Question

Which one of the following options is correct?
For Al - 4% Ag alloy (composition in atom %), the correct sequence of operations involved in precipitation hardening is:

• A. Quenching \(\rightarrow\) Aging \(\rightarrow\) Solution treatment
• B. Solution treatment \(\rightarrow\) Aging \(\rightarrow\) Quenching
• C. Aging \(\rightarrow\) Solution treatment \(\rightarrow\) Quenching
• D. Solution treatment \(\rightarrow\) Quenching \(\rightarrow\) Aging

Hint:

Ask what each stage needs to already exist before it can happen: aging needs supersaturation, and supersaturation needs a fast quench right after dissolving the solute.

Answer 1

The Correct Option is D

Step 1: Recall what precipitation hardening does.
Precipitation hardening (also called age hardening) strengthens an alloy by creating a fine, uniform dispersion of small second-phase particles inside the matrix. These particles obstruct dislocation motion, since a moving dislocation must either cut through them or bow around them (Orowan looping), and both mechanisms need extra applied stress, raising the yield strength.

Step 2: Recall the three stages, in order.
(i) Solution treatment: The alloy, here Al with 4 atom% Ag, is heated above the solvus temperature (but below the eutectic melting point) and held there. At this temperature Ag is fully soluble in the Al matrix, so the alloy becomes a single-phase, homogeneous solid solution, with all the Ag atoms dissolved.
(ii) Quenching: The alloy is then rapidly cooled, usually in water, from the solution treatment temperature down to room temperature. This rapid cooling does not give the Ag atoms time to diffuse and precipitate out, so the alloy is frozen into a supersaturated solid solution (SSSS), a metastable single phase holding far more Ag in solution than the room-temperature equilibrium allows.
(iii) Aging: The supersaturated solid solution is then held at room temperature (natural aging) or reheated to a moderate temperature (artificial aging). This gives the excess Ag atoms enough mobility to cluster and form fine coherent or semi-coherent precipitate particles throughout the matrix, which is what actually hardens the alloy.

Step 3: Explain why the order cannot be changed.
Aging must always come last, because it needs a supersaturated solid solution to precipitate from, and that supersaturation only exists right after quenching. Quenching must always come right after solution treatment, because any slow cooling would let precipitation start uncontrolled, coarse, and non-uniform, defeating the purpose. So the sequence must be solution treatment, then quenching, then aging.

Step 4: Eliminate the other options.
(A) Quenching, then Aging, then Solution treatment: Starting with quenching makes no sense, since nothing is yet dissolved into a single phase to quench in. Incorrect.
(B) Solution treatment, then Aging, then Quenching: Aging before quenching would let the Ag precipitate out slowly and uncontrolled right after solution treatment, giving coarse particles, and quenching afterward serves no purpose. Incorrect.
(C) Aging, then Solution treatment, then Quenching: Aging cannot happen first, since there is no supersaturated solid solution yet to age. Incorrect.

Step 5: Final Answer.
The correct order is solution treatment, followed by quenching, followed by aging.
\[ \boxed{\text{Solution treatment} \rightarrow \text{Quenching} \rightarrow \text{Aging}} \]