CME-260: Chapter 9

Phase Diagram

Diagram showing what phases and microstructures exist at given temperature, pressure, and composition in equlibrium.

Microstructure

Arrangement of phases within a material.

Phase

Chemically and structurally homogeneous region of a material.

Component

Chemically distinct substance that makes up a phase

State

Specific combination of phases present in a system.

State Variables

Independent parameters (temperature, pressure, composition) that define a system.

Degrees of Freedom

Number of independent variables that change without altering number of phases.

Gibbs Phase Rule

Relationship: F = C - P + 1 (or +2 if pressure included)

Binary Diagram

Diagram showing phases as a function of temperature and composition for two components.

Complete Solid Solution

System where components are fully soluble in each other in the solid state.

Phase Field

Region of a phase diagram where a specific phase or combination of phases exists.

State Point

Specific temperature and composition on a phase diagram.

Tie Line

Horizontal line used to determine compositions of phases in a two-phase region.

Mass Balance

Total mass equals sum of masses of all phases in a system.

Lever Rule

Method to calculate relative amounts of phases using tie-line distances.

Liquidus

Boundary above which material is completely liquid.

Solidus

Boundary below which material is completely solid.

Eutectic Diagram

Phase diagram where components have limited/no solid solubility.

Eutectic Temperature

Temperature where liquid transforms into two solid phases simultaneously.

Eutectic Composition

Composition at which eutectic reaction occurs.

Eutectic Reaction

L → α + β (liquid transforms into two solid phases)

Hypoeutectic Composition

Composition less than eutectic composition.

Hypereutectic Composition

Composition greater than eutectic composition.

Proeutectic

Phase that forms before eutectic reaction.

Eutectoid Diagram

Phase diagram involving solid-state transformation (no liquid).

Eutectoid Reaction

γ → α + β (solid transforms into two solid phases)

Peritectic Diagram

Diagram where liquid and solid form a new solid phase.

Peritectic Reaction

L + solid → new solid

Intermediate Compound

Compound formed between two components with fixed composition.

Congruent melting

Solid melts into liquid of same composition.

Incongruent Melting

Solid melts into liquid + different solid

Invariant Point

Point where degrees of freedom = 0; system cannot change without phase change.

Microstructural Development

Formation of internal structure during cooling from liquid.

Austenite

FCC iron phase (γ-iron) that dissolves carbon

Metastable

Phase that is not most stable but persists for long time.

White Cast Iron

Fe-C alloy (>2% C) where carbon exists as cementite

Gray Cast Iron

Fe-C alloy (>2% C) where carbon exists as graphite flakes

Zone Refining

Purification method using moving molten zone to remove impurities.

General Diagram

Combination of multiple binary diagrams involving intermediate compounds.

Phase Field

Region in a phase diagram where a specific phase or phase mixture exists.