Question

The solidification of an alloy starts in a phase diagram from

• A. Liquidus line
• B. Solidus line
• C. At equilibrium
• D. At freezing point

Hint:

For alloys: {Liquidus = start of solidification}, {Solidus = end of solidification}.

Answer 1

The Correct Option is A

Using the lever rule just below the liquidus line for a given alloy composition, the fraction of solid present is \[ f_s = \frac{C_L - C_0}{C_L - C_S} \] where \( C_0 \) is the overall alloy composition and \( C_L, C_S \) are the liquidus and solidus compositions at that temperature.

1. Step 1: Just above the liquidus, \( C_L = C_0 \), giving \( f_s = 0 \), fully liquid.
2. Step 2: Just below the liquidus, \( f_s \) becomes a small positive value, showing that solid has just started to appear.
3. Step 3: This transition from \( f_s = 0 \) to \( f_s > 0 \) happens exactly at the liquidus line, confirming it as the start of solidification.

So solidification of an alloy begins at the liquidus line.

Answer 2

Comparing how a pure metal solidifies against how an alloy solidifies highlights exactly why "liquidus line" is the right description and the other options are not.

1. Liquidus line: unlike a pure metal, an alloy does not have one single solidification temperature. Solidification instead starts wherever the cooling path first meets the liquidus line for that composition, marking the onset of the freezing range.
2. Solidus line: this instead marks the temperature where the alloy has become fully solid, the closing boundary of the freezing range, not its start.
3. At equilibrium: equilibrium is simply the condition assumed while reading the phase diagram, it is not itself a starting boundary for any transformation.
4. At freezing point: a single freezing point is a feature of pure substances and eutectics only; a general alloy composition freezes over a range rather than at one point, so this description does not fit.

Because general alloys solidify over a range beginning at the liquidus and ending at the solidus, the correct starting boundary is the liquidus line.

Therefore, the correct answer is Liquidus line.