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Objectives

1. Define “phase” 

2. Fully characterize the state of a

material at a point on a phase

diagram.

3. Identify the invariant points on a

given phase diagram.

4. Utilize the Fe-C diagram todescribe the microstructure of a

given alloy.

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Solubility Limit

Maximum fraction of a substance which willdissolve in another under a given condition.

Example: Sugar in Water

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Phase:

Homogeneous volume with uniformphysical and chemical characteristics

Phase Diagram A map of the solubility limits for thedifferent phases in an alloy as a

function of composition andtemperature.

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Which of the following constitute phases and what is

the basis for defining or not defining them as a phase?

1. Magnesium dissolved in seawater

2. Tramp impurities collected at the grain

boundaries in cast iron

3. Copper poor grains in Brass (Cu-Zn alloy)

4. Amorphous regions in largely crystalline

polyethylene polymer

5. Carbon atoms in iron crystals

6. Water vapor in air

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Phase:

Homogeneous volume with uniformphysical and chemical characteristics

Phase Diagram A map of the solubility limits for thedifferent phases in an alloy as a

function of composition andtemperature.

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Terms:Greek = solid phases

Liquid phases = L

 Two phase regions notalways labeled  

Combination of neighboring 

(horizontal) single phase

regions

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- alpha

-beta

-gamma

-delta-zeta

-eta

-theta

-epsilon

Greek Alphabet

(lower case)-iota

-kappa

-lambda

-mu-nu

-omicron

-pi

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Equilibrium

 The transformation temperatures change with

different heating/cooling rates

 To avoid an infinity of phase diagrams, all

diagrams are Equilibrium diagrams.

Equilibrium implies extremely slow processes.

 Thus these diagrams are the behavior as thecooling/heating rate approaches zero.

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Isomorphous

One phase due tocomplete solid

solution solubility 

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 What happens if you have poor Solid

Solution Solubility?

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Composition of Component Phases

 The Tie Line

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Proportion of Phases Present: The

Reverse Lever Law 

43-35 8

43-32 11PL = =

Pa = =35-32 3

43-32 11

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 The Composition of the Al3Ni2 is most

nearly:

 A. 43 % Ni B. 50% Ni C. 57 % Ni

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 The Proportion of Al3Ni2 is most nearly:

 A. 5/7 B. 8/14 C. 6/14

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RXN NAME

Peritectic

Eutectoid

Peritectoid

Eutectic

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Pearlite:

Ferrite (white)

Cementite (dark)

Eutectoid Equilibrium Structure

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 What’s Up with Al3Ni? 

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Intermediate Compound

 A compound which forms at a single ratio of 

constituents.

Phase region for IC is a vertical line

IC will be bordered by two phase regions

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A State is Characterized by 4 Things:

1. Composition and Temperature

2. Number and Names of phases

3. Composition of phases

4. Proportion of phases