Phase Diagram

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Phase Diagram
IE 2302 Engineering materials
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Grain Boundaries

• Grain boundaries are defects which have higher
  energy than the grains and are more active with
  chemicals.
• Help to stop the dislocation.

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Macroscopic Defects

• Holes, bubbles, surface imperfections, cracks,
  and macroscopic impurities

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phase

• A phase is a state of matter that is distinct in
  some way from the matter around it.
• Eg. A mixture of ice and water 2 phases
• The distinction between single- and multiple
  phase alloys is important to the strength,
  corrosion, biocompatibility, and other alloy
  properties.

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Phase Diagram

• Phase of a family of alloys of a general metal
  composition are defined by the Temperature-Composi
  tion (Phase) diagram for that family of alloys.

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Cooling Curves
Cooling Curves

• Pure Metals

• Alloys

• Pure Metals

• Alloys

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Cooling Curves and Phase Diagram
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Hume-Rothery Rules for Alloying

• The lattice parameters of the two metals must be
  similar.
• Same type of crystal lattice (FCC,..etc)
• The relative size of the atoms must not exceed
  15-20.
• (gt15 ? multiple phases)
• Large differences in valence state preclude
  solubility.
• The chemical affinity of the atoms should be
  similar.
• A high degree of chemical affinity ? form an
  intermetallic compound on solidification

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Classification of Alloy Systems
1. Solid Solutions 2. Intermetallic Compound 3.
Eutectic Alloy
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Solid solutions

• Two metals are completely miscible in the liquid
  state, and they remain completely mixed on
  solidification.
• L ? S
• A single -phase system
• Always have a range of possible compositions
• e.g. the solid phase in the copper-gold (Cu-Au)
  system has a wide range of compositions between
  100 Cu and 100 Au

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Phase Diagram of a Solid Solution
Phase Diagram of a Solid Solution

• All compositions above the liquidus line are
  liquid, and those below the solidus line are
  solid.
• Solid and liquid exist in the area between both
  line.
• The solid has only one phase.

• All compositions above the liquidus line are
  liquid, and those below the solidus line are
  solid.
• Solid and liquid exist in the area between both
  line.
• The solid has only one phase.

LIQUID
liquidus
Temperature
solidus
SOLID
Metal A (100)
Metal B (100)
composition
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Intermetallic Compounds

• The resulting phase has a fixed chemical
  composition or a narrow range of compositions.
• e.g. in an amalgam alloy,
• 73.2 Ag and 26.8 Sn ? Ag3Sn (one phase)
• Silver and tin atoms occupying definite positions
  in the space lattice.

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Phase Diagram of an Intermetallic Compound
Ag3Sn, 73.2 Ag and 26.8 Sn
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Eutectic Alloys

• The metals are soluble in the liquid state, but
  separate into two phases in the solid state.
• L ? S1 S2 ( 2 solid solutions)

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Phase Diagram of a Eutectic Alloy
Silver-copper system

• L ? a-solid solution b-solid solution
• The lowest temperature at which any alloy
  composition is entirely liquid Eutectic Temp
  (779.4C, E)
• The eutectic temperature is lower than the fusion
  temperature of either Ag and Cu.
• At eutectic point, there is no solidification
  range. (pure metal)
• At eutectic composition (72Ag 28 Cu), the two
  phases often precipitate as very fine layers of
  one phase over the other one.

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Silver-copper system
Silver-copper system
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Ex. Lead-Tin Alloy
Eutectic composition
High tin content
b eutectic
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How to read a simple phase diagram?
(1) Composition of Liquid and Solid Phases at
Various Temp. (2) Amount of Liquid and Solid
Phases at Various Temp.
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Composition of Liquid and Solid Phases at Various
Temp.
Alloy (80A 20B)
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Amount of Liquid and Solid Phases at Various Temp.

• The relative amounts of the two phases in the
  liquid-solid region can be determined at a given
  temperature by the inverse lever rule.
• At 560C for 60A and 40B composition
• Liquid XY/XZ
• Solid YZ/XZ

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Ex. Silver-Palladium System
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