Discussion
Fluidity of a molten alloy during sand casting depends on its solidification range. The phase diagram of a hypothetical binary alloy of components A and B is shown in the figure with its eutectic composition and temperature. All the lines in this phase diagram, including the solidus and liquidus lines, are straight lines. If this binary alloy with 15 weight % of B is poured into a mould at a pouring temperature of 800 °C, then the solidification range is
Answer: D
The solidification range is 150 °C.
The solidification range is the temperature span between the start of freezing (the liquidus) and the end of freezing (the solidus) for the alloy's composition.
Liquidus at 15 % B. The liquidus is a straight line from 700 °C at 0 % to the eutectic at 400 °C and 30 %, falling 10 °C per 1 % B:
\[ T_{L} = 700 - 10 \times 15 = 550\ ^{\circ}\text{C} \]
Solidus at 15 % B. The alpha solidus ends at 10 % B on the eutectic line. An alloy richer than 10 % B finishes freezing at the eutectic temperature, 400 °C.
So the range is \(550 - 400 = 150\) °C. 250 °C is the superheat, 800 minus 550; 400 °C is the eutectic temperature itself; and 800 °C is the pouring temperature, which plays no part in the freezing range.
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