Phase

1
Lecture 9

• Phase
• Diagrams

8-1
2
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.

8-2
3
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.

Figure 8.2
Figure 8.1
8-3
After W. G. Moffatt, et al., The Structure and
Properties of Materials, vol I Structure,
Wiley, 1965, p.151
4
Gibbs Phase Rule
P number of phases that coexist in a system C
Number of components F Degrees of freedom

• PF C2
• For pure water, at triple point, 3 phases
  coexist.
• There is one component (water) in the system.
• Therefore 3 F 1 2 F 0.
• Degrees of freedom indicate number of variables
  that can be changed without changing number of
  phases.

8-4
5
Cooling Curves

• Used to determine phase transition temperature.
• Temperature and time data of cooling molten
  metal is recorded and plotted.
• Thermal arrest heat lost heat supplied by
  solidifying metal
• Alloys solidify over a range of temperature (no
  thermal arrest)

Pure Metal
Iron
6
Binary Isomorphous Alloy Systems
Mixture of two systems
Two component system

• Binary alloy
• Isomorphous system Two elements completely
  soluble in each other in liquid and solid state.
• Example Cu-Ni solution.

Composition at liquid and solid phases at any
temperature can be determined by drawing a tie
line.
Figure 8.3
8-5
Adapted from Metals Handbook, vol. 8, 8th ed.,
American society of Metals, 1973, p. 294.
7
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.
• The greater the number of cooling curves the more
  accurate the phase diagram.

Figure 8.4
8-6
8
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 Xl ws w0
ws w1
W0 is the weight percentage of the alloy. Ws is
the weight percentage within the solid phase Wl
is the weight percentage in the liquid phase
Figure 8.5
8-7
9
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.

W030 Cu
Figure 8.7
Figure 8.8
8-8
10
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
Eutectic temperature
a solid solution ß solid solution
Liquid
Cooling
8-9
11
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.

Figure 8.12
Figure 8.13

• Further cooling lowers alpha Sn content and beta
  Pb.
• (They try to move to equilibrium)

8-10
From J. Nutting and R. G. Baker, Microstructure
of Metals, Institute of Metals, London,
1965,p.19.
12
Various Eutectic Structures

• Structure depends on factors like minimization
  of free energy at a / ß 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.
13
Binary Peritectic Alloy System

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

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.

Figure 8.16
cooling
Liquid(5.4 wt Ni) d (4.0 wt Ni)
? 4.3 wt Ni
8-12
14
Peritectic Alloy System

• At 42.4 Ag 14000C
• Phases present Liquid Alpha
• Composition 55 Ag 7Ag
• Amount of Phases 42.4 7 55-42.4
• 55 7
  55 - 7
• 74
  26
• At 42.4 Ag and 11860C ?T
• Phase Present Beta only
• Composition 42.4 Ag
• Amount of Phase 100
• At 42.4 Ag and 11860C ?T
• Phases present Liquid Alpha
• Composition 66.3 Ag 10.5Ag
• Amount of Phases 42.4 10.5 66.3-42.4
• 66.3 10.5
  66.310.5
• 57
  43

Figure 8.17
Figure 8.18
8-13
15
Rapid Solidification in Peritectic System

• Surrounding or Encasement During peritectic
  reaction, L a ß , the beta phase created
  surrounds primary alpha.
• Beta creates diffusion barrier resulting in
  coring.

Figure 8.19
Figure 8.20
8-14
After F Rhines, Phase Diagrams in
Metallurgy,McGraw- Hill, 1956, p. 86.
16
Binary Monotectic Systems

• Monotectic Reaction Liquid phase transforms
  into solid phase and another liquid.
• L1 a L2

Cooling

• Two liquids are immiscible.
• Example- Copper Lead
• system at 9550C and 36 Pb.

Table 8.1
Eutectic Eutectoid Peritectic Peritectoid Mono
tectic
Figure 8.23
8-15
Metals Handbook, vol. 8 Metallography
Structures and Phase Diagrams, 8th ed., American
Society of Metals, 1973, p. 296.
17
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
Metals Handbook, vol. 8 Metallography
Structures and Phase Diagrams, 8th ed., American
Society of Metals, 1973, p. 301
8-16
18
Intermediate Phases in Ceramics

• In Al2O2 SiO2 system, an intermediate phase
  called Mullite is formed, which includes the
  compound 3Al2O3.2SiO2.

Figure 8.26
8-17
After A. G. Guy, Essentials of Materials
Science, McGraw-Hill, 1976
19
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
8-18
Metals Handbook, vol. 8 American Society of
Metals, 1973, p. 314.
20
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.

Figure 8.28
Temperature can be represented as uniform
throughout the Whole Diagram
Isothermal section.
8-19
21
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
8-20
After Metals Handbook, vol. 8 American Society
of Metals, 1973, p. 425.