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The binary phase diagram of metals P and Q is shown in the figure. An alloy X containing 60% P and 40% Q by weight is cooled from liquid to solid state. The fractions of solid and liquid, respectively, at 1250 °C are

1. 77.8% and 22.2%
2. 22.2% and 77.8%
3. 68.0% and 32.0%
4. 32.0% and 68.0%

Correct answer: 77.8% and 22.2%

Solution

At 1250 °C, the alloy lies in the two-phase region, so both solid and liquid coexist. Using the lever rule with the compositions from the phase diagram gives the solid fraction as 77.8% and the liquid fraction as 22.2%.

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