Non-ferrous Alloys Questions and Answers

Answer: C

The hardness first increases and then decreases.

Ageing a supersaturated Al-Cu solid solution forms coherent GP zones and then the transition phases \(\theta''\) and \(\theta'\). These fine, closely spaced precipitates obstruct dislocations and raise the hardness up to a peak. With longer ageing the precipitates coarsen, lose coherency and move towards the equilibrium \(\theta\) (CuAl₂); dislocations can then bow around them more easily and the hardness falls (overageing).

A monotonic change in either direction misses one of these two stages, and an initial drop is not part of the age-hardening curve.

Answer: C

The purpose is to form supersaturated solid solution.

Age hardening (precipitation hardening) has three stages. Solution treatment heats the alloy into the single-phase region so that the alloying element, such as copper in Al-Cu, dissolves completely. Quenching then cools it too fast for the solute to come out, trapping it in a supersaturated solid solution. Ageing, at room or a moderate temperature, finally lets fine coherent precipitates form throughout the grains, and these block dislocations.

Martensite forms in quenched steels, not in aluminium alloys. Growing precipitates is what over-ageing does, and it softens the alloy. Precipitates at grain boundaries are undesirable and are what the quench is meant to prevent.

Answer: A

The sequence is GP zone → θ″ → θ′ → θ.

In a supersaturated Al-4.5% Cu alloy, copper first clusters into Guinier-Preston zones, thin disc-shaped copper-rich layers fully coherent with the matrix. These give way to the coherent θ″ (sometimes called GP II), then to the semi-coherent θ′, and finally to the equilibrium, incoherent θ phase, CuAl₂.

Each step lowers the free energy, and the coherent phases form first because their low interfacial energy makes them easiest to nucleate. The other options place θ′ before θ″ or the equilibrium θ before the transition phases, which reverses this order.

Answer: A

Gun metal is an alloy of copper, tin and zinc.

Gun metal is a bronze, typically about 88% copper, 10% tin and 2% zinc. It resists corrosion and wear, which is why it is used for bearings, valve bodies, pump parts and marine fittings.

Copper, lead and zinc describes a leaded brass. The options built on lead, zinc and nickel or tin, zinc and nickel leave out copper, the base metal of gun metal.

Answer: D

The element not found in non-ferrous alloys is iron.

'Ferrous' comes from ferrum, Latin for iron. Ferrous alloys, such as steel and cast iron, are based on iron; non-ferrous alloys are those whose base metal is something else, such as copper, aluminium, lead, tin or zinc, and iron is not a main constituent.

Lead, tin and zinc are all common in non-ferrous alloys: solder is tin and lead, brass is copper and zinc, and bronze is copper and tin.