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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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.
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.
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.
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.
Segregation: Composition Gradient.
Fast Cooling is the key for Segregation
There is no enough time for the diffusion of elements
Example : Pb-Sn alloy.
α solid solution + β solid solution
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
From J. Nutting and R. G. Baker, “Microstructure of Metals,” Institute of Metals, London, 1965,p.19.
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: 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
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
Liquid + α βBinary Peritectic Alloy System
Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni) γ 4.3 wt % Ni
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
Beta creates diffusion barrier resulting in coring.Rapid Solidification in Peritectic System
After F Rhines, “ Phase Diagrams in Metallurgy,”McGraw- Hill, 1956, p. 86.
Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 296.
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
After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 1976
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.
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