Post on 19-Jan-2017
Phase Transformation MYO ZIN AUNG 28J16121 Ship Design Lab. (NAOE)
Phase Transformation - Contents
1. Change of Crystal Structure (Micro)
2. Shape Memory
3. Temperature Dependency of Linear Expansion
Coefficient (Macro)
2
Why STUDY Phase Transformation?• Tensile strength of iron-carbon alloy of eutectoid
composition can be varied between 700 MPa and 2000 MPa depending on heat treatment employed. • This shows that the desirable mechanical properties
of a material can be obtained as a result of phase transformations using heat treatment processes.• The time and temperature dependencies of phase transformations are represented on phase diagrams.• It is important to know how to use these phase diagrams in order to design a heat treatment for alloy to obtain the desired room-temperature mechanical properties.
3
Phase Diagram for Water
4
3 Phases1.Solid2.Liquid3.Vapor
Crystal Structure
5Face Centered Cubic
Crystal Structure (FCC)
Body-centered cubic crystal structure (BCC)
Hexagonal close-packed crystal structure (HCP)
Atomic Packing Factor
6
Structure APF
BCC 0.68
FCC 0.74
HCP 0.74
3 Classifications1. Diffusion-dependent transformation (Simple)
No change in number or composition of the phases presentSolidification of a pure metalAllotropic TransformationsRecrystallization and Grain Growth
2. Diffusion-dependent transformationSome alternation in phase compositionsOften alternation in the number of phases presentFinal microstructure ordinarily consists of 2 phasesEutectoid reaction
3. Diffusionless transformationMetastable phase is producedMartensitic transformation in some steel alloys 7
Polymorphism or Allotropy
8Iron exists in both BCC and FCC form depending on the temperature.
Metals exist in more than one crystalline formChange of these forms is called Allotropic Transformation
Phase Diagram of Pure Iron
9
3 Solid Phases1.α Fe (BCC)2.γ Fe (FCC)3.δ Fe (BCC)
Cooling Curve of Pure Iron
10Take times between Phases
White to Gray Tin
11
Body-centered tetragonal Crystal structure similar to diamond
The rate at which this change takes place is extremelyslow; however, the lower the temperature (below13.2 C) the faster the rateIncrease in volume (27%), a decrease in density (from 7.30 g/cm3 to 5.77 g/cm3). This volume expansion results in the disintegration of the white tin metal into a coarse powder of the grey allotrope
How transform?• Most phase transformations do not occur
instantaneously• They begin by the formation of numerous small
particles of the new phase(s), which increase in size until the transformation has reached completion• 2 stages of Phase Transformation
1. Nucleation• Nucleation involves the appearance of very small particles,
or nuclei of the new phase which are capable of growing.2. Growth• During the growth stage these nuclei increase in size,
which results in the disappearance of some (or all) of the parent phase. 1
2
Nucleation & Growth
13
↑ t
“For sufficientUndercooling”
Iron-Carbon System (Steel)• Fe-Fe3C (Iron-Iron Carbide) Phase Diagram
14
Type CrystalStructure Temperature
Ferrite α-iron BCC Room Temperature(Stable Form)
Austenite γ-iron FCC @ 912 ˚C – 1394 ˚C δ-ferrite BCC @ 1394 ˚C – 1538 ˚C
Liquid No CrystalStructure
@1538 ˚C - above
Cementite Compound
Phases of Iron-Carbon Alloys
15
Steel is stronger than pure iron because of the carbon atoms in the void space of unit cell.
16
α-ferrite Austenite (γ-iron)
Fe-Fe3C (Iron-Iron Carbide) Phase Diagram
17
6.7 wt% C means 100% Fe3C
Not interested in more than 6.7 wt% C
Mechanically, cementite is very hard and brittle; the strength of some steels is greatly enhanced by its presence.
Steel
Eutectoid composition - 0.76 wt% C
Eutectoid temperature – 727 ˚C
Cast IronIron Cementite0.008% 2.14% 6.7%
Eutectoid Alloys (0.76 wt% C)
18
Pearlite: a micro-constituent consisting of alternating layers of ferrite and cementite.
𝛾→𝛼+𝐹 𝑒3𝐶
Nucleation & growth of pearlite
19
Hypoeutectoid Alloys (< 0.76 wt% C)
20
𝛾→𝛾+𝛼 (𝑃𝑟𝑜𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 )
𝛼 (𝐸𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 )+𝐹𝑒3𝐶+𝛼(𝑃𝑟𝑜𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑)
Hypereutectoid Alloys (> 0.76 wt% C)
21
𝛾→𝛾+𝐹𝑒3𝐶 (𝑃𝑟𝑜𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 )
𝛼+𝐹𝑒3𝐶 (𝐸𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑 )+𝐹𝑒3𝐶 (𝑃𝑟𝑜𝑒𝑢𝑡𝑒𝑐𝑡𝑜𝑖𝑑)
Ferrite/Cementite Transformation
22
Properties of Different Phases of Steel
TypeTensile
Strength (psi)
Hardness(Rockwell)
Elongation
(2 in.)
Ferrite 40,000 C 0 or B 90 40 %softest structure on the diagramsmall amount of carbon dissolved in α (BCC) ironFerromagnetic & Fairly ductile
Pearlite 120,000 C 20 or B 95-100 20 %
α-Ferrite + Cementite
Austenite 150,000 ~ C 40 10 %
normally not stable at room temperature. But, under certain conditions it is possible to obtain austenite at room temperatureCarbon dissolved in γ (F.C.C.) ironNon-magnetic & ductile
Cementite ~ 5,000
Hardest structure in the diagram and BrittleClassified as ceramic in pure formOrthorhombic Crystal Structure 2
4
How to do “Phase Transformations”?• By varying Temperature, Composition, and the external Pressure• Temperature Changes by means of Heat Treatments are most
conveniently utilized• Crossing a Phase Boundary on the Composition–Temperature phase diagram as an alloy of given composition is heated or cooled• Most phase transformations require some finite time to go to
completion (to get the equilibrium state) – need to wait to finish• The speed or rate is often important in the relationship between
the heat treatment and the development of microstructure• One limitation of phase diagrams is their inability to indicate the time period required for the attainment of equilibrium 2
6
Equilibrium vs Metastable• The rate of approach to equilibrium for solid systems is so slow.• Equilibrium conditions are maintained only if heating or cooling
is carried out at extremely slow and unpractical rates.• For other-than-equilibrium cooling, transformations are shifted to
lower temperatures than indicated by the phase diagram. (Supercooling)• for heating, the shift is to higher temperatures (Superheating)• For many technologically important alloys, the preferred state or
microstructure is a metastable one (e.g. Martensite)• Intermediate between the initial and equilibrium states• It thus becomes imperative to investigate the influence of time on phase transformations.
27
Austenite to Pearlite
28Austenite
Pearlite
Eutectoid Steel (0.76 wt% C)Eutectoid Temp = 727 ˚C
Isothermal transformation diagram ( TTT Diagram )
29
With superimposed isothermal heat treatment curve (ABCD)
30Shortest time interval for Transformation
31
Coarse & Fine Pearlite
Coarse Pearlite Fine Pearlite
Bainite
32
The microstructure of bainite consists offerrite and cementite phases, and thus diffusional processes are involved in its formation
Spheroidite• If a steel alloy having either pearlitic or bainitic
microstructures is heated to, and left at, a temperature below the eutectoid for a sufficiently long period of time—for example, at about 700C (1300F) for between 18 and 24 h—yet another microstructure will form called spheroidite• Instead of the alternating ferrite and cementite lamellae
(pearlite) or the microstructure observed for bainite, the Fe3C phase appears as spherelike particles embedded in a continuous a–phase matrix.• The kinetics of spheroidite formation is not included on
isothermal transformation diagrams.33
Spheroidite microstructure
34
Martensite• Martensite is formed when austenite alloys are rapidly cooled (or quenched) to a relatively low temperature (in the vicinity of the ambient).• Martensite is a nonequilibrium single-phase structure
that results from a diffusionless transformation of austenite.• It may be thought of as a transformation product that is
competitive with pearlite and bainite.• The martensitic transformation occurs when the
quenching rate is rapid enough to prevent carbon diffusion.• Any diffusion whatsoever results in the formation of
ferrite and cementite phases. 35
Unit Cell of Martensite
36
Body-centered tetragonal (BCT) Structure
37
Ferrite
Cementite
Ferrite matrix and elongated particles of Fe3C
Pearlite
Bainite Diffusion Dependent
Austenite (FCC) Martensite (BCT)
• Diffusionless Transformation
• No enough time to form Pearlite or Bainite
Very Hard and Brittle
Austenite
Very Rapid Cooling
(Quenching)
Moderate Cooling
Slow Cooling
Cooling
Super-saturated solid solution of carbon in ferrite
Martensite
38
The needleshape grains are the Martensite phase, and the white regions are austenite that failed to transform during the rapid quench
Cooling Rate
39
Continuous-cooling transformation diagram for a eutectoid iron–carbon alloy and superimposed cooling curves, demonstrating the dependence of the final microstructure on the transformations that occur during cooling
Tempered Martensite• In the as-quenched state, martensite, is very hard, but
so brittle • So it cannot be used for most applications• Any internal stresses that may have been introduced
during quenching have a weakening effect.• The ductility and toughness of martensite may be
enhanced and these internal stresses relieved by a heat treatment known as tempering.• By heating to a temperature below the eutectoid for a
specified time period
40
between 250˚C and 650˚C
Diffusion Process
Isothermal transformation diagram for an alloy steel (type 4340)
41
42
Continuous-cooling transformation diagram for an alloy steel (type 4340) and several superimposed cooling curves demonstrating dependence of the final microstructure of this alloy on the transformations that occur during cooling
Different transformed products of Austenite
Austenite Slow Cooling
43
Quenching
ReheatReheat
Bainite Temper Martensite
Martensite
PearliteCoarse Fine
Spheroidite
Moderate CoolingIsothermal
Treatment
Alloy SteelPlain Carbon
Steel
44
Mechanical Properties of Plain carbon steels having microstructures consisting of fine pearlite
Mechanical Properties of Different Microstructures
45
Microstructures and Mechanical Properties for Iron–Carbon Alloys
46
Shape Memory Alloys (SMA)• SMA recover predefined shape when subjected to
appropriate heat treatment.• Recovers strain and exerts forces• Examples: AuCd, Cu-Zn-Al, Cu-Al-Ni, Ni-Ti• Processed using hot and cold forming techniques and
heat treated at 500-800 0C at desired shape.• At high temperature ---Regular cubic microstructure • (Austenite)• After cooling – Highly twinned platelets (Martensite)
47
Shape Memory Effect
48
• SMA easily deformed in martensite state due to twin boundaries and deformation is not recovered after load is removed.• Heating causes Martensite Austenite
transformation so shape is recovered.• Effect takes place over a range of temperature.
Heated(Austenite)
Cooled(Martensite)
Deformed(Martensite)
Heated(Austenite)
NiTi
The Shape Memory Effect
49
s
e
T
Cooling
Detwinning
Heating/Recovery
Stress
Temperature
Strain/ Defromation
50
51
52
53
54
Shape Memory Alloys
55
AlloyTransformation
CompositionTransformation Temp. Rang (°C)
Hysteresis (°C)
Ag-Cd 44/49 at % Cd -190 to -50 ~15Au-Cd 46.5/50 at % Cd 30 to 100 ~15
Cu-Al-Ni14/14.5 wt %Al, 3/4.5 wt %Ni
-140 to 100 ~35
Cu-Sn ~15 at % Sn -120 to 30 −Cu-Zn 38.5/41.5 wt % Zn -180 to -10 ~10Cu-Zn-X (X=Si,Sn,Al)
few wt % X -180 to 200 ~10
In-Ti 18/23 at % Ti 60 to 100 ~4Ni-Al 36/38 at % Al -180 to 100 ~10Ni-Ti ~49/51 at % Ni -50 to 110 ~30Fe-Pt ~25 at % Pt ~-130 ~4Mn-Cu 5/35 wn % Cu -250 to 180 ~25Fe-Mn-Si 32 wt % Mn -200 to 150 ~100
SMA Applications
56
• Micro-actuators• Mobile phone antennas• Orthodontic archwires• Penile implant• Pipe couplings• Robot actuators• Rock splitting• Root canal drills• Satellite antenna deployment• Scoliosis correction• Solar actuators• Spectacle frames• Steam valves• Stents• Switch vibration damper• Thermostats• Underwired bras• Vibration dampers• ZIF connectors
• Aids for disabled• Aircraft flap/slat adjusters• Anti-scald devices• Arterial clips• Automotive thermostats• Braille print punch• Catheter guide wires• Cold start vehicle actuators• Contraceptive devices• Electrical circuit breakers• Fibre-optic coupling• Filter struts• Fire dampers• Fire sprinklers• Gas discharge• Graft stents• Intraocular lens mount• Kettle switches• Keyhole instruments• Key-hole surgery instruments
Applications of Shape Memory Alloys
57
58
Existing and potential SMA applications in the biomedical domain
59
SMAs in Bio-medical Devices
60Bone Anchors
Robotic arms
Medical Stents
61
Existing and potential SMA applications in the automotive domain
62
Existing and potential SMA applications in the aerospace domain
Temperature Dependency of Linear Expansion Coefficient
63
Substances that expand at the same rate in every direction are called isotropic
Expansion Joints
64
If the body is constrained so that it cannot expand, then internal stress will be caused (or changed) by a change in temperature.
Linear Expansion
• This equation works well as long as the linear-expansion coefficient does not change much over the change in temperature , and the fractional change in length is small 1. • If either of these conditions does not hold, the equation must be
integrated. 65
• The change in the linear dimension can be estimated to be:
66The linear expansion coefficient α vs. temperature for ceramic AlN
samples
67
Effect of High Pressure Heat Treatment on Microstructure and Thermal Expansion Coefficients of CuAl Alloy
68
High pressure heat treatment involves three values: 1, 3 and 6 GPa.
The samples were held at 750°C under pressure for 10 min and subsequently cooled to room temperature by cutting off the power supply with the holding pressure unchanged.
Finally, the pressure was taken off.
Thermal expansion coefficients of CuAl alloy vs Temperature
69
Same Material (Cu-Al Alloy)
Different Heat Treatments
Different Microstructures
Different Thermal Expansion Coefficients for Different Temperature
Effects on strain
70
References1. Material Science & Engineering - An Introduction 9th
Edition (William D. Callister, Jr. & David G. Rethwisch)2. An Introduction to Shape Memory Alloys (SMAs)
(Mehrshad Mehrpouya)3. Thermal Expansion (Wikipedia)4. Effect of High Pressure Heat Treatment on
Microstructure and Thermal Expansion Coefficients of CuAl Alloy (Ma Yu-quan)
71
72