Age hardening (precipitation strengthening) — identify the alloy system Age hardening is characteristically associated with which of the following alloys due to precipitation of finely dispersed phases after solution treatment and ageing?

Introduction / Context:
Age hardening (precipitation strengthening) produces significant increases in yield and hardness through controlled formation of nanoscale precipitates from a supersaturated solid solution. Recognizing alloy families that respond strongly to this treatment is essential for selecting appropriate heat treatments and anticipating property changes over time.

Given Data / Assumptions:

• Heat treatment sequence: solution treat → quench → age (natural or artificial).
• Alloy families considered: Al–Cu (duralumin), Cu–Zn (brass), commercially pure copper, elemental silver.

Concept / Approach:
Duralumin (Al–Cu with minor Mg, Mn) is the classic age-hardenable aluminium alloy. After solution treatment and quenching, a supersaturated Al matrix decomposes via GP zones → metastable precipitates → stable theta phases, raising strength markedly. By contrast, common brasses and pure copper are not strongly precipitation hardenable in standard compositions (brasses typically strengthen by cold work). Silver is an elemental metal and does not age harden in the absence of specific alloying additions.

Step-by-Step Solution:

Verification / Alternative check:
Mechanical property charts show duralumin's yield strength doubling or more after artificial ageing compared with the solutionized condition.

Why Other Options Are Wrong:

• Brass: limited or no precipitation response in common compositions.
• Copper: elemental metal; needs alloying for precipitation response.
• Silver: elemental; no age hardening without alloying additions.
• Cast iron: responds to heat treatment differently (graphite/carbides), not standard precipitation hardening.

Common Pitfalls:
Confusing natural ageing in Al–Cu alloys with strain ageing in low-carbon steels; expecting brass to behave like age-hardenable aluminium.

Final Answer: