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

Concept: Precipitation always takes the path of least resistance first.
When a supersaturated Al-Cu alloy is aged, it does not form the stable, low energy $\theta$ phase ($CuAl_2$) right away, because that phase is incoherent with the aluminium matrix and costly to nucleate. Instead the system builds up to it through a chain of intermediate structures, each one a little closer to $\theta$ but each one easier to form than jumping there directly.

Step 1: Start with the easiest structure.
The very first thing to form is a GP zone: just a thin, fully coherent, copper rich cluster on a $\{100\}$ plane of the matrix. It needs almost no interface energy to form, so it appears fastest even though it is far from the stable phase.

Step 2: Track how coherency is slowly given up.
As the copper rich region grows, keeping it perfectly coherent with the surrounding lattice becomes harder. The structure reorganizes into $\theta''$, still fully coherent but more ordered than a GP zone. With more aging it becomes $\theta'$, which is only semi-coherent, larger, and carries its own distinct tetragonal structure. Finally the strain becomes too much to keep any coherency at all, and the precipitate turns into the stable, incoherent phase $\theta$.

Step 3: Write the chain in order.
\[ \text{GP zone} \to \theta'' \to \theta' \to \theta \]
Coherency strain energy drops step by step while the precipitate gets closer and closer to the true equilibrium phase.

Step 4: Rule out the other three options by their starting or ending point.
Any sequence that starts from $\theta$ or from $\theta''$ instead of the GP zone is going in the wrong direction, since the GP zone is always the first thing to nucleate, not the last. Any sequence that reaches $\theta$ before passing through both $\theta''$ and $\theta'$ skips real intermediate stages that are always observed experimentally. Only one option starts at the GP zone and ends at $\theta$ while keeping $\theta''$ ahead of $\theta'$.

Step 5: Conclude.
\[ \boxed{\text{GP zone} \to \theta'' \to \theta' \to \theta} \]