Diagramas de fase - WordPress.com...• Diagramas de fase : Representan las fases presentes en el metal a diferentes condiciones (Temperatura, presión y composición). Indica la solubilidad

CHAPTER

8

PhaseDiagrams

8-1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display

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.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 faseEjemplo : Agua, Hierro puro.

• Diferentes fases coexisten en el punto triple.Figure 8.1 Figure 8.2

After W. G. Moffatt, et al., “The Structure and Properties of Materials,” vol I: “Structure,” Wiley, 1965, p.1518-3


REGLA DE LAS FASES DE GIBBS

• P+F = C+2

• 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.

P = número de fase que coexisten en un sistemaC = Numero de componentesF = Grados de libertad

• 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

• Meseta térmica :pérdida de calor = calor necesario para solidifacar el metal.

• Las aleaciones solidifican en un rango de temperatura (sin meseta térmica)

Pure Metal

Iron


Sistema de aleación Binario Isomorfo

Aleación binaria

• Sistema Isomorfo :Los dos elementos son completamente solubles en estado líquido y en estado sólido. Ejemplo: Cu-Ni solución.

Sistema de dos componentes

La Composición de las fasespuede ser determinada A cualquier temperatura en forma gráfica.

Figure 8.3

Adapted from “Metals Handbook,” vol. 8, 8th ed., American society of Metals, 1973, p. 294.8-5


Phase Diagram from Cooling Curves

• Series of cooling curvesat different metal composition are first constructed.

• Points of change of slopeof cooling curves (thermal arrests) are noted and phase diagram is constructed.

• More the number of cooling curves, more accurate is the phase diagram.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= Xs = w0 – w1

w – wws – w1

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 coolingdelays solidification.

• Homogenization:Cast ingots heatedto elevated temperature to eliminatecored structure.

• Temperature of homogenization• Temperature of homogenizationmust be lower than lowest meltingpoint 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.

• Eutectic composition freezesat lower temperature than allother compositions.

Example : Pb-Sn alloy.

• This lowest temperature iscalled eutectic temperature.

Liquid α solid solution + β solid solutionEutectic temperature

Cooling

Figure 8.11

8-9


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

Figure 8.12composition of alpha follows solidus and composition of sn varies from 40% to 61.9%.

• At eutectic temperature, all the remaining liquid solidifies.

• Further cooling lowersalpha Sn content and beta Pb.

Figure 8.13

From J. Nutting and R. G. Baker, “Microstructure of Metals,” Institute of Metals, London, 1965,p.19.8-10


Various Eutectic Structures

• Structure depends on factors like minimization of free energyat α / β interface.

• Manner in which two phases nucleateand grow also affects structures.

Figure 8.14

After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.8-11


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 occurswhen a slowly cooled alloyof Fe-4.3 wt% Ni passesof Fe-4.3 wt% Ni passesthrough Peritectic temperature of 15170C.

• Peritectic point is invariant .

Liquid(5.4 wt% Ni) + δ (4.0 wt% Ni) γ 4.3 wt % Nicooling

Figure 8.16

8-12


Peritectic Alloy System• At 42.4 % Ag & 14000CPhases present Liquid AlphaComposition 55% Ag 7%AgAmount of Phases 42.4 –7 55-42.4

55 – 7 55 - 7= 74% = 26%

• At 42.4% Ag and 11860C –∆TPhase Present BetaonlyPhase Present BetaonlyComposition 42.4% AgAmount of Phase 100%

• At 42.4% Ag and 11860C + ∆TPhases present Liquid AlphaComposition 66.3% Ag 10.5%AgAmount of Phases 42.4 –10.5 66.3-42.4

66.3 – 10.5 66.3–10.5 = 57% =43%

Figure 8.17

Figure 8.18

8-13


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.

Figure 8.20Figure 8.19

After F Rhines, “ Phase Diagrams in Metallurgy,”McGraw- Hill, 1956, p. 86.8-14


Binary Monotectic Systems

• Monotectic Reaction:Liquid phase transforms into solid phase and another liquid.

L1 α + L2Cooling

• Two liquids are immiscible.• Example:- Copper – Lead

system at 9550C and 36% Pb.

Eutectic

Eutectoid

Peritectic

Peritectoid

Monotectic

Figure 8.23

Table 8.1

Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 296.8-15


Intermediate Phases and Compounds

• Terminal phases:Phases occur at the end of phase diagrams.

• Intermediate phases:Phases occur in acomposition range inside phase diagram.phase diagram.

• Examples:Cu-Zn diagram has both terminal and intermediate phases.

• Five invariant peritectic points and one eutectic point.

Figure 8.25“Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 3018-16


Intermediate Phases in Ceramics

• In Al 2O2 – SiO2 system, an intermediate phase called Mullite is formed, which includes the compound 3Al2O3.2SiO2.

Figure 8.26

After A. G. Guy, “Essentials of Materials Science, “McGraw-Hill, 19768-17


Intermediate Compounds

• In some phase diagrams, intermediate compoundare formed – Stoichiometric

• Percent Ionic/Covalent bond depends on electronegativeness• Example:- Mg-Ni phase diagram contains

� Mg2Ni : Congruently melting compound� MgNi2 : Incongruently melting compound.� MgNi2 : Incongruently melting compound.

Figure 8.27

Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 314.8-18


Ternary Phase Diagrams

• Three components• Constructed by using a equilateral triangle as base.

• Pure components at eachend of triangle.

• Binary alloy compositionrepresented on edges.represented on edges.

Temperature can be represented as uniform throughout the Whole Diagram Isothermal section.

Figure 8.28

8-19


Ternary Phase Diagram (Cont..)

• Example:- Iron-Chromium -Nickel phase diagrams.

•Isothermal reaction at 6500Cfor this system• Composition of any metalat any point on the phaseat any point on the phasediagram can be found by drawing perpendicularfrom pure metal corner toapposite side and calculatingthe % length of line at thatpoint

Figure 8.30

After “Metals Handbook,” vol. 8: American Society of Metals, 1973, p. 425.8-20

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Diagramas de fase - WordPress.com...• Diagramas de fase : Representan las fases presentes en el metal a diferentes condiciones (Temperatura, presión y composición). Indica la solubilidad