GATE 2021 MT – Question 60
An A–B alloy with 5 wt.% B solidifies without convection, with the shown hypothetical eutectic phase diagram. Under steady state, the critical temperature gradient to maintain a planar front is ______ K/mm (nearest integer). Liquid diffusivity of B is $10^{-9}$ m$^2$/s and front speed is 4 μm/s.
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Correct answer: 298 to 302
Explanation
From the phase-diagram construction, the equilibrium temperature difference associated with solute pile-up is 775−700=75 K. The steady diffusion length is D/v=10⁻⁹/(4×10⁻⁶)=0.25 mm. Threshold gradient is 75/0.25=**300 K/mm**.