Question

In age-hardening of an aluminium alloy, the purpose of solution treatment followed by quenching is to

• A. form martensitic structure
• B. increase the size of the precipitates
• C. form supersaturated solid solution
• D. form precipitates at the grain boundaries

Hint:

Remember: solution treatment dissolves alloying elements; quenching traps them in a supersaturated solid solution; ageing then allows fine precipitates to form, which strengthen the alloy.

Answer 1

The Correct Option is C

Step 1: Recall the process of age-hardening.
Age-hardening (also called precipitation hardening) is a strengthening method used in aluminium and other alloys. It involves three stages: 1. Solution treatment – heating the alloy to a high temperature so that all the alloying elements dissolve in a single-phase solid solution. 2. Quenching – rapidly cooling the alloy to retain this high-temperature solid solution at room temperature. 3. Ageing – reheating at a lower temperature to allow controlled precipitation of fine particles that strengthen the alloy.
Step 2: Effect of quenching.
Quenching "freezes in" the solid solution, preventing immediate precipitation. This results in a supersaturated solid solution because the solute atoms are trapped in higher concentrations than would normally be stable at room temperature.
Step 3: Evaluate each option.
(A) {Martensitic structure} – this occurs in steels, not in aluminium alloys. Incorrect.
(B) {Increase the size of precipitates} – solution treatment and quenching aim to dissolve precipitates, not grow them. Incorrect.
(C) {Form supersaturated solid solution} – correct, this is the immediate result of quenching after solution treatment.
(D) {Form precipitates at the grain boundaries} – that happens during over-ageing, not during solution + quench. Incorrect. Final Answer: \[ \boxed{\text{Form supersaturated solid solution}} \]