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GATE 2026 MT – Question 22

Physical Metallurgy · Diffusion in solids · 1 mark · Multiple choice

Which one of the following options is correct? In a binary alloy system, during spinodal decomposition, uphill diffusion occurs:

  1. From higher to lower concentration and from higher to lower chemical potential
  2. From higher to lower concentration and from lower to higher chemical potential
  3. From lower to higher concentration and from lower to higher chemical potential
  4. From lower to higher concentration and from higher to lower chemical potential

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Correct answer: (D) From lower to higher concentration and from higher to lower chemical potential

Explanation

Within the spinodal, the free-energy curvature with composition is negative. A composition fluctuation therefore makes the solute chemical potential lower in the locally richer region. Atoms move down chemical-potential gradients into that region, increasing the concentration fluctuation. Thus concentration moves low→high while chemical potential moves high→low: **D**.