CHAPTER 8. Phase Diagrams 8-1

CHAPTER 8 Phase Diagrams 8-1

Introducción Fase: Una region en un material que difiere en estructura y función de otra región. Diagramas de fase : Representan las fases presentes en el metal a diferentes condiciones (Temperatura, presión y composición). Indica la solubilidad de un sólido en equilibrio en otro elemento. Indica el rango de temperatura en el cual ocurre la solidificación. Indica la temperatura a la cual las diferentes fases inician su fusión. 8-2

Diagram de fase de substancias puras Las sustancias puras existen en estado sólido, líquido y vapor. Las fases estan separadas por límites de fase Ejemplo : Agua, Hierro puro. Figure 8.1 Figure 8.2 Diferentes fases coexisten en el punto triple. 8-3 After W. G. Moffatt, et al., The Structure and Properties of Materials, vol I: Structure, Wiley, 1965, p.151

REGLA DE LAS FASES DE GIBBS P+F = C+2 P = número de fase que coexisten en un sistema C = Numero de componentes F = Grados de libertad Para el agua pura, en el punto triple, coexisten 3 fases. Hay un componente (agua) en el sistema. Así que 3 + F = 1 + 2 F = 0. Los grados de libertad indican el número de variable que pueden ser cambiadas sin que cambie el número de fases. 8-4

Curvas de enfriamiento en equilibrio Usedas para determinar la temperatura de transición de la fase. Pure Metal Iron Meseta térmica : pérdida de calor = calor necesario para solidifacar el metal. Las aleaciones solidifican en un rango de temperatura (sin meseta térmica)

Sistema de aleación Binario Isomorfo Aleación binaria Sistema de dos componentes Sistema Isomorfo : Los dos elementos son completamente solubles en estado líquido y en estado sólido. Ejemplo: Cu- Ni solución. La Composición de las fases puede ser determinada A cualquier temperatura en forma gráfica. Figure 8.3 8-5 Adapted from Metals Handbook, vol. 8, 8 th ed., American society of Metals, 1973, p. 294.

Phase Diagram from Cooling Curves 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. More the number of cooling curves, more accurate is the phase diagram. Figure 8.4 8-6

The Lever Rule The Lever rule gives the weight % of phases in any two phase regions. Wt fraction of solid phase = X s = w 0 w 1 w w w s w 1 Wt fraction of liquid phase = X1 = ws w0 ws w1 Figure 8.5 8-7

Non Equilibrium Solidification of Alloys Very slow cooling (equilibrium) gives rise to cored structure. Rapid cooling delays solidification. 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. Figure 8.7 Figure 8.8 8-8

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. Figure 8.11 Liquid Eutectic temperature Cooling α solid solution + β solid solution 8-9

Slow Cooling of 60% Pb 40% Sn alloy Figure 8.13 Figure 8.12 Liquid at 300 0 C. At about 245 0 C first solid forms proeutectic solid. Slightly above 183 0 C composition of alpha follows solidus and composition of sn varies from 40% to 61.9%. At eutectic temperature, all the remaining liquid solidifies. 8-10 Further cooling lowers alpha Sn content and beta Pb. 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. Figure 8.14 8-11 After W. C. Winegard, An Introduction to the Solidification of Metals, Institute of Metals, London, 1964.

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 Figure 8.16 Peritectic reaction occurs when a slowly cooled alloy of Fe-4.3 wt% Ni passes through Peritectic temperature of 15170C. Peritectic point is invariant. 8-12 cooling Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni) γ 4.3 wt % Ni

Figure 8.17 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display Peritectic Alloy System At 42.4 % Ag & 1400 0 C 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 1186 0 C T Phase Present Beta only Composition 42.4% Ag Amount of Phase 100% 8-13 Figure 8.18 At 42.4% Ag and 1186 0 C + T 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%

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. 8-14 Figure 8.20 After F Rhines, Phase Diagrams in Metallurgy, McGraw- Hill, 1956, p. 86. Figure 8.19

Binary Monotectic Systems Monotectic Reaction: Liquid phase transforms into solid phase and another liquid. Cooling L 1 α + L 2 Two liquids are immiscible. Example:- Copper Lead system at 9550C and 36% Pb. Table 8.1 Figure 8.23 8-15 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. Figure 8.25 8-16 Metals Handbook, vol. 8: Metallography Structures and Phase Diagrams, 8 th ed., American Society of Metals, 1973, p. 301

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. 8-17 Figure 8.26 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 Mg 2 Ni : Congruently melting compound MgNi 2 : Incongruently melting compound. Figure 8.27 8-18 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. 8-19 Figure 8.28 Temperature can be represented as uniform throughout the Whole Diagram Isothermal section.

Ternary Phase Diagram (Cont..) Example:- Iron-Chromium-Nickel phase diagrams. Figure 8.30 Isothermal reaction at 650 0 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 8-20 After Metals Handbook, vol. 8: American Society of Metals, 1973, p. 425.