Question

4. Briefly explain why, upon solidification, an alloy of eutectic composition forms a microstructure consisting of alternating layers of the two solid phases. What is this structure called? What kind of effect does this structure have on the melting point of the alloy and why? (5 points)

Answer 1

• Consider a lead-tin alloy having a composition of 61.9 wt% Sn in liquid phase region
• This alloy is cooled from this liquid phase region (say at 250^oC). This is represented by a vertical line in the figure

300 L 61.9 wt% sn) 2007 d ath BAL 1839€ Temperature --> Q+B 100 + + 20 40 60 CS) Composition (wt% 50) ►

• When it is cooled, there will be no change until it reaches the eutectic temperature, 183^oC.
• When it is cooled beyond eutectic temperature, the liquid transforms into $\alpha$ and $\beta$ phases.
• Here the composition of both $\alpha$ and $\beta$ are different from that of the liquid phase. This redistribution is due to atomic diffusion
• The microstructure of solid formed by this transformation consists of alternate layers of $\alpha$ and $\beta$ phases
• The microstructure thus formed is called eutectic structure

Interface R & Liquid eo a PL B
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• During this transformation process, $\alpha - \beta$ eutectic layer grows by replacing the liquid phase
• The process of redistribution of lead and tin occurs by the atomic diffusion of the liquid region just ahead of the interface.
• Eutectic temperature is the lowest possible melting point for the alloy