The GATE Grind

GATE 2020 MT – Question 25

Physical Metallurgy · Nucleation solidification and transformations · 1 mark · Multiple choice

The precipitation sequence during aging of Al–4 wt.% Cu is

  1. GP zone → $\theta^{\prime\prime}$ → $\theta^\prime$ → $\theta$
  2. GP zone → $\theta$ → $\theta^\prime$ → $\theta^{\prime\prime}$
  3. GP zone → $\theta^\prime$ → $\theta^{\prime\prime}$ → $\theta$
  4. $\theta^{\prime\prime}$ → $\theta^\prime$ → GP zone → $\theta$

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Correct answer: (A) GP zone → $\theta^{\prime\prime}$ → $\theta^\prime$ → $\theta$

Explanation

Supersaturated Al–Cu first forms solute-rich coherent Guinier–Preston zones. These evolve through metastable θ″ and then θ′ precipitates before the equilibrium θ (Al₂Cu) phase. The progression reflects changing structure, coherency and stability during aging. Thus A.

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