The GATE Grind

GATE 2026 MT – Question 23

Physical Metallurgy · Heat treatment and recrystallization · 1 mark · Multiple choice

In an Al–4 wt.% Cu alloy during isothermal aging, the correct precipitation sequence is ________.

  1. $\theta\prime\prime\to\theta\prime\to\theta\to\text{GP zone}$
  2. $\text{GP zone}\to\theta\to\theta\prime\to\theta\prime\prime$
  3. $\theta\to\theta\prime\to\theta\prime\prime\to\text{GP zone}$
  4. $\text{GP zone}\to\theta\prime\prime\to\theta\prime\to\theta$

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Correct answer: (D) $\text{GP zone}\to\theta\prime\prime\to\theta\prime\to\theta$

Explanation

The supersaturated alloy first forms coherent solute-rich GP zones, then the metastable $\theta\prime\prime$ and $\theta\prime$ precipitates, and finally the equilibrium $\theta$ phase.

This progression moves toward the stable phase while reducing coherency. The correct sequence is **D**.