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

Mechanical & Aerospace Engineering West Virginia University 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

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University 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. W 0 =30% Cu

Mechanical & Aerospace Engineering West Virginia University Eutectic Reaction Liquid   (solid) +  (solid)

Mechanical & Aerospace Engineering West Virginia University 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".

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University Eutectic Structure

Mechanical & Aerospace Engineering West Virginia University 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

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University Eutectic Structure - Dendrites

Mechanical & Aerospace Engineering West Virginia University Segregation Behaviors Segregation: Composition Gradient. Fast Cooling is the key for Segregation There is no enough time for the diffusion of elements

Mechanical & Aerospace Engineering West Virginia University 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 solution Eutectic temperature Cooling Example : Pb-Sn alloy. 8-9

Mechanical & Aerospace Engineering West Virginia University Slow Cooling of 60% Pb – 40% Sn alloy Liquid at C. At about C first solid forms – proeutectic solid. Slightly above C 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.

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University 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

Mechanical & Aerospace Engineering West Virginia University Fe-C Phase Diagram

Mechanical & Aerospace Engineering West Virginia University Eutectoid Structures  Similar as Eutectic Structure, finer in scale  Origin solid decomposes into two new solid phase  Need diffusion of solute atoms

Mechanical & Aerospace Engineering West Virginia University Eutectoid Structures Pearlite in a plain-carbon steel, 0.05mm

Mechanical & Aerospace Engineering West Virginia University 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 

Mechanical & Aerospace Engineering West Virginia University 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 % Ni cooling

Mechanical & Aerospace Engineering West Virginia University Peritectic Alloy System At 42.4 % Ag & C Phases present Liquid Alpha Composition 55% Ag 7%Ag Amount of Phases 42.4 – – = 74% = 26% At 42.4% Ag and C – ΔT Phase Present Beta only Composition 42.4% Ag Amount of Phase 100% At 42.4% Ag and C + ΔT Phases present Liquid Alpha Composition 66.3% Ag 10.5%Ag Amount of Phases 42.4 – – –10.5 = 57% =43% 8-13

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University Binary Monotectic Systems Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. L 1 α + L 2 Cooling Two liquids are immiscible. Example:- Copper – Lead system at C 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.

Mechanical & Aerospace Engineering West Virginia University 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

Mechanical & Aerospace Engineering West Virginia University Intermediate Phases in Ceramics In Al 2 O 2 – SiO 2 system, an intermediate phase called Mullite is formed, which includes the compound 3Al 2 O 3.2SiO 2. After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 1976

Mechanical & Aerospace Engineering West Virginia University Intermediate Compounds In some phase diagrams, intermediate compound are formed – Stoichiometric Percent Ionic/Covalent bond depends on electronegativeness Example:- Mg-Ni phase diagram contains  Mg 2 Ni : Congruently melting compound  MgNi 2 : Incongruently melting compound. Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.

Mechanical & Aerospace Engineering West Virginia University 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.

Mechanical & Aerospace Engineering West Virginia University Ternary Phase Diagram (Cont..) Example:- Iron-Chromium-Nickel phase diagrams. Isothermal reaction at C 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.

Mechanical & Aerospace Engineering West Virginia University SUMMARY Phase Reactions Eutectic Type: Phase1  Phase2 + Phase3 Eutectic Reaction: Liquid  Solid1 + Solid2 Eutectoid Reaction: Solid1  Solid2 + Solid3 Peritectic Reaction: Liquid + Solid   Solid 