the easiest structure to nucleate since it needs the least amount of atomic rearrangement and interfacial energy, so it always appears first.

Step 3: Recall the intermediate coherent and semi-coherent phases.
As aging continues, the GP zones grow and reorganize into a more ordered, fully coherent phase called \(\theta''\).
Further aging turns \(\theta''\) into \(\theta'\), which is a larger, partially coherent (semi-coherent) precipitate with its own tetragonal structure and a higher strain energy than \(\theta''\), but a lower chemical energy penalty since the structure is closer to the stable phase.
With enough time and temperature, \(\theta'\) finally transforms into the stable, incoherent equilibrium phase \(\theta\) (\(CuAl_2\)), and the precipitate loses coherency with the matrix completely.

Step 4: Write out the full sequence.
\[ \text{GP zone} \rightarrow \theta'' \rightarrow \theta' \rightarrow \theta \]
Each step trades a small amount of coherency strain energy against a lower chemical free energy, moving the system step by step toward the stable phase rather than jumping there directly.

Step 5: Analyze the options.
(A) \(\theta'' \rightarrow \theta' \rightarrow \theta \rightarrow\) GP zone: runs the sequence backward, ending at the zone that should form first. Incorrect.
(B) GP zone \(\rightarrow \theta \rightarrow \theta' \rightarrow \theta''\): starts correctly but then jumps straight to the stable \(\theta\) phase before the intermediate phases, and even reverses the order of \(\theta'\) and \(\theta''\). Incorrect.
(C) \(\theta \rightarrow \theta' \rightarrow \theta'' \rightarrow\) GP zone: also runs backward from the stable phase to the zone. Incorrect.
(D) GP zone \(\rightarrow \theta'' \rightarrow \theta' \rightarrow \theta\): matches the true aging sequence exactly. Correct.

Final Answer:
The precipitation sequence in Al - 4 wt.% Cu during isothermal aging is GP zone, then \(\theta''\), then \(\theta'\), then the stable \(\theta\) phase.
\[ \boxed{\text{GP zone} \rightarrow \theta'' \rightarrow \theta' \rightarrow \theta} \]