Mechanical & Aerospace Engineering West Virginia University 9 – Phase Diagram (2) (Phase...

Mechanical & Aerospace Engineering

West Virginia University

9 – Phase Diagram (2)(Phase Reactions)



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.

Phase Reaction



Solidification• 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.



Non-Equilibrium Solidification of Alloys• 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

• . • 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 SegregationThere is no enough time for the diffusion of elements



Binary Eutectic Alloy System

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

• Eutectic composition freezes at lower temperature than all other compositions. • This lowest temperature is called eutectic temperature.

Liquid α solid solution + β solid solutionEutectic temperature

Cooling

Example : Pb-Sn alloy.

8-9



Slow Cooling of 60% Pb – 40% Sn alloy

• 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.

• 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.



Various Eutectic Structures

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

• Manner in which two phases nucleate and grow also affects 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



Eutectoid Structures

Pearlite in a plain-carbon steel, 0.05mm



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



Binary Peritectic Alloy System

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

Liquid + α β

cooling • 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 % Nicooling



Peritectic Alloy System• At 42.4 % Ag & 14000C

Phases present Liquid Alpha

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 Liquid Alpha

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%

8-13



Rapid Solidification in Peritectic System

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

• Beta creates diffusion barrier resulting in coring.

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



Binary Monotectic Systems

• Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. L1 α + L2Cooling

• Two liquids are immiscible.• Example:- Copper – Lead system at 9550C and 36% Pb.

Eutectic

Eutectoid

Peritectic

Peritectoid

Monotectic

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



Intermediate Phases and Compounds

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

• Intermediate phases: Phases occur in a composition range inside phase diagram.

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

• Five invariant peritectic points and one eutectic point.

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



Intermediate Phases in Ceramics

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

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



Intermediate Compounds

• 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.

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



Ternary Phase Diagrams

• Three components

• Constructed by using a equilateral triangle as base.

• 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.



Ternary Phase Diagram (Cont..)

• Example:- Iron-Chromium-Nickel phase diagrams.•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.



SUMMARY

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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