Question

During the ageing of a homogenized Al-Cu alloy (1 to 4 wt.% Cu) below the GP zone solvus, hardness of the alloy:

• A. increases monotonically
• B. decreases monotonically
• C. first increases and then decreases
• D. first decreases and then increases

Hint:

Ageing curves for precipitation-hardenable alloys typically show: underaged → peak aged → overaged behaviour.

Answer 1

The Correct Option is C

During ageing below the GP zone solvus, Guinier–Preston (GP) zones form within the Al–Cu alloy.
Initially, fine coherent GP zones strengthen the alloy through coherency strain hardening.
This causes the hardness to increase with ageing time.
As ageing continues, GP zones grow and transform into θ'' and θ' precipitates.
Overageing occurs when precipitates coarsen and lose coherency with the matrix.
Loss of coherency reduces the resistance to dislocation motion.
Thus hardness decreases after reaching a peak value.
Therefore the hardness first increases and then decreases.