05 Phase Diagrams and Phase Transformations Free Online FlashCards

What is a phase?

A physically and chemically distinct region of material, such as a liquid, a solid solution, or a compound.

What does a binary phase diagram show?

At fixed pressure, it plots temperature against composition and shows which phase or phases are stable at equilibrium.

What does the liquidus mark?

Above the liquidus, the alloy is entirely liquid. Cooling through it marks the start of solidification.

What does the solidus distinguish?

Below the solidus, the alloy is entirely solid; between the solidus and liquidus, liquid and solid coexist.

What does the solvus separate?

It separates a single solid solution from a region where two solid phases coexist.

What do tie-line endpoints identify?

Its endpoints give the compositions of the two coexisting phases at that temperature. They do not give the phases’ relative amounts.

What can rapid cooling cause during solidification?

Rapid cooling may outpace diffusion, leaving composition gradients called segregation or coring; it can also produce finer structures.

How does an off-eutectic alloy solidify?

They first form a primary solid phase. The remaining liquid later transforms into the eutectic mixture.

Why are TTT and CCT diagrams useful alongside phase diagrams?

An equilibrium phase diagram does not specify transformation speed or the resulting microstructure. TTT and CCT diagrams help predict transformations over time and during continuous cooling.

What commonly forms when eutectoid steel cools slowly?

On slow cooling, austenite decomposes into ferrite and cementite. Near the eutectoid composition, these often form layered pearlite.

What can a rapid quench form from austenite?

A sufficiently rapid quench transforms austenite into martensite, a hard, nonequilibrium structure formed largely without long-range diffusion.

What is the purpose of tempering martensitic steel?

Reheating martensitic steel below the austenite range allows limited diffusion, reducing brittleness while retaining useful strength.