1. ME 330 Engineering Materials
Lecture 12
Phase Diagrams, Solidification, Phase transformations
Solidification
Solidification microstructures
Iron-Carbon alloys
Phase transformations
Read Chapter 10

2. Kinetics of phase transformations
Reaction kinetics … what happens when time is introduced to solidification
TTT curves
CCT curve
Hardenability and Jominy end quench test
Surface hardening treatments
Please read chapter 10

3. Development of microstructure in single or
two phase alloys usually involves
phase transformations –
alteration in the number and/or character of phases.

4. Phase transformations
At least one new phase is formed that has different physical/chemical characteristics and/or different structure that the parent phase
They do not occur instantaneously
Transformation rate (dependence of reaction progress on time)
Phase transformations have two stages
Nucleation
(appearance of very small particles of new phase which are capable of growing)
Growth

5. Phase transformations
Simple diffusion-dependent transformation (no change in either the number or composition of phases present) Ex: solidification of pure metal, recrystallization and grain growth
Diffusion dependent transformation (some alteration in phase compositions and often in number of phases present) Ex: eutectoid reaction
Diffusionless trasformation (metastable phase is produced) Ex: martensitic transformation

6. Nucleation Homogeneous phase Heterogeneous phase
Nuclei of new phase form uniformly throughout the parent phase
Heterogeneous phase
Nuclei form preferentially at structural inhomogeneities (contain surfaces, grain boundaries, dislocations, etc)

7. Homogeneous Nucleation
Free Energy Change
Solid-liquid interface
Solid
Find critical radius for maximum free energy change
Activation free energy required
for the formation of stable nucleus

8. heat given up during solidification
is zero at Tm
heat given up during solidification
These two quantities decrease
as temperature decreases.
Lowering of temperature at temperatures below the
equilibrium solidification temperature, nucleation
occurs more easily.

9. So far we considered temperature dependence.
f10_03_pg316.jpg
So far we considered temperature dependence.
Next, we consider time dependence.

10. General Solid State Reaction Kinetics
Time dependence of rate
Phase transformations generally time & temperature dependent
Time split between nucleation & growth
Measure transformation % versus time for various temperatures
Transformation rate (Avrami equation)
Typical kinetics behavior
1.0
Fraction of transformation,y
0.5
t0.5
t0.5
Fraction of transformation
Growth
Nucleation
log of heating time, t

11. f10_11_pg323.jpg
Fig in Callister

12. Iron-Iron Carbide Phase Diagram
Recall:
Iron-Iron Carbide Phase Diagram
Ferrite, 
BCC structure
Carbon dissolved in iron (0.02% max.)
Soft, weak, ductile
Austenite, 
High temperature phase, above 727°C
FCC crystal structure
Carbon dissolved in iron (2.14% max.)
Ferrite, 
Cementite
Hard, strong, brittle
, Ferrite
(BCC)
, Austenite
(FCC)
, Ferrite
(BCC)
Cementite
(Fe3C)

13. Eutectoid Solidification
Recall:
Eutectoid Solidification
1 %
1000
800
600
Temperature °C g
a + g
g + Fe3C
a + Fe3C a
1076 Steel
6.70%
Fe3C
727
Composition (wt % C)
Start with pure austenite, 
Eutectoid Reaction
0.76 % Carbon at 727 °C
Develop pearlite microstructure
Similar to eutectic microstructure from last time
Alternating lamellae of  and Fe3C
Properties intermediate between constituents 

14. Eutectoid Composition
Recall:
Eutectoid Composition
1076 Steel
1000 g
Temperature °C
g + Fe3C
800
a + g
727 a
0.022
0.76
Fe3C
600
6.70%
a + Fe3C
1 %
Pearlite % C
Ferrite (white)
Cementite (black)
Composition (wt % C)

15. Time-Temperature-Transformation (TTT)
Eutectoid composition
0.76 % C
Transformation rate has a strong temperature dependence
Rapidly cool to given temperature
Hold to solidification
More convenient to plot time versus temperature
Plot initiation and completion lines
Eutectoid plotted as horizontal line
Valid only for
Given composition
Isothermal transformation
End
Begin
transformed
Percent  50
100
650 °C
675 °C
Austenite (stable)
Austenite
(unstable)
Pearlite Te
time (s)
Temperature °C 1 10
102
103
104
105
400
500
600
700
50 % completion
100 % completion
0 % completion

16. f10_14_pg327.jpg
Fig in Callister

17. Completing the Plots
Other than eutectoid compositions have proeutectoid phases
Cementite (%C > 0.76)
, Ferrite (%C < 0.76)
At low enough temperatures, this phase is suppressed
Bainite
Forms below “knee” of curve
Not really a new phase
Ferrite and cementite phases
No longer lamellar structure
Martensite
Quench fast enough to avoid other transformations
Forms at very low temperatures
Nonequilibrium and diffusionless
time (s) 1 10
102
103
104
105
10-1
Temperature °C
400
500
600
700
300
200
800 Te 
+C (Hypereutectoid)
(Hypoeutectoid)
+ 
 +P
Pearlite, P
 +B
Bainite, B 
M (start)
M (50%) 0%
50%
100%
M (90%)
Martensite, M

18. Pearlite, Bainite, Martensite
Formed by diffusion
Ferrite and cementite
Lamellar structure
Stronger than ferrite
Bainite
Not as much diffusion
Not lamellar structure
Harder that pearlite
Martensite
Diffusionless transformation
Speed of sound
BCT Structure (body-centered tetragon) with carbon interstitials
Strong and brittle
Austenite
FCC Structure
Above 725 C
Transforms to other phases
Ferrite
Iron + C in solid solution
Max. C is 0.022%
Ductile
Cementite
Compound, Fe3C
Hard and Brittle
Contains 6.7% C

19. Bainite & Martensite Upper bainite Martensite Lower bainite
From: Callister

20. Pearlite Formation Course pearlite Fine pearlite
Back to eutectoid composition
Above knee form pearlite as described in last lecture
Thickness of lamellae depends on isotherm
Course pearlite
Higher temperatures
Diffusion rate high
Carbon travels larger distances
Fine pearlite
Close to 540 °C
Diffusion suppressed
time (s) 1 10
102
103
104
105
10-1
Temperature °C
400
500
600
700
300
200
800 Te 
(Coarse pearlite) 
 +P
Pearlite, P
(Fine pearlite) 
 +B
Bainite, B
M (start)
M (50%)
M (90%)
Martensite, M

21. Bainite Formation ~300 - 540 °C
Below knee form bainite
Upper bainite
~ °C
Ferrite grows first, then Fe3C drops out
Needles of ferrite separated by elongated cementite particles
Lower bainite
~ °C
Thin plates of ferrite containing fine blades of cementite
Cannot transform pearlite to bainite
Can coexist with each other
time (s) 1 10
102
103
104
105
10-1
Temperature °C
400
500
600
700
300
200
800 Te  
 +P
Pearlite, P
(upper Bainite) 
 +B
Bainite, B
(lower
Bainite)
M (start)
M (50%)
M (90%)
Martensite, M

22. Martensite Formation Happens quickly (velocity of sound)
Nonequilibrium (metastable) phase
Due to FCC/BCT transition
Happens quickly (velocity of sound)
Little atomic motion
Diffusionless transformation
Time independent
–46 °C for complete transformation
Lath: long thin plates (%C < 0.6)
Lenticular: needlelike
Characteristically brittle, strong, hard
time (s) 1 10
102
103
104
105
10-1
Temperature °C
400
500
600
700
300
200
800 Te  
 +P
Pearlite, P 
 +B
Bainite, B
M (start)
M (50%)
Martensite, M
M (90%)

23. Representative TTT Diagrams
Presence of other alloying elements
1021 Steel
1045 Steel
1095 Steel
4140 Steel
Mn 0.77
Cr 0.98
Mo 0.21
4340 Steel
Mn 0.78
Cr 0.80
Mo 0.33
Ni 1.79

24. Alloying Effects Higher carbon content
Shifts curve to right (slightly!)
Change proeutectoid phase from ferrite to cementite
If %C < 0.4, steel is not “hardenable”
Necessary cooling rate would be far too quick to form martensite
Book says 0.25%, but realistically very difficult below 0.4%
Alloying other than carbon
Shift austenite nose to longer times
Formation of separate bainite nose
What does this mean?
To form equilibrium products, cooling rate must be much slower
Easier to form martensite
Thicker parts will have more uniform hardness

25. Continuous Cooling Transformation (CCT)
Isothermal heat treatment not common
Practically, want to cool steadily to room temperature
Isothermal curves shifted to longer times and lower temperatures
Bainite will not form for plain carbon steel
Alloying agents shifts the pearlite transformation curve to the right
Now possible to obtain bainite
2 “knees” appear on curve
time (s) 1 10
102
103
104
105
10-1
Temperature °C
400
500
600
700
300
200
800 Te 
 +P 
Pearlite, P 
Bainite, B
 +B
M (start)
M (50%)
Martensite, M
M (90%)

26. Example: 4340 Steel 1. Martensite 2. Martensite Banite 3. Martensite
Bainite
Pearlite
1. Martensite 1
2. Martensite
Banite 2
4. Ferrite
Pearlite 4
5. Pearlite 5
3. Martensite
Ferrite
Banite 3

27. Linking Important Concepts
The composition of an alloy determines the phase change kinetics
The cooling rate determines the microstructure of an alloy
The microstructure determines the mechanical properties

28. Mechanical Properties (Pearlite & Bainite)
Cementite harder & more brittle than ferrite
Spheroidite: spherical Fe3C particles in a matrix, very soft & ductile
Pearlite: Fine is harder & more brittle than coarse
Bainite: Stronger & harder than pearlite, good ductility
120
240
280
160
200 80 60 40 20
Brinell Hardness
%RA
wt% C
Bainite
Pearlite
600
300
500
400
100
200
700
Transformation temperature (°C)
Brinell Hardness
Spheroidite
Fine Pearlite
Coarse Pearlite

29. Mechanical Properties (Martensite)
Very strong and hard
Brittle
C interstitials
Few slip systems
Large internal stresses due to volume change from austenite phase
To recover ductility, need to do special heat treatment called tempering…
700
600
500
400
300
200
100
Brinell Hardness
0.2
0.4
0.6
0.8
1.0
0.0
Carbon, %
Martensite
Pearlite
Tempered martensite

30. Mechanical Properties (Tempered Martensite)
Tempering: Reheating martensite up to a sub-eutectoid temperature for long time
Trade strength for ductility
1800
1600
1400
1200
1000
800
Strength, MPa 60 50 40 30
% RA
Tensile
Yield
200
300
400
500
600
Tempering temperature, °C
4340 steel

31. Austenitic Transformations

32. Objectives for Chapter 11
Have basic understanding of types of steels, relative C content, strength, and applications.
Know fundamental differences between the types of cast iron. Describe the purpose of and procedures for process annealing, stress relief annealing, normalizing, full annealing, and spheroidizing.
Define hardenability and list the class steels that have low hardenability.
Know how to use the Jominy End Quench Test - see lab on hardenability.
Explain the procedure to precipitation harden an alloy and what characteristics on the phase diagram allow the alloy to be hardened by this mechanism.
Explain the shape of the strength vs time curve for precipitation hardened alloys in terms of the mechanism of hardening in each stage.
Distinguish between the four types of forming operations.
Distinguish between the four types of casting techniques.

33. Ferrous Alloys

34. Metal Fabrication

35. Heat Treating Steel Full Annealing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Full Annealing
Heat to °C above phase transition line, hold for diffusion, and slow cool (usually in a furnace, often takes number of hours).
Returns microstructure to coarse pearlite.
Often performed on steels to be machined or formed to increase ductility

36. Heat Treating Steel Normalizing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Normalizing
Heat to the fully austenite region and air cool.
Fine pearlite microstructure
Refine grain size and distribution (often done after a rolling operation)

37. Heat Treating Steel Spheroidizing
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Spheroidizing
Heat a medium-high carbon steel below the eutectoid line and hold for several hours
Forms small Fe3C spherical particles in  matrix.
For higher carbon steels, even pearlite is often brittle and difficult to deform
Spheroidizing minimizes hardness and is highly machinable

38. Heat Treating Steel Quenching Heat to fully austenitize steel
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Quenching
Heat to fully austenitize steel
Rapidly cool (quench in water, oil, etc)
No time for carbon to diffuse
Produces a non-equilibrium microstructure called martensite
Very hard, strong, brittle with large internal stresses

39. Heat Treating Steel Tempering After quenching to form martensite
Temperature °C
1 %
1000
800
600 g
a + g
g + Fe3C
a + Fe3C a
% Carbon
Tempering
After quenching to form martensite
Reheat below eutectoid temperature
Hold for several hours to precipitate carbides and relieve residual stresses
Forms tempered martensite
Relatively hard, strong, much more ductile
Often ideal microstructure ...

40. Hardenability Hardness vs. Hardenability
Hardness – resistance to deformation
Hardenability – ability to be form martensite (to be hardened)
Formal definition:
Hardenability – ability of an alloy to be hardened by formation of martensite due to given heat treatment
High hardenability  martensite forms (i.e. hardens) far into specimen, not just at surface

41. Jominy End Quench Test
Standard test for measuring hardness of any material
Sample of fixed geometry suspended over water jet
Geometric factors all constant
Only effect on hardness is alloying content
Hardness measured as function of distance from quenched end
Cooling rate
Distance
Hardness, HRC
Distance from quenched end

42. Distance from quenched end, mm
Hardness Profiles 10 20 30 40 50
Distance from quenched end, mm
Cooling rate at 700 °C (°C/s ) 60
Hardness, HRC
8660
8630
8640
8620
Cooling rate
Highest at quenched end
Smoothly decreases with distance
Microstructure
Martensite
Bainite
Fine Pearlite
Course Pearlite
Often plot cooling rate and distance
Increasing carbon content
Maximum possible hardness increases
Higher overall hardness (more hardenable)
Slower
Cooling

43. Severity of Quench
Rate of cooling!
Depends on
Medium- Fluid in which an austenetized part is plunged
Most common: air, oil, water
Agitation - Relative motion of quenching medium during cooling
Agitation rate influences cooling rate
Surface area to volume ratio -
All heat removed at surface
More surface area provides more opportunity to remove heat
Irregular shapes with edges have large quenching surfaces
For highly stressed part, want 80% martensite throughout part
Usually from a “guideline” for specific company

44. Using Cooling Curves Normal quenching takes place on radial surface
Can predict hardness across radius of a bar from Jominy tests.

45. Review: Heat Treatment of Steel
Annealing- Heat, holding at temperature, gradual cool
General term, but also used for specific processes
Tempering -Heat martensite to get diffusional transformation:
Still have ultra-fine microstructure, but more ductile
Process annealing -Reverse effects of cold work; recovery, recrystallization, but little grain growth
Stress relief - Lower temperature anneal to undo thermal stress or transformation mismatch stress
Spheroidizing - Heat pearlite just below eutectoid temp to produce spheroidal structure; makes steel easier to machine

46. Surface Hardening Techniques
Many applications, especially wear
Strong, hard, wear-resistant surface
Tough, fracture resistant inner core
Two different heat treatments - through, then surface
Methods
Chose material with steep cooling curve
Case Hardening - Change chemical composition of surface
Carburizing
Nitriding
Carbonitriding
Decremental Hardening – Very localized heat treatment
Flame Hardening
Induction Hardening

47. Alloys for Surface Hardness10 20 30 40 50
Distance from quenched end, mm
Cooling rate at 700 °C (°C/s ) 60
Hardness, HRC
100 80
Percent Martensite
4340
4140
5140
8640
1040 A B A B
Prescribe:
Minimum hardness at A
Maximum hardness at B Must know the cooling rates at each point!

48. Carburizing
Hardens steel by causing carbon to diffuse into the surface.
Furnace heat to a temperature at which carbon will diffuse
Hold until diffusion creates the proper case depth.
Must be a carbon rich environment
For steels with low carbon content (%C < 0.2)
Used extensively on gears and shafts to harden them yet maintain the core toughness.
Decarburization is opposite process for high carbon steels - softer case
From: Callister p. 92, 224

49. Flame Hardening For steels of hardenable carbon content (%C  0.4)
A high intensity oxy-acetylene flame is applied to the bring the region of interest to an austenite transformation.
The interior never reaches high temperature.
The heated region is quenched to achieve the desired hardness.
The depth of hardening can be increased by increasing the heating time.
In addition, large parts, which will not normally fit in a furnace, can be heat-treated
From: Scheer p. 43

50. Precipitation Hardening
Form extremely small, dispersed particles of a second phase
Small second phase particles called precipitates
Age hardening - strength develops as the alloy ages
Time dependent
There is a maximum attainable effect
Different than tempering martensite
Similar heat treatment procedure
Different strengthening mechanism

51. Precipitation Hardening
a Ti in b Ti matrix
Primary strengthening mechanism for
Aluminum
Nickel based superalloys
Titanium
Examples
Al/Cu
Cu/Sn
Mg/Al
Ni3Al in Ni matrix
From: Socie

52. Phase Diagrams & Treatment
Precipitation hardenable alloys have:
Appreciable maximum solubility
Decreasing solubility with temperature
Composition less than maximum solubility L
L + a
Temperature
L + b a b
Solution Treating:
Heat into the single phase region and rapidly quench to room temperature to produce a supersaturated solid solution
a + b
Aging:
Heat to a temperature below the phase transition to allow time for precipitates to form
Composition

53. Why Precipitation Harden?
Look at Aluminum rich side of Al/Cu system
a is a substitutional solid solution of copper in aluminum
q is an intermetallic compound CuAl2
One slow cool in is NOT helpful in strengthening
Coarse q phase weakens the alloy
q + a a °C
600
700
500
400
300
200
100 10 5
time
100% a
(95.5% Al, 4.5% Cu)
Temperature
Coarse q precipitates
At a grain boundaries Al
wt % Cu

54. How to Precipitation Harden
Two reheating treatments are needed:
Solution treatment
Age hardening
Fine precipitates strengthen & harden material
q + a a °C
600
700
500
400
300
200
100 10 5
wt % Cu
100% a solid solution
Equilibrium
Microstructure
Temperature
Fine precipitates in grains
(retained after cooling) Al
time

55. Overaging If precipitates get too big, strengthening/hardening is lost
Process sped up by temperature
Some alloys age at room temperature
Why is strength lost?
100% a solid solution
Temperature
Fine precipitates in grains
Coarse precipitates
in grains
taging
time

56. Strength Development
Fine precipitates (Guinier-Preston zones  q’’ phase) are coherent with lattice
Deformation of crystal impedes dislocation motion
At optimal aging time, precipitates are dispersed, small, and coherent.
After optimal aging time, precipitates get too big, incoherent with matrix.
From: Callister

57. Age vs. Material Properties
Must age carefully to avoid overaging
Too high temperature
Too long age time
Precipitation rate maximized at intermediate temperatures
Near solvus  no driving force for nucleation
Low temperatures  diffusion is slow t T
Nucleation
Diffusion
From: Callister

58. Strengthening Mechanisms
Dislocation Looping
Dislocation cutting
From: Hertzberg

59. What is Attractive About Aluminum?
Upside
Lightweight ( )
Ductile
Heat treatable
Castable
Corrosion resistant
Good conductor
Downside
Formability is lower
Simpler shapes
For same stiffness, need larger section
Longitudinal stiffness(~EA/L)
Bending stiffness (~EI)
Rect. section, constant b b h

60. Aluminum Designations
Tempering Designations
-O: Annealed
-F: As fabricated
-H1: Strain Hardened
-H2: Strain Hardened/ Partial Annealed
-T: Thermally Processed
-T2: Annealed (cast products)
-T3: Solution treated, CW, naturally aged
-T4: Solution treated, naturally aged
-T5: Artificially aged, no solution treatment
-T6: Solution treated, artificially aged
-T7: Solution treated, over-aged
-T8: Solution treated, CW, artificially aged
-T9: Solution treated, artificially aged , CW
Alloying Composition
Aluminum 1xxx
Aluminum
Copper 2xxx
Manganese 3xxx
Silicon 4xxx
Magnesium 5xxx
Mg and Si 6xxx
Zinc 7xxx
Common alloys: T4 ,
6061-T6,
7075-T6

61. New Concepts & Terms Applications of hardness profiles (Jominy curves)
Precipitation hardening
Heat treatment process
Effect of overaging
Microstructures
Effect on strength
Applications of aluminum
Know tradeoffs, especially with steels
Which alloys are precipitation hardenable?