9 – Phase Diagram (2)
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9 – Phase Diagram (2) (Phase Reactions). Phase Reaction. Phase Reaction: The compositions of the phases with temperature, it is called Phase Reaction takes place.

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9 – Phase Diagram (2) (Phase Reactions)

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9 – Phase Diagram (2)

(Phase Reactions)

Phase Reaction

Phase Reaction: The compositions of the phases with temperature, it is called Phase Reaction takes place.

Eutectic Reaction: A three-phase reaction, by which, on L+cooling, a liquid transforms into two solid phases at the same time.

Series of cooling curves at different metal composition are first constructed.

Points of change of slope of cooling curves (thermal arrests) are noted and phase diagram is constructed.

The greater the number of cooling curves the more accurate the phase diagram.


Very slow cooling (equilibrium) gives rise to cored structure as composition of melt continuously changes.

Rapid cooling delays solidification but also leads to cored structure


Non-Equilibrium Solidification of Alloys

  • Homogenization: Cast ingots heated

  • to elevated temperature to eliminate

  • cored structure.

  • Temperature of homogenization

  • must be lower than lowest melting

  • point of any of the alloy components.

W0=30% Cu

Eutectic Reaction

Liquid   (solid) +  (solid)

Eutectic Reaction

Liquidus : The line (or lines) on a phase diagram above which only liquid is present. Technically it is "the locus of temperatures above which only liquid is stable".

Solidus : The line (or lines) on a phase diagram below which only solid is present. Technically it is "the locus of temperatures below which only solid is stable".

Eutectic Reaction

Eutectic Point: A point defines the temperature and composition at which the eutectic reaction occurs. It is the bottom point of the "V" formed by two liquidus lines.

Eutectic Line (Eutectic Tie Line): The constant temperature line through the eutectic point.

Eutectic Structure

Eutectic Structure

As an alloy cools the first phase to solidify (unless the alloy's overall composition is eutectic) is called the Primary Phase.

Secondary Phase

Eutectic Structure

Eutectic Structure - Dendrites

Primary phases often form as dendrites. These are solid structures forming from a liquid, which solidify in a branched manner because it is energetically favorable.

Eutectic Structure - Dendrites

Segregation Behaviors

Segregation: Composition Gradient.

Fast Cooling is the key for Segregation

There is no enough time for the diffusion of elements

In some binary alloy systems, components have limited solid solubility.

Binary Eutectic Alloy System

Example : Pb-Sn alloy.

  • Eutectic composition freezes

  • at lower temperature than all

  • other compositions.

  • This lowest temperature is

  • called eutectic temperature.

Eutectic temperature

α solid solution + β solid solution




Liquid at 3000C.

At about 2450C first solid forms – proeutectic solid.

Slightly above 1830C composition of alpha follows solidus and composition of sn varies from 40% to 61.9%.

At eutectic temperature, all the remaining liquid solidifies.

Slow Cooling of 60% Pb – 40% Sn alloy

  • Further cooling lowers alpha Sn content and beta Pb.

  • (They try to move to equilibrium)

From J. Nutting and R. G. Baker, “Microstructure of Metals,” Institute of Metals, London, 1965,p.19.

Structure depends on factors like minimization of free energy at α / β interface.

Manner in which two phases nucleate and grow also affects structures.

Various Eutectic Structures

After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.

Eutectoid Reaction

Eutectoid Reaction: A three-phase reaction by which, on cooling, a solid transforms into two other solid phases at the same time. If bottom of a single-phase solid field closes (and provided the adjacent two-phase fields are solid also), it does so with a eutectoid point.

Solid-phase 1  Solid-phase 2 + Solid-phase 3

Fe-C Phase Diagram

Eutectoid Structures

  • Similar as Eutectic Structure, finer in scale

  • Origin solid decomposes into two new solid phase

  • Need diffusion of solute atoms

Pearlite in a plain-carbon steel, 0.05mm

Eutectoid Structures

Peritectic Reaction

Peritectic Reaction: A three-phase reaction by which, on cooling, two phases (one of them liquid) react to give a single new solid phase.

Liquid + Solid   Solid 

Peritectic reaction: Liquid phase reacts with a solid phase to form a new and different solid phase.

Liquid + α β

Binary Peritectic Alloy System


  • Peritectic reaction occurs

  • when a slowly cooled alloy

  • of Fe-4.3 wt% Ni passes

  • through Peritectic

  • temperature of 1517C.

  • Peritectic point is invariant.


Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni) γ 4.3 wt % Ni

At 42.4 % Ag & 14000C

Phases present LiquidAlpha

Composition 55% Ag 7%Ag

Amount of Phases 42.4 –7 55-42.4

55 – 7 55 - 7

= 74% = 26%

At 42.4% Ag and 11860C – ΔT

Phase Present Beta only

Composition 42.4% Ag

Amount of Phase 100%

At 42.4% Ag and 11860C + ΔT

Phases present LiquidAlpha

Composition 66.3% Ag 10.5%Ag

Amount of Phases 42.4 –10.5 66.3-42.4

66.3 – 10.5 66.3–10.5

= 57% =43%

Peritectic Alloy System


Surrounding or Encasement: During peritectic reaction, L+ α β , the beta phase created surrounds primary alpha.

Beta creates diffusion barrier resulting in coring.

Rapid Solidification in Peritectic System

After F Rhines, “ Phase Diagrams in Metallurgy,”McGraw- Hill, 1956, p. 86.

Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. L1α + L2

Binary Monotectic Systems


  • Two liquids are immiscible.

  • Example:- Copper – Lead

  • system at 9550C and 36% Pb.






Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 296.

Terminal phases: Phases occur at the end of phase diagrams.

Intermediate phases: Phases occurin acomposition range inside phase diagram.

Examples: Cu-Zn diagram has both terminal and intermediate phases.

Five invariant peritectic points and one eutectic point.

Intermediate Phases and Compounds

“Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 301

In Al2O2 – SiO2 system, an intermediate phase called Mullite is formed, which includes the compound 3Al2O3.2SiO2.

Intermediate Phases in Ceramics

After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 1976

In some phase diagrams, intermediate compound are formed – Stoichiometric

Percent Ionic/Covalent bond depends on electronegativeness

Example:- Mg-Ni phase diagram contains

Mg2Ni : Congruently melting compound

MgNi2 : Incongruently melting compound.

Intermediate Compounds

Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.

Three components

Constructed by using a equilateral triangle as base.

Ternary Phase Diagrams

  • Pure components at each

  • end of triangle.

  • Binary alloy composition

  • represented on edges.

Temperature can be represented as uniform throughout the

Whole Diagram Isothermal section.

Example:- Iron-Chromium-Nickel phase diagrams.

Ternary Phase Diagram (Cont..)

  • Isothermal reaction at 6500C

  • for this system

  • Composition of any metal

  • at any point on the phase

  • diagram can be found by

  • drawing perpendicular

  • from pure metal corner to

  • apposite side and calculating

  • the % length of line at that

  • point

After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.


Phase Reactions

Eutectic Type: Phase1  Phase2 + Phase3

Eutectic Reaction: Liquid  Solid1 + Solid2

Eutectoid Reaction: Solid1  Solid2 + Solid3

Peritectic Reaction: Liquid + Solid   Solid 

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