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:
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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.
Precipitation hardening works by locking a supersaturated solid solution into the metal and then letting it throw out a fine, closely spaced set of second-phase particles that block dislocations from sliding through the lattice. Getting this sequence backwards makes the process meaningless, so let's check each option against what physically has to happen first.
Quenching then Aging then Solution treatment: quenching is meant to freeze in a solid solution that was just created at high temperature. Doing it first, before any solution treatment, leaves nothing dissolved to freeze in. This order is physically backwards.
Solution treatment then Aging then Quenching: after solution treatment the Ag atoms are fully dissolved in the hot Al matrix. If aging (a slow, controlled hold) happens next instead of a fast quench, the Ag begins precipitating immediately during the slow cool, forming coarse, widely spaced particles instead of the fine dispersion needed for strengthening. Quenching afterward achieves nothing.
Aging then Solution treatment then Quenching: aging needs a supersaturated solid solution to draw precipitates from. Before solution treatment, that supersaturated state does not exist, so there is nothing for aging to act on.
Solution treatment then Quenching then Aging: solution treatment dissolves the Ag fully into the Al matrix at high temperature. The fast quench then traps this dissolved Ag in a supersaturated solid solution at room temperature, since there is no time for it to come back out. Aging finally gives the trapped Ag atoms just enough mobility to form the fine precipitates that block dislocation motion and raise strength. Every step has exactly what it needs from the step before it.
Only the fourth order lines up with the physics of each stage: dissolve first, freeze the dissolved state in second, then let fine particles form last.
Let's summarize:
Solution treatment dissolves the alloying element into a single-phase solid solution.
Quenching freezes that solution in a supersaturated, metastable state.
Aging lets fine, hardening precipitates form out of the supersaturation.
So the correct sequence for the Al - 4% Ag alloy is solution treatment, then quenching, then aging.
