Phase Diagrams Short

8/10/2019 Phase Diagrams Short

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Materials Science

Lecture 7 : Phase Diagrams

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Reading: Chapter 9

9.1-9.14, 9.18-9.20


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Introduction

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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 inanother.

Indicates temperature rangeunder which solidification

occurs.

Indicates temperature at which different phases start to

melt.


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Basic Concepts

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Component: Pure metals of which an alloy iscomposed.

System: Relate to the series of possible

alloys consisting of the same components,but may have different compositions.

e.g. Fe-C system: Fe-1%C, Fe-5% CComponents: Pure Fe and pure C


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Basic Concepts

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Solute: impurity atoms Solvent: matrix

Solubility limit

maximum concentration of solute atomsthat may dissolve in solvent to still form asolid solution

(without the formation of a new or secondphase)


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Basic Concepts

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Phasea homogeneous portion of asystem that has uniform physical (e.g.

crystal structure) and chemical

characteristics (e.g. composition). Eg., In sugar-water system, two phases:

1. Syrup (water and sugar) liquid phase

2. Sugar solid phase


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Microstructure of each phase is typically different

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Aluminum-Copper Alloy

0.38 Wt C steel


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Phase DiagramWater-Sugar System

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What is the

solubility limit at

20oC?

How many phases does the system have when

(1) Co < 65wt% sugar

(2) Co > 65wt% sugar


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Phase diagrams of pure substances

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Pure substance exist as solid, liquid and vapor.

Phases are separated by phase boundaries.

Example : Water, Pure Iron.

Different phases coexist at triple point.


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Cooling Curves

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Used to determine phase transitiontemperature.

Temperature and time data of cooling molten metalis recorded and plotted.

Thermal arrest :heat lost = heat supplied bysolidifying metal

Alloys solidify over a range of temperature(nothermal arrest)

Pure Metal

Iron


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What can we find from phase diagrams?

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For any alloy at any temperature

1. What phases are present Type of phases (liquid, solid (which solid))

2. What are their compositions? Composition of each phase is how much of an alloying element

that phase have.

Composition could be in weight% or in Atomic%

3. What is the amount of each phase? Weight fraction or volume fraction of the phase

Or volume fraction

4. What will happen if the temperature or composition ofthe system is changed?


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Phase Diagrams

Binary alloy

We will consider two types of binary phase diagrams: Isomorphous phase diagrams

Eutectic phase diagrams

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Two component system

Isomorphous Eutectic


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Binary Isomorphous Alloy System

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Isomorphous system:Two

elements completely soluble

in each other in liquid and

solid state.

Example: Cu-Ni solution.


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Phase Diagrams from Cooling Curves

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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.


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Weight Fraction of phases (Lever Rule)

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The Lever rulegives the weight % (fraction) of phases in

any two phase regions.

Wt fraction of solid phase

= Xs= w0w1

ws

w1

Wt fraction of liquid phase

= X1 = wsw0

wsw1


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# 1: Number and type of phases

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A (1100

o

C, 60 Ni)

1 phase -

B (1250

o

C, 35 Ni)

2 phases: L +


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# 2. Composition of phases

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For CO= 35wt% Ni

at TA:

CL=CO =35wt%Ni

(only L phase)

at TD:

C

=CO=35wt%Ni

(only phase)

at TB: both L phase and phase (Tie Line)

CL= 32 wt% Ni, C

= 43 wt% Ni


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# 3. Amount of phases (wt%)

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For CO= 35wt% Ni

at TA: only L phase

WL

= 100 wt%

W = 0 wt%

at TD: only phase

W = 100 wt%WL= 0 wt%


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#3. cont. Amount of phases (wt%)

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At TB: both L phase and phase

WL

S

R S

W R

R S

43 35

43 32

73wt %

= 27wt%


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Equilibrium Cooling

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Binary Eutectic Systems

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Analysis of eutectic Cu-Ag phase diagram

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Invariant point E, coordinate = (TE, CE)

TEno liquid phase below TE (minimum Tm)

CEconcentration at minimum Tm

line BEG also called eutectic isotherm

Eutectic reaction

L (CE) (C E) + (C E)cooling

heating


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Eutectic Reaction

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Eutectic reaction in Cu-Ag system

L (71.9%Ag) (8%Ag) + (91.2%Ag)

Important observations:(1) Within a phase field, only one or at most two phases may

be in equilibrium.

(2) Three phase in equilibrium only along eutectic isotherm

(3) Single-phase regions are always separated by a two-phase

region.

cooling

heating


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Another Example of Binary Eutectic system: Pb-Sn

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A i i ( b S h di )


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Team Activity (Pb-Sn phase diagram)

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Identify the invariant point using coordinate

Identify the eutectic isotherm

What is the eutectic reaction in Pb-Sn system

For Pb-40wt%Sn, at 150o

C,(1) what phases are present?

(2) what are their compositions?

(3) what is the mass fraction of each phase?

For Pb 20wt%Sn, at 175oC, answer the above threequestions.

S E l i E l Pb S


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Structure Evolution: Example Pb-Sn system

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S E l i 1


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Structure Evolution 1

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Co< 2wt%Sn

Result:--polycrystal of grains.

S E l i 2


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2wt%Sn < Co< 18.3wt%Sn

Result:

--polycrystal with finecrystals.

St t E l ti 3


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18.3wt%Sn < Co< 61.9wt%Sn

Result: crystals and a eutectic microstructure

Mi t t E l ti 3


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Microstructure Evolution 3

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Just above TE :

WL= (1-W)=50wt%

C= 18.3wt%SnCL= 61.9wt%Sn

S

R+ SW= =50wt%

Just below TE :

C= 18.3wt%SnC= 97.8wt%Sn

S

R+ SW= =73wt%

W= 27wt%

St t E l ti 4 E t ti t t


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Structure Evolution 4: Eutectic structure

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Co= CE

Result: Eutectic microstructure

--alternating layers of and crystals.

E t t id R ti


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Eutectoid Reaction

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E t t id R ti


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Eutectoid Reaction

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+

Eutectoid (eutectic like reactions)

Eutectoid: solid solid1 + solid 2

Eutectic: liquid solid1 + solid 2

cooling

heating

F C t


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Fe-C system

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Iron Carbide (Fe Fe C) Phase Diagram


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Iron-Carbide (Fe-Fe3C) Phase Diagram

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Equlibrium Phases in Iron-Carbide phase diagram

Ferrite:Very low solubility

of carbon. Max 0.02 % at 7230C

and 0.005% at 00C.

Austenite:Interstitial solid

solution of carbon in

iron. Solubility of C is

2.08% at 11480C and 0.8%

at 7230C.

Cementite (Fe3C):Intermetallic compound.

6.67% C and 93.3% Fe.

Pearlite:Consists of alternating layers

of Ferrite and Cementite.

Fe C System


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Fe-C System

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For pure Fe (100 wt% Fe)Polymorphic transformations:

ferrite - austenite ferrite

Fe C system (up to 6.7 wt% C) or

Fe-Fe3C system (up to 100wt% Fe3C)

912oC 1394oC

Solubility of C in different phases


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Solubility of C in different phases

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Maximum solubility of C in - ferrite (bcc):

0.022wt%C

Maximum solubility of C in austenite (fcc):

2.14wt%C The solubility of C in austenite is much higher.

This is because that interstitial position in FCC

crystal is larger than that in BCC crystals

Octahedral Interstices


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Octahedral Interstices

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Invariant Reactions in Fe Fe C diagram


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Invariant Reactions in Fe-Fe3C diagram

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Peritectic reaction:

Eutectic reaction:

Eutectoid reaction:

1495 CLiquid (0.53% ) (0.09% ) (0.17% )C C C

1148

3Liquid (4.3% ) austenite (2.14% ) (6.67% )C

C C Fe C C

723

3Austenite (0.76% ) Ferrite (0.022% ) (6.67% )C

C C Fe C C

Development of Microstructure in Iron Carbon alloy


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Development of Microstructure in Iron-Carbon alloy

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0.8% C

Eutectoid Steel

Hypoeutectoid

Steel

Hypereutectoid

Steel

Less than 0.8% More than 0.8%

Slow Cooling of Plain Carbon Steel


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Slow Cooling of Plain Carbon Steel

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Eutectoid plain carbon steel:If a sample is heated up to7500C and held for sufficient time, structure will become

homogeneous austenite.

Below eutectoid temperature,

layersof ferrite and cementite

are formed. Pearlite.

Eutectoid steel

Slow Cooling of Plain Carbon Steel (cont )


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Slow Cooling of Plain Carbon Steel (cont..)

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Hypoeutectoid plain carbon steel:If a sample of 0.4% C is

heated up to 9000C, it getsaustenitized.

Further cooling gives rise to and pearlite.

Slow Cooling of Plain Carbon Steel (cont )


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Slow Cooling of Plain Carbon Steel (cont..)

Hypereutectoid plain carbon steel:If for example a 1.2% C sample is heated up

to 9000C and held for sufficient time, it entirely gets austenitized.

Further cooling results in eutectoid cementiteand pearlite.

Phase Diagrams Short

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