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

Step 1: Understanding solidification in alloys.
Solidification of an alloy begins when the first solid phase starts to form from the liquid during cooling. In a phase diagram, this transformation is represented by a specific boundary line.
Step 2: Role of the liquidus line.
The liquidus line separates the fully liquid region from the liquid + solid region. As soon as the temperature crosses this line during cooling, solidification begins.
Step 3: Elimination of other options.
(B) Solidus line: Represents the completion of solidification, not the beginning.
(C) At equilibrium: Equilibrium is a condition, not a boundary for phase change.
(D) At freezing point: Pure metals freeze at a point, but alloys solidify over a range of temperatures.
Step 4: Conclusion.
Hence, the solidification of an alloy starts from the liquidus line.

Answer 2

Following the cooling curve of a molten alloy as it slowly loses heat is a direct way to see exactly where solidification actually begins, so let's trace through it and check each option.

1. Liquidus line: As the melt cools, it remains fully liquid until its temperature and composition reach the liquidus line on the phase diagram. At that exact point, the first tiny amount of solid phase appears, and the cooling curve shows its first change of slope (due to the latent heat now being released). This is the physical onset of solidification.
2. Solidus line: The solidus line marks the temperature at which the very last trace of liquid disappears and the alloy becomes fully solid. This is the end of solidification, not the beginning.
3. At equilibrium: Equilibrium describes a state where the phases present are stable and not changing, it is not a line or boundary on the phase diagram, and it does not by itself mark when solidification starts.
4. At freezing point: A single, sharp freezing point strictly applies to pure metals or eutectic compositions, which solidify at one fixed temperature. A general alloy solidifies gradually over a range of temperatures between the liquidus and solidus, so "freezing point" is not the correct description here.

Tracing the cooling curve shows solidification physically begins the moment the liquidus line is crossed.

Therefore, the correct answer is Liquidus line.