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SE104 Structural Materials

Phase Diagram

Dr. Yu QiaoDepartment of Structural Engineering, UCSD


Introduction

• Phase: A region in a material that differs in structure and function from other regions.

• Phase diagrams: Represents phases present in metal at different conditions

(Temperature, pressure and composition).

Indicates equilibrium solid solubility of one element in another.

Indicates temperature range under which solidification occurs.

Indicates temperature at which different phases start to melt.

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Temperature, pressure, composition… Crystalline Structure

8-2(http://getasword.com)

Samurai sword

Jade steel: 1-1.5% carbon – hard yet brittle

Jacket steel

Core steel: < 0.2% carbon – tough yet soft


Phase Diagram of Pure Substances

• Pure substance exist as solid, liquid and vapor.

• Phases are separated by phase boundaries.

• Example : Water, Pure Iron.

• Different phases coexist at triple point.

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

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Cooling Curves

Iron at 1 atm pressure

Thermal arrest : heat lost = heat supplied by solidifying metal


Binary Isomorphous Alloy Systems

• Binary alloy

• Isomorphous system: Two elements completely soluble in each other in liquid and solid state.

• Example: Cu-Ni solution.

Mixture of two systems

Two componentsystem

Composition at liquid and solid phases at any temperature can be determined by drawinga tie line.

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

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Binary Isomorphous Alloy Systems

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

The four Hum-Rothery rules for the solid solubility of one element in another are:

1. The crystal structure of each element of the solid solution must be the same.

2. The size of the atoms of each of the two elements must not differ by more than fifteen percent.

3. The elements should not form compounds with each other; there should be no appreciable difference in the electronegativities of the two elements.

4. The elements should have the same electron valence.


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.

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20% Ni

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Binary Eutectic Alloy System

• In some binary alloy systems, components have limited solid solubility.

• Eutectic composition freezesat lower temperature than allother compositions.

• This lowest temperature iscalled eutectic temperature.

Liquid α solid solution + β solid solutionEutectic temperature

Cooling

Example : Lead-Tin (Pb-Sn) alloy.

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• In some binary alloy systems, components have limited solid solubility.

Liquid α solid solution + β solid solutionEutectic temperature

Cooling

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40% Tin

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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 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.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 energy at α / β interface.

• Manner in which two phases nucleate and 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

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The Lever Rule

• The Lever rule gives the weight % of phases in any two phase regions.

Wt fraction of solid phase= Xs = w0 – w1

ws – w1

Wt fraction of liquid phase= X1 = ws – w0

ws – w1

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Content of B in liquid phase at T

Content of B insolid (at T)


Problem 8.15 Problem 8.17


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More Complicated Phase Diagrams

(http://www.mrl.ucsb.edu/~edkramer)

Bronze: Copper + Tin7000 yrs ago: Arsenical bronze

(Mesopotamia; today’s Iran)6500 yrs ago: Bronze (today’s Serbia;

The “Old Europe” civilization)

Lower melting point than CuHigher hardness/strength than Cu

Satisfactory stiffness & toughnessHigh resistance to oxidization

Halberd

[Cu]=5/6

Pike

[Cu] = 4/5Kao-Gong-Ji (~500 B.C.E):[Cu] = 6/7: Bell

[Cu] = 3/4: Falchion[Cu] = 1/2: Mirror[Cu] = 5/7:

Arrowhead


Steel

After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.8-11 http://www.fgg.uni-lj.si/

Annealed: Ferrite (white) + pearlite (dark)

Quenched: Austenite Martensite

L+

Carbon content:Low carbon steel: 0.05-0.3%Medium carbon steel: 0.3-0.6%High carbon steel: 0.6-1%Ultrahigh carbon steel: 1-2%Cast iron: > 2%

Steel

Cast Iron

Two phased, lamellar structure of alternating layers of phase (88 wt%) and cementite (12 wt%).

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Steel

After W. C. Winegard, “An Introduction to the Solidification of Metals,” Institute of Metals, London, 1964.8-11http://www.m4040.com/

+Fe3C

+Fe3C

Iron “bloom”

Perfused steel


Steel

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

Early Steels: Iron Bloom

T is only ~ 1200 oC

Solid state reduction:

Highly pure iron is directly reduced from iron ore, without melting

Carbon is added through diffusion later on, during iron working

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Binary Peritectic Alloy System

• Peritectic reaction: Liquid phase reacts with a solid phase to form a new and different solid phase.

Liquid + α βcooling

• Peritectic reaction occurswhen a slowly cooled alloyof 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

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Peritectic Alloy System• At 42.4 % Ag & 14000CPhases present Liquid AlphaComposition 55% Ag 7%Ag

Amount of Phases 42.4 –7 55-42.455 – 7 55 - 7= 74% = 26%

• At 42.4% Ag and 11860C – ΔTPhase Present Beta onlyComposition 42.4% Ag

Amount of Phase 100%

• At 42.4% Ag and 11860C + ΔTPhases present Liquid AlphaComposition 66.3% Ag 10.5%Ag

Amount of Phases 42.4 –10.5 66.3-42.466.3 – 10.5 66.3–10.5 = 57% =43%

Figure 8.17

Figure 8.18

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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

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Intermediate Phases and Compounds

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

• Intermediate phases:Phases occur in acomposition 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“Metals Handbook,” vol. 8: “Metallography Structures and Phase Diagrams,” 8th ed., American Society of Metals, 1973, p. 3018-16


Intermediate Phases in Ceramics

• In Al2O2 – 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

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Intermediate Compounds

• 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.

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.

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

Figure 8.28

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Ternary Phase Diagram (Cont..)

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

•Isothermal reaction at 6500C

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 Figure 8.30

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


Binary peritectic iridium-osmium (Prob. 8-21)

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

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