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 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
Liquid (solid) + (solid)
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 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.
As an alloy cools the first phase to solidify (unless the alloy's overall composition is eutectic) is called the Primary Phase.
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: 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.
Example : Pb-Sn alloy.
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.
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.
After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.
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
Pearlite in a plain-carbon steel, 0.05mm
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
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%
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.
Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. L1α + L2
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.
“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.
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.
Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.
Constructed by using a equilateral triangle as base.
Temperature can be represented as uniform throughout the
Whole Diagram Isothermal section.
Example:- Iron-Chromium-Nickel phase diagrams.
After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.
Eutectic Type: Phase1 Phase2 + Phase3
Eutectic Reaction: Liquid Solid1 + Solid2
Eutectoid Reaction: Solid1 Solid2 + Solid3
Peritectic Reaction: Liquid + Solid Solid