material science slide of addis ababa university

1. Chapter 11
Phase
Transformations
Fe3C (cementite)- orthorhombic
Martensite - BCT
Austenite - FCC
Ferrite - BCC

2. Phase Transformations
• 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

3.  In the study of phase transformations we will
be dealing with the changes that can occur
within a given system e.g.an alloy that can exist
as a mixture of one or more phases
 A phase can be defined as a portion of the
system whose properties and composition are
homogeneous and which is physically distinct
from other parts of the system
 The components of a system are the different
elements or chemical compound which make up
the system
Phase Transformations

4. Equilibrium
Any transformation that results in a decrease in Gibbs free energy is
possible

5. 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,
recrystallization, grain growth
 More complicated diffusion-dependent
 Change in # of phases
 Change in composition
 Example: eutectoid reaction
 Diffusionless
 Example: metastable phase - martensite

6. Phase Transformations
 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 80-300°C).
• Heterogeneous nucleation - form at structural
inhomogeneities (container surfaces, impurities, grain
boundaries, dislocations) in liquid phase much easier since
stable “nucleating surface” is already present; requires
slight supercooling (0.1-10ºC).

7. 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 supercooling (or
undercooling.
 The driving force to nucleate increases as T
increases
 Small supercooling  slow nucleation rate - few
nuclei - large crystals
 Large supercooling  rapid nucleation rate -
many nuclei - small crystals

8. Nucleation of a spherical solid particle in a liquid
Liquid
The change in free energy G (a function of the
internal energy and enthalpy of the system) must
be negative for a transformation to occur.
Assume that nuclei of the solid phase form in the
interior of the liquid as atoms cluster together-
similar to the packing in the solid phase.
Also, each nucleus is spherical and has a radius r.
Free energy changes as a result of: 1) the
difference between the solid and liquid phases
(volume free energy, GV); and
2) the solid-liquid phase boundary (surface free
energy, GS).
Transforming one phase into another takes time.
G = GS + GV
Fe
g
(Austenite)
Eutectoid
transformation
C FCC
Fe3C
(cementite)
a
(ferrite)
+
(BCC)

9. r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy)
Homogeneous Nucleation & Energy Effects
GT = Total Free Energy
= GS + GV
Surface Free Energy- destabilizes
the nuclei (it takes energy to make
an interface)
g 2
4 rGS
g = surface tension
Volume (Bulk) Free Energy –
stabilizes the nuclei (releases energy)
 GrGV
3
3
4
volumeunit
energyfreevolume
 G

10. Effect of Temperature

11. Solidification
TH
T
r
f
m

g

2
*
Note: Hf and g are weakly dependent on T
 r* decreases as T increases
For typical T r* ~ 10 nm
Hf = latent heat of solidification (fusion)
Tm = melting temperature
g = surface free energy
T = Tm - T = supercooling
r* = critical radius

12. Effect of Temperature

13. Growth
• It begins once an embryo has exceeded the
critical size r*
• nucleation will continue to occur simultaneously
with growth
• The growth process will cease in any region
where particles of the new phase meet
• Growth occurs by long-range atomic diffusion
– diffusion through the parent phase, across a
phase boundary, and then into the nucleus.

14. growth rate

16. Computation of Critical Nucleus Radius and Activation
Free Energy
(a) For the solidification of pure gold, calculate the critical radius
r*and the activation free energy ΔG* if nucleation is homogeneous.
Values for the latent heat of fusion and surface free energy are -1.16
x109 J/m3 and 0.132 J/m2 , respectively. Use the super-cooling value
found in Table 10.1.
(b) Now calculate the number of atoms found in a nucleus of critical
size. Assume a lattice parameter of 0.413 nm for solid gold at its
melting temperature.

17. Transformations & Undercooling
• For transformation to occur, must
cool to below 727°C
• Eutectoid transformation (Fe-Fe3C system): g  a + Fe3C
0.76 wt% C
0.022 wt% C
6.7 wt% C
Fe3C(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a +Fe3C
L+Fe3C
d
(Fe) C, wt% C
1148°C
T(°C)
a
ferrite
727°C
Eutectoid:
Equil. Cooling: Ttransf. = 727ºC
T
Undercooling by Ttransf. < 727C
0.76
0.022

18. 18
Rate of Phase Transformation
Avrami equation => y = 1- exp (-ktn)
transformation complete
log t
Fractiontransformed,y
Fixed T
fraction
transformed
time
0.5
By convention rate = 1 / t0.5
Fraction
transformed
depends on
time
maximum rate reached – now amount
unconverted decreases so rate slows
t0.5
rate increases as surface area increases
& nuclei grow
Avrami relationship - the rate is defined as the inverse of the time to complete half of the
transformation. This describes most solid-state transformations that involve diffusion.

19. • In general, rate increases as T 
r = 1/t0.5 = A e -Q/RT
– R = gas constant
– T = temperature (K)
– A = ‘preexponential’ rate factor
– Q = activation energy
• r is often small so equilibrium is not possible.
Arrhenius expression
Adapted from Fig. 10.11,
Callister 7e. (Fig. 10.11
adapted from B.F. Decker and
D. Harker, "Recrystallization in
Rolled Copper", Trans AIME,
188, 1950, p. 888.)
135C 119C 113C 102C 88C 43C
1 10 102 104
Temperature Dependence of
Transformation Rate

20. Generation of Isothermal Transformation Diagrams
• The Fe-Fe3C system, for Co = 0.76 wt% C
• A transformation temperature of 675°C.
100
50
0
1 102 104
T = 675°C
%transformed
time (s)
400
500
600
700
1 10 102 103 104 105
Austenite (stable)
TE (727C)Austenite
(unstable)
Pearlite
T(°C)
time (s)
isothermal transformation at 675°C
Consider:

21. Coarse pearlite  formed at higher temperatures – relatively soft
Fine pearlite  formed at lower temperatures – relatively hard
• Transformation of austenite to pearlite:
ga
a
a
a
a
a
pearlite
growth
direction
Austenite (g)
grain
boundary
cementite (Fe3C)
Ferrite (a)
g
• For this transformation,
rate increases with ( T)
[Teutectoid – T ].
675°C
(T smaller)
0
50
%pearlite
600°C
(T larger)
650°C
100
Diffusion of C
during transformation
a
a
g
g
a
Carbon
diffusion
Eutectoid Transformation Rate ~ T

22. Isothermal Transformation Diagrams
2 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.

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

24. Transformations Involving
Noneutectoid Compositions
Hypereutectoid composition – proeutectoid cementite
Consider C0 = 1.13 wt% C
Fe3C(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a+Fe3C
L+Fe3C
d
(Fe)
C, wt%C
T(°C)
727°C
T
0.76
0.022
1.13

25. 25
Transformations Involving
Noneutectoid Compositions
Hypereutectoid composition – proeutectoid cementite
Consider C0 = 1.13 wt% C
a
TE (727°C)
T(°C)
time (s)
A
A
A
+
C
P
1 10 102 103 104
500
700
900
600
800
A
+
P
Adapted from Fig. 11.16,
Callister & Rethwisch 3e.
Adapted from Fig. 10.28,
Callister & Rethwisch 3e.
Fe3C(cementite)
1600
1400
1200
1000
800
600
400
0 1 2 3 4 5 6 6.7
L
g
(austenite)
g+L
g +Fe3C
a+Fe3C
L+Fe3C
d
(Fe)
C, wt%C
T(°C)
727°C
T
0.76
0.022
1.13

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

28. Coarse pearlite (high diffusion rate) and (b) fine pearlite
- Smaller T:
colonies are
larger
- Larger T:
colonies are
smaller

29. 10 103
105
time (s)
10-1
400
600
800
T(°C)
Austenite (stable)
200
P
B
TEA
A
Bainite: Non-Equil Transformation Products
 elongated Fe3C particles in a-ferrite matrix
 diffusion controlled
 a lathes (strips) with long rods of Fe3C
100% bainite
100% pearlite
Martensite
Cementite
Ferrite

30. Bainite Microstructure
• 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 (considered
semi-stable below 150°C).

31. 10
 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
Spheroidite: Nonequilibrium Transformation

32. Pearlitic Steel partially transformed to Spheroidite

33.  single phase
 body centered tetragonal (BCT) crystal structure
 BCT if C0 > 0.15 wt% C
 Diffusionless transformation
 BCT  few slip planes  hard, brittle
 % transformation depends only on T of rapid cooling
Martensite Formation
• Isothermal Transformation Diagram
10 103
105
time (s)10-1
400
600
800
T(°C)
Austenite (stable)
200
P
B
TEA
A
M + A
M + A
M + A
0%
50%
90%
Martensite needles
Austenite

34. 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.

35. Martensite
 The martensitic transformation occurs without composition change
 The transformation occurs by shear without need for diffusion
 The atomic movements required are only a fraction of the
interatomic
spacing
The amount of martensite formed is a function of the temperature to
which the sample is quenched and not of time
 Hardness of martensite is a function of the carbon content
→ but high hardness steel is very brittle as martensite is brittle
 Steel is reheated to increase its ductility
→ this process is called TEMPERING

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

37. Example 11.2:
 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 (a)
 Rapidly cool to 350 ˚C
 Hold for 104 seconds
 Quench to room temperature
Bainite,
100%

38. Martensite,
100%
Example 11.2:
 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%

39. Bainite, 50%
Example 11.2:
 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 (c)
 Rapidly cool to 650˚C
 Hold for 20 seconds
 Rapidly cool to 400˚C
 Hold for 103 seconds
 Quench to room temperature
Austenite,
100%
Almost 50% Pearlite,
50% Austenite
Final:
50% Bainite,
50% Pearlite

40. class quiz (bonus)
1. Describe the microstructure present in a 1045 steel after each
step in the following heat treatments:
a) heat at 820°C, quench to 650°C and hold for 90s, and
quench to 25°C;
b) heat at 820°C, quench to 450°C and hold for 90s, and
quench to 25°C;
c) heat at 820°C, and quench to 25°C;
d) heat at 820°C, quench to 680°C and hold for 100s, and
quench to 25°C;
e) heat at 820°C, quench to 720°C and hold for 100s, quench
to 400°C and hold for 500 s, and quench to 25°C;
f) heat at 820°C, quench to 720°C and hold for 100s, quench
to 400°C and hold for 10 s, and quench to 25°C; and
g) heat at 820°C, quench to 25°C, heat to 500°C and hold

42. ALLOY STEELS
 Various elements like Cr, Mn, Ni, W, Mo etc are added to plain
carbon
steels to create alloy steels
 The alloys elements move the nose of the TTT diagram to the right
→ this implies that a slower cooling rate can be employed to
obtain
martensite → increased HARDENABILITY
 The ‘C’curves for pearlite and bainite transformations overlap in
the case of plain carbon steels
→ in alloy steels pearlite and bainite
transformations can be represented by separate ‘C’curves

43. ROLE OF ALLOYING ELEMENTS
• + Simplicity of heat treatment and lower cost
•  Low hardenability
•  Loss of hardness on tempering
•  Low corrosion and oxidation resistance
•  Low strength at high temperatures
Plain Carbon Steel
Element Added
Solid solution
• ↑ hardenability
• Provide a fine distribution of alloy carbides during
tempering
• ↑ resistance to softening on tempering
• ↑ corrosion and oxidation resistance
• ↑ strength at high temperatures
• Strengthen steels that cannot be quenched
• Make easier to obtain the properties throughout a larger
section
• ↑ Elastic limit (no increase in toughness)
Alloying elements
• Alter temperature at
which the transformation
occurs
• Alter solubility of C in
a or g Iron
• Alter the rate of various
reactions
Interstitial
Substitutional

44. Austenite Pearlite
Bainite
Martensite
100
200
300
400
600
500
800
Ms
Mf
t →
T→
TTT diagram for Ni-Cr-Mo low alloy steel
~1 min

45.  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;
Effect of Adding
Other Elements
4340 Steel
plain
carbon
steel
nose
 Plain carbon steel: primary
alloying element is carbon.

46. 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 (dashed 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.

47.  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.

48.  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

49.  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.

50. Tempering
 Heat below Eutectoid temperature → wait→
slow cooling
The microstructural changes which take place
during tempering
are very complex
 Time temperature cycle chosen to optimize
strength and toughness
Cementite
ORF
Ferrite
BCC
Martensite
BCT Temper )(Ce)()(' 3
 
aa

51. Austenite
Pearlite
Pearlite + Bainite
Bainite
Martensite100
200
300
400
600
500
800
723
0.1 1 10 102 103 104
105
Eutectoid temperature
Ms
Mf
t (s) →
T→
a + Fe3C
MARTEMPERING
AUSTEMPERING
 To avoid residual stresses generated during quenching
 Austenized steel is quenched above Ms for homogenization of temperature
across the sample
 The steel is then quenched and the entire sample transforms simultaneously
 Tempering follows
 To avoid residual stresses generated during quenching
 Austenized steel is quenched above Ms
 Held long enough for transformation to Bainite
Martempering
Austempering

52. % Carbon →
Hardness(Rc)→
20
40
60
0.2 0.4 0.6
Harness of Martensite as a
function of Carbon content
Properties of 0.8% C steel
Constituent Hardness (Rc) Tensile strength (MN / m2)
Coarse pearlite 16 710
Fine pearlite 30 990
Bainite 45 1470
Martensite 65 -
Martensite tempered at 250 oC 55 1990

53. Examples
• Unusual combinations of properties can be
obtained by producing a steel with a microstructure
containing 50% ferrite and 50% martensite. The
martensite provides strength, and the ferrite
provides ductility and toughness. Design a heat
treatment to produce a dual phase steel in which
the composition of the martensite is 0.60% C.

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

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

56. Mechanical Properties: Fe-C System

57.  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.
Tempered Martensite
4340 steel

58. Hardness as a function of carbon
concentration for steels

59. Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080) that
has been rapidly quenched to form martensite.
Rockwell C and Brinell Hardness

61. 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.
• 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

62. 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).
Phase Diagram for Precipitation Hardened Alloy

63.  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.
Precipitation Heat Treatment

64. Solution Heat Treatment
• Heat treatable aluminum alloys gain strength from
subjecting the material to a sequence of processing steps
called solution heat treatment, quenching, and aging.
• The primary goal is to create sub-micron sized particles in
the aluminum matrix, called precipitates that in turn
influence the material properties.
• While simple in concept, the process variations required
(depending on alloy, product form, desired final property
combinations, etc.) make it sufficiently complex that heat
treating has become a professional specialty.
• The first step in the heat treatment process is solution heat
treatment. The objective of this process step is to place the
elements into solution that will eventually be called upon for
precipitation hardening.
• Developing solution heat treatment times and temperatures
has typically involved extensive trial and error, partially due
to the lack of accurate process models.

65. Aging-microstructure
• The supersaturated solid solution is
unstable and if, left alone, the excess q
will precipitate out of the a phase. This
process is called aging.
• Types of aging:
–Natural aging process occurs at room
temperature
–Artificial aging If solution heat treated,
requires heating to speed up the
precipitation

66. Overaging
• After solution heat treatment the material is ductile,
since no precipitation has occurred. Therefore, it may
be worked easily.
• After a time the solute material precipitates and
hardening develops.
• As the composition reaches its saturated normal state,
the material reaches its maximum hardness.
• The precipitates, however, continue to grow. The fine
precipitates disappear. They have grown larger, and as
a result the tensile strength of the material decreases.
This is called overaging.

67. Precipitation Heat Treatment
 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.
Guinier-Preston (GP) zones - Tiny clusters
of atoms that precipitate from the matrix in
the early stages of the age-hardening
process.

68. Hardness vs. Time
The hardness and tensile strength vary
during aging and overaging.

69. • 2014 Al Alloy:
• TS peak with precipitation time.
• Increasing T accelerates
process.
Influence of Precipitation Heat Treatment on
Tensile Strength (TS), %EL
precipitation heat treat time
tensilestrength(MPa)
200
300
400
100
1min 1h 1day 1mo 1yr
204°C
149°C
• %EL reaches minimum
with precipitation time.
%EL(2insample)
10
20
30
0
1min 1h 1day 1mo 1yr
204°C 149°C
precipitation heat treat time

70. 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.

71. Aluminum rivets
 Alloys that experience significant
precipitation hardening at room
temp, 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.

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

73. 0 10 20 30 40 50
wt% Cu
L
a+La
aq
q
q+L
300
400
500
600
700
(Al)
T(°C)
composition range
available for precipitation hardening
CuAl2
A
Precipitation Hardening
• Particles impede dislocation motion.
• Ex: Al-Cu system
• Procedure:
-- Pt B: quench to room temp.
(retain a solid solution)
-- Pt C: reheat to nucleate
small q particles within
a phase.
Temp.
Time
-- Pt A: solution heat treat
(get a solid solution)
Pt A (solution heat treat)
B
Pt B
C
Pt C (precipitate q)
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).

74. 24
• Hard precipitates are difficult to shear.
Ex: Ceramics in metals (SiC in Iron or Aluminum).
• Result: y ~
1
S
PRECIPITATION STRENGTHENING

75. Aging
• Aging either at room or moderately elevated
temperature after the quenching process is used to
produce the desired final product property
combinations.
• The underlying metallurgical phenomenon in the
aging process is precipitation hardening. Due to the
small size of the precipitate particles, early
understanding was hampered by the lack of
sufficiently powerful microscopes to actually see
them.
• With the availability of the transmission electron
microscope (TEM) with nanometer-scale resolution,
researchers were able to actually image many
precipitate phases and build on this knowledge to
develop improved aluminum alloy products.

76. Aluminum
• Aluminum is light weight,
but engineers want to
improve the strength for
high performance
applications in
automobiles and
aerospace.
• To improve strength,
they use precipitation
hardening.
Age-hardening heat treatment phase diagram

77. Quenching
• Quenching is the second
step in the process.
• Its purpose is to retain the
dissolved alloying elements
in solution for subsequent
precipitation hardening.
• Generally the more rapid the
quench the better, from a
properties standpoint, but
this must be balanced
against the concerns of part
distortion and residual stress
if the quench is non-uniform.
Changes in Microstructure due to quenching