Transcript Slide 1

West Virginia University
9 – Phase Diagram (2)
(Phase Reactions)
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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.
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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.
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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
• .
W0=30% Cu
• 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.
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Eutectic Reaction
Liquid   (solid) +  (solid)
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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".
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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.
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Eutectic Structure
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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
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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.
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Eutectic Structure - Dendrites
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Segregation Behaviors
Segregation: Composition Gradient.
Fast Cooling is the key for Segregation
There is no enough time for the diffusion of elements
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Binary Eutectic Alloy System
•
In some binary alloy systems, components have limited solid solubility.
Example : Pb-Sn alloy.
• Eutectic composition freezes
at lower temperature than all
other compositions.
• This lowest temperature is
called eutectic temperature.
Eutectic temperature
Liquid
8-9
Cooling
α solid solution + β solid solution
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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)
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From J. Nutting and R. G. Baker, “Microstructure of Metals,” Institute of Metals, London, 1965,p.19.
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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.
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After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.
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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
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Fe-C Phase Diagram
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Eutectoid Structures
 Similar as Eutectic Structure, finer in scale
 Origin solid decomposes into two new solid phase
 Need diffusion of solute atoms
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Eutectoid Structures
Pearlite in a plain-carbon steel,
0.05mm
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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 
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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)
cooling
γ 4.3 wt % Ni
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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
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Rapid Solidification in Peritectic System
•
Surrounding or Encasement: During peritectic reaction, L+ α
phase created surrounds primary alpha.
• Beta creates diffusion barrier resulting in coring.
β , the beta
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After F Rhines, “ Phase Diagrams in Metallurgy,”McGraw- Hill, 1956, p. 86.
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Binary Monotectic Systems
• Monotectic Reaction: Liquid phase transforms into solid phase and another
liquid. L1 Cooling α + L2
• Two liquids are immiscible.
• Example:- Copper – Lead
system at 9550C and 36% Pb.
Eutectic
Eutectoid
Peritectic
Peritectoid
Monotectic
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Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8 th ed., American Society of Metals, 1973, p. 296.
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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.
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“Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 301
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Intermediate Phases in Ceramics
• In Al2O2 – SiO2
system, an
intermediate phase
called Mullite is
formed, which
includes the
compound
3Al2O3.2SiO2.
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After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 1976
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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.
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Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.
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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.
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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
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After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.
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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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