1. Phase Transformations in Metals
Ferrite - BCC
Martensite - BCT
Austenite - FCC
Chapter 10
Phase Transformations in Metals
Fe3C (cementite)- orthorhombic

2. Why do we study phase transformations?
The tensile strength of an Fe-C alloy of eutectoid composition can be varied between MPa depending on HT process adopted.
Desirable mechanical properties of a material can be obtained as a result of phase transformations using the right HTprocess.
In order to design a HT for some alloy with desired RT properties, time & temperature dependencies of some phase transformations can be represented on modified phase diagrams.
Based on this, we will learn:
Phase transformations in metals
Microstructure and property dependence in Fe-C alloy system
Precipitation Hardening, Crystallization, Melting, and Glass Transition

3. Topics to be covered: Transformation rate
Kinetics of Phase Transformation
Nucleation: homogeneous, heterogeneous
Free Energy, Growth
Isothermal Transformations (TTT diagrams)
Pearlite, Martensite, Spheroidite, Bainite
Continuous Cooling
Mechanical Behavior
Precipitation Hardening

4. Phase Transformations
Phase transformations – change in the number or character of phases.
Simple diffusion-dependent
No change in # of phases
No change in composition
Example: solidification of a pure metal, allotropic transformation, re-crystallization, grain growth
More complicated diffusion-dependent
Change in # of phases
Change in composition
Example: eutectoid reaction
Diffusion-less
Example: meta-stable phase : martensite

5. Phase Transformations -Stages
Most phase transformations begin with the formation of numerous small particles of the new phase that increase in size until the transformation is complete.
Nucleation is the process whereby nuclei (seeds) act as templates for crystal growth.
Homogeneous nucleation - nuclei form uniformly throughout the parent phase; requires considerable supercooling (typically °C).
Heterogeneous nucleation - form at structural in-homogeneities (container surfaces, impurities, grain boundaries, dislocations) in liquid phase much easier since stable “nucleating surface” is already present; requires slight super-cooling (0.1-10ºC).

6. Supercooling
During the cooling of a liquid, solidification (nucleation) will begin only after the temperature has been lowered below the equilibrium solidification (or melting) temperature Tm. This phenomenon is termed super-cooling or under-cooling.
The driving force to nucleate increases as T increases
Small super-cooling  slow nucleation rate - few nuclei - large crystals
Large super-cooling  rapid nucleation rate - many nuclei - small crystals

7. Kinetics of Solid State Reactions
Transformations involving diffusion depend on time.
Time is also necessary for the energy increase associated with the phase boundaries between parent and product phases.
Moreover, nucleation, growth of the nuclei, formation of grains and grain boundaries and establishment of equilibrium take time.
As a result we can say the transformation rate is a function of time.
The fraction of reaction completed is measured as a function of time at constant T.
Tranformation progress can be measured by microscopic examination or measuring a physical property (e.g., conductivity).
The obtained data is plotted as fraction of the transformation versus logarithm of time.

8. FRACTION OF TRANSFORMATION
• Fraction transformed depends on time.
• Transformation rate depends on T.
• r often small: equil not possible 2

9. Transformations & Undercooling
Eutectoid transformation (Fe-Fe3C system): g Þ a +
Fe3C
For transformation to occur, must cool to below 727°C
0.76 wt% C
6.7 wt% C
0.022 wt% C
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
C, wt% C
1148°C
T(°C)
ferrite
727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC DT
Undercooling by Ttransf. < 727C
0.76
0.022

10. Generation of Isothermal Transformation Diagrams
• The Fe-Fe3C system, for Co = 0.76 wt% C
• A transformation temperature of 675°C.
100
T = 675°C
% transformed 50 1 10 2 10 4
time (s)
400
500
600
700 1 10 2 3 4 5
0%pearlite
100%
50%
Austenite (stable)
TE (727C)
Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C

11. Eutectoid Transformation Rate ~ DT
• Transformation of austenite to pearlite: g a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
Diffusion of C
during transformation a g
Carbon diffusion
• For this transformation,
rate increases with ( DT)
[Teutectoid – T ].
675°C
(DT smaller) 50
% pearlite
600°C
(DT larger)
650°C
100
Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard

12. Eutectoid Transformation Rate
At T just below 727°C, very long times (on the order of 105 s) are required for 50% transformation and therefore transformation rate is slow.
The transformation rate increases as T decreases, for example, at 540°C 3 s is required for 50% completion.
This observation is in clear contradiction with the equation of
This is because in T range of 540°C-727°C, the transformation rate is mainly controlled by the rate of pearlite nucleation and nucleation rate decreases with T increase. Q in this equation is the activation energy for nucleation and it increases with T increase.
It has been found that at lower T, the austenite decomposition is diffusion controlled and the rate behavior can be calculated using Q for diffusion which is independent of T.

13. Nucleation and Growth
• Reaction rate is a result of nucleation and growth of crystals.
• Examples: 5

14. Isothermal Transformation Diagrams
c11f13
Isothermal Transformation Diagrams
solid curves are plotted:
one represents the time required at each temperature for the start of the transformation;
the other is for transformation completion.
The dashed curve corresponds to 50% completion.
The austenite to pearlite transformation will occur only if the alloy is supercooled to below the eutectoid temperature (727˚C).
Time for process to complete depends on the temperature.

15. Isothermal Transformation Diagram
c11f14
• Eutectoid iron-carbon alloy; composition, Co = 0.76 wt% C
• Begin at T > 727˚C
• Rapidly cool to 625˚C and hold isothermally.
Austenite-to-Pearlite

16. Transformations Involving Noneutectoid Compositions
Consider C0 = 1.13 wt% C
Fe3C (cementite)
1600
1400
1200
1000
800
600
400 1 2 3 4 5 6
6.7 L g
(austenite) +L
+Fe3C a
L+Fe3C d
(Fe)
C, wt%C
T(°C)
727°C DT
0.76
0.022
1.13
Hypereutectoid composition – proeutectoid cementite 16

17. Possible Transformations
c11f37
Strength
Ductility
Martensite
T Martensite
bainite
fine pearlite
coarse pearlite
spheroidite
General Trends

18. Coarse pearlite (high diffusion rate) and (b) fine pearlite
c11f15
Coarse pearlite (high diffusion rate) and (b) fine pearlite

19. Bainite: Non-Equilibrium Transformation Products
elongated Fe3C particles in a-ferrite matrix
diffusion controlled
a lathes (strips) with long rods of Fe3C 10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50% A
Martensite
100% pearlite
100% bainite
Cementite
Ferrite

20. Bainite Microstructure
Bainite: formed as a result of transformation of austenite.
Bainite consists of ferrite and cementite and diffusion processes take place as a result.
This structure looks like needles or plates. There is no proeutectoid phase in bainite.
Bainite consists of acicular (needle-like) ferrite with very small cementite particles dispersed throughout.
The carbon content is typically greater than 0.1%.
Bainite transforms to iron and cementite with sufficient time and temperature.

21. Spheroidite: Nonequilibrium Transformation
Fe3C particles within an a-ferrite matrix
diffusion dependent
heat bainite or pearlite at temperature just below eutectoid for long times
driving force – reduction of a-ferrite/Fe3C interfacial area 10

22. Pearlitic Steel partially transformed to Spheroidite
c11f20
Pearlitic Steel partially transformed to Spheroidite

23. Martensite Formation TE T(°C) time (s) Martensite needles Austenite A10 3 5
time (s) -1
400
600
800
T(°C)
Austenite (stable)
200 P B TE 0%
100%
50% A
M + A
90%
Martensite needles
single phase
body centered tetragonal (BCT) crystal structure
BCT if C0 > 0.15 wt% C
Diffusion-less transformation
BCT  few slip planes  hard, brittle
% transformation depends only on T of rapid cooling
Austenite

24. An micrograph of austenite that was polished flat and then allowed to transform into martensite. The different colors indicate the displacements caused when martensite forms.

25. Isothermal Transformation Diagram
Iron-carbon alloy with eutectoid composition.
A: Austenite
P: Pearlite
B: Bainite
M: Martensite

26. Effect of Adding Other Elements
c11f24
Effect of Adding Other Elements
4340 Steel
Other elements (Cr, Ni, Mo, Si and W) may cause significant changes in the positions and shapes of the TTT curves:
Change transition temperature;
Shift the nose of the austenite-to-pearlite transformation to longer times;
Shift the pearlite and bainite noses to longer times (decrease critical cooling rate);
Form a separate bainite nose;
nose
plain carbon
steel
Plain carbon steel: primary alloying element is carbon.

27. c11f23
Example 1:
Iron-carbon alloy with eutectoid composition.
Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (a)
Rapidly cool to 350 ˚C
Hold for 104 seconds
Quench to room temperature
Bainite, 100%

28. c11f23 Example 2: Iron-carbon alloy with eutectoid composition.
Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (b)
Rapidly cool to 250 ˚C
Hold for 100 seconds
Quench to room temperature
Austenite, 100%
Martensite, 100%

29. c11f23 Austenite, 100% Example 3:
Iron-carbon alloy with eutectoid composition.
Specify the nature of the final microstructure (% bainite, martensite, pearlite etc) for the alloy that is subjected to the following time–temperature treatments:
Alloy begins at 760˚C and has been held long enough to achieve a complete and homogeneous austenitic structure.
Treatment (c)
Rapidly cool to 650˚C
Hold for 20 seconds
Rapidly cool to 400˚C
Hold for 103 seconds
Quench to room temperature
Almost 50% Pearlite, 50% Austenite
Bainite, 50%
Final:
50% Bainite,
50% Pearlite

30. Continuous Cooling Transformation Diagrams
c11f26
Continuous Cooling Transformation Diagrams
Isothermal heat treatments are not the most practical due to rapidly cooling and constant maintenance at an elevated temperature.
Most heat treatments for steels involve the continuous cooling of a specimen to room temperature.
TTT diagram (dotted curve) is modified for a CCT diagram (solid curve).
For continuous cooling, the time required for a reaction to begin and end is delayed.
The isothermal curves are shifted to longer times and lower temperatures.

31. c11f27
Moderately rapid and slow cooling curves are superimposed on a continuous cooling transformation diagram of a eutectoid iron-carbon alloy.
The transformation starts after a time period corresponding to the intersection of the cooling curve with the beginning reaction curve and ends upon crossing the completion transformation curve.
Normally bainite does not form when an alloy is continuously cooled to room temperature; austenite transforms to pearlite before bainite has become possible.
The austenite-pearlite region (A---B) terminates just below the nose. Continued cooling (below Mstart) of austenite will form martensite.

32. c11f28
For continuous cooling of a steel alloy there exists a critical quenching rate that represents the minimum rate of quenching that will produce a totally martensitic structure.
This curve will just miss the nose where pearlite transformation begins

33. c11f29
Continuous cooling diagram for a 4340 steel alloy and several cooling curves superimposed.
This demonstrates the dependence of the final microstructure on the transformations that occur during cooling.
Alloying elements used to modify the critical cooling rate for martensite are chromium, nickel, molybdenum, manganese, silicon and tungsten.

34. Mechanical Properties
Hardness
Brinell, Rockwell
Yield Strength
Tensile Strength
Ductility
% Elongation
Effect of Carbon Content

35. Mechanical Properties: Influence of Carbon Content
C0 > 0.76 wt% C
Hypereutectoid
Pearlite (med)
Cementite
(hard)
c11f30
Pearlite (med)
ferrite (soft)
C0 < 0.76 wt% C
Hypoeutectoid

36. Mechanical Properties: Fe-C System
c11f31
Mechanical Properties: Fe-C System

37. c11f34 Tempered Martensite
Martensite is hard but also very brittle so that it can not be used in most of the applications.
Any internal stress that has been introduced during quenching has a weakening effect.
The ductility and toughness of the material can be enhanced by heat treatment called tempering. This also helps to release any internal stress.
Tempering is performed by heating martensite to a T below eutectoid temperature (250°C-650°C) and keeping at that T for specified period of time.
The formation of tempered martensite is by diffusion.

38. c11f34 Tempered Martensite
Tempered martensite is less brittle than martensite; tempered at 594 °C.
Tempering reduces internal stresses caused by quenching.
The small particles are cementite; the matrix is a-ferrite. US Steel Corp.
4340 steel

39. c11f34 Tempered Martensite
Tempered martensite may be nearly as hard and strong as martensite, but with substantially enhanced ductility and toughness.
The hardness and strength may be due to large area of phase boundary per unit volume of the material.
The phase boundary acts like a barrier for dislocaitons. The continuous ferrite phase in tempered martensite adds ductility and toughness to the material.
The size of the cementite particles is important factor determining the mechanical behavior.
As the cementite particle size increases, material becomes softer and weaker. The temperature of tempering determines the cementite particle size. Since martensite-tempered martensite transformation involves diffusion, Increasing T will accelerate the diffusion and rate of cementite particle growth and rate of softening as a result.

40. Hardness as a function of carbon concentration for steels
c11f33

41. Rockwell C and Brinell Hardness
c11f36
Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080); room temperature.

42. c11tf02

43. Precipitation Hardening
The strength and hardness of some metal alloys may be improved by the formation of extremely small, uniformly dispersed particles (precipitates) of a second phase within the original phase matrix.
Other alloys that can be precipitation hardened or age hardened:
Copper-Beryllium (Cu-Be)
Copper-Tin (Cu-Sn)
Magnesium-Aluminum (Mg-Al)
Aluminum-Copper (Al-Cu)
High-strength Aluminum alloys

44. c11f40 Phase Diagram for Precipitation Hardened Alloy Criteria:
Maximum solubility of 1 component in the other (M);
Solubility limit that rapidly decreases with decrease in temperature (M→N).
Process:
Solution Heat Treatment – first heat treatment where all solute atoms are dissolved to form a single-phase solid solution.
Heat to T0 and dissolve B phase.
Rapidly quench to T1
Nonequilibrium state (a phase solid solution supersaturated with B atoms; alloy is soft, weak-no ppts).

45. c11f43 Precipitation Heat Treatment – the 2nd stage
The supersaturated a solid solution is usually heated to an intermediate temperature T2 within the a+b region (diffusion rates increase).
The b precipitates (PPT) begin to form as finely dispersed particles. This process is referred to as aging.
After aging at T2, the alloy is cooled to room temperature.
Strength and hardness of the alloy depend on the ppt temperature (T2) and the aging time at this temperature.

46. Precipitation Hardening
• Particles impede dislocation motion.
• Ex: Al-Cu system
• Procedure: 10 20 30 40 50
wt% Cu L
+L a
a+q q
+L
300
400
500
600
700
(Al)
T(°C)
composition range
available for precipitation hardening
CuAl2 A
-- Pt A: solution heat treat (get a solid solution) B
Pt B
-- Pt B: quench to room temp. (retain a solid solution) C
-- Pt C: reheat to nucleate small q particles within a phase.
Temp.
Time
At room temperature the stable state of an aluminum-copper alloy is an aluminum-rich solid solution (α) and an intermetallic phase with a tetragonal crystal structure having nominal composition CuAl2 (θ).
Pt A (solution heat treat)
Pt C (precipitate )

47. c11f43 Precipitation Heat Treatment – the 2nd stage
PPT behavior is represented in the diagram:
With increasing time, the hardness increases, reaching a maximum (peak), then decreasing in strength.
The reduction in strength and hardness after long periods is overaging (continued particle growth).
Small solute-enriched regions in a solid solution where the lattice is identical or somewhat perturbed from that of the solid solution are called Guinier-Preston zones.

48. PRECIPITATION STRENGTHENING
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
• Result: 24

49. c11f44
Several stages in the formation of the equilibrium PPT (q) phase.
supersaturated a solid solution;
transition (q”) PPT phase;
equilibrium q phase within the a matrix phase.

50. Influence of Precipitation Heat Treatment on Tensile Strength (TS), %EL
• 2014 Al Alloy:
• TS peak with precipitation time.
• Increasing T accelerates
process.
• %EL reaches minimum
with precipitation time.
%EL (2 in sample) 10 20 30
1min 1h
1day
1mo
1yr
204°C
149 °C
precipitation heat treat time
precipitation heat treat time
tensile strength (MPa)
200
300
400
100
1min 1h
1day
1mo
1yr
204°C
non-equil.
solid solution
many small
precipitates
“aged”
fewer large
“overaged”
149°C

51. Effects of Temperature
c11f45
Effects of Temperature
Characteristics of a 2014 aluminum alloy (0.9 wt% Si, 4.4 wt% Cu, 0.8 wt% Mn, 0.5 wt% Mg) at 4 different aging temperatures.

52. Aluminum rivets
Alloys that experience significant precipitation hardening at room temp and after short periods must be quenched to and stored under refrigerated conditions.
Several aluminum alloys that are used for rivets exhibit this behavior. They are driven while still soft, then allowed to age harden at the normal room temperature.