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-hardenable alloys like Al–Cu, metastable phases appear in a definite order: GP zones $\rightarrow$ $\theta''$ $\rightarrow$ $\theta'$ $\rightarrow$ equilibrium phase. The maximum hardness corresponds to $\theta''$.

Answer 1

The Correct Option is A

Step 1: Recall precipitation sequence in Al–Cu alloys.
Aluminium–copper alloys undergo age hardening (precipitation hardening). The process occurs in a sequence of metastable precipitates before reaching the equilibrium phase. The general sequence is: \[ \text{GP zones} \;\; \rightarrow \;\; \theta'' \;\; \rightarrow \;\; \theta' \;\; \rightarrow \;\; \theta. \]

Step 2: Explanation of each stage. - GP zones (Guinier–Preston zones): These are clusters of solute atoms (Cu) that form at room temperature. They are coherent with the Al matrix. - $\theta''$: Transition phase, coherent with Al lattice, responsible for major strengthening effect. - $\theta'$: Semi-coherent precipitate, less strengthening than $\theta''$. - $\theta$: Equilibrium phase (CuAl$_2$), incoherent, final stable phase.

Step 3: Verify options.
- (A) matches the correct sequence. - (B), (C), (D) are incorrect because they alter the experimentally observed order. Final Answer:
\[ \boxed{\text{GP zone} \;\; \rightarrow \;\; \theta'' \;\; \rightarrow \;\; \theta' \;\; \rightarrow \;\; \theta} \]