Phase transformation (Material Science)

Phase transformation (Material Science)

1. Phase Transformation MYO ZIN AUNG 28J16121 Ship Design Lab. (NAOE)

2. Phase Transformation - Contents Change of Crystal Structure (Micro) Shape Memory Temperature Dependency of Linear Expansion Coefficient (Macro ) 2

3. 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

4. Phase Diagram for Water 4 3 Phases Solid Liquid Vapor

5. Crystal Structure 5 Face Centered Cubic Crystal Structure (FCC) Body-centered cubic crystal structure (BCC) Hexagonal close-packed crystal structure (HCP)

6. Atomic Packing Factor 6 Structure APF BCC 0.68 FCC 0.74 HCP 0.74

7. 3 Classifications Diffusion-dependent transformation (Simple) No change in number or composition of the phases present Solidification of a pure metal Allotropic Transformations Recrystallization and Grain Growth Diffusion-dependent transformation Some alternation in phase compositions Often alternation in the number of phases present Final microstructure ordinarily consists of 2 phases Eutectoid reaction Diffusionless transformation Metastable phase is produced Martensitic transformation in some steel alloys 7

8. Polymorphism or Allotropy 8 Iron exists in both BCC and FCC form depending on the temperature. Metals exist in more than one crystalline form Change of these forms is called Allotropic Transformation

9. Phase Diagram of Pure Iron 9 3 Solid Phases α Fe (BCC) γ Fe (FCC) δ Fe (BCC)

10. Cooling Curve of Pure Iron 10 Take times between Phases

11. White to Gray Tin 11 Body-centered tetragonal  Crystal structure similar to diamond The rate at which this change takes place is extremely slow; however, the lower the temperature (below 13.2 C ) the faster the rate Increase 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

12. 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 Nucleation Nucleation involves the appearance of very small particles, or nuclei of the new phase which are capable of growing. Growth During the growth stage these nuclei increase in size, which results in the disappearance of some (or all) of the parent phase . 12

13. Nucleation & Growth 13 ↑ t “For sufficient Undercooling”

14. Iron-Carbon System (Steel) Fe-Fe3C (Iron-Iron Carbide) Phase Diagram 14 Type Crystal Structure 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 Crystal Structure @1538 ˚C - above Cementite Compound Type Crystal Structure 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 Crystal Structure @1538 ˚C - above Cementite Compound

15. 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. 16 α -ferrite Austenite ( γ- iron)

17. 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 Iron Iron Cementite 0.008% 2.14% 6.7%

18. Eutectoid Alloys (0.76 wt % C) 18 Pearlite : a micro-constituent consisting of alternating layers of ferrite and cementite.  

19. Nucleation & growth of pearlite 19

20. Hypoeutectoid Alloys (< 0.76 wt % C) 20    

21. Hypereutectoid Alloys (> 0.76 wt % C) 21    

22. Ferrite/Cementite Transformation 22

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24. Properties of Different Phases of Steel Type Tensile Strength (psi) Hardness (Rockwell) Elongation (2 in.) Ferrite 40,000 C 0 or B 90 40 % softest structure on the diagram small amount of carbon dissolved in α (BCC) iron Ferromagnetic & 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 temperature Carbon dissolved in γ (F.C.C.) iron Non-magnetic & ductile Cementite ~ 5,000 Hardest structure in the diagram and Brittle Classified as ceramic in pure form Orthorhombic Crystal Structure 24

25. Microstructures and Mechanical Properties for Iron–Carbon Alloys 25

26. 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 26

27. 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

28. Austenite to Pearlite 28 Austenite Pearlite Eutectoid Steel (0.76 wt % C) Eutectoid Temp = 727 ˚C

29. Isothermal transformation diagram ( TTT Diagram ) 29

30. With superimposed isothermal heat treatment curve (ABCD) 30 Shortest time interval for Transformation

31. 31 Coarse & Fine Pearlite Coarse Pearlite Fine Pearlite

32. Bainite 32 The microstructure of bainite consists of ferrite and cementite phases, and thus diffusional processes are involved in its formation

33. 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

34. Spheroidite microstructure 34

35. 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

36. Unit Cell of Martensite 36 Body-centered tetragonal (BCT) Structure

37. 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

38. Martensite 38 The needleshape grains are the Martensite phase, and the white regions are austenite that failed to transform during the rapid quench

39. 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

40. 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

41. Isothermal transformation diagram for an alloy steel (type 4340) 41

42. 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

43. Different transformed products of Austenite Austenite Slow Cooling 43 Quenching Reheat Reheat Bainite Temper Martensite Martensite Pearlite Coarse Fine Spheroidite Moderate Cooling Isothermal Treatment Alloy Steel Plain Carbon Steel

44. 44 Mechanical Properties of Plain carbon steels having microstructures consisting of fine pearlite

45. Mechanical Properties of Different Microstructures 45

46. Microstructures and Mechanical Properties for Iron–Carbon Alloys 46

47. 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

48. 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 ) Ni Ti

49. The Shape Memory Effect 49 s e T Cooling Detwinning Heating/Recovery Stress Temperature Strain/ Defromation

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55. Shape Memory Alloys 55 Alloy Transformation Composition Transformation Temp. Rang (°C) Hysteresis (°C) Ag-Cd 44/49 at % Cd -190 to -50 ~15 Au-Cd 46.5/50 at % Cd 30 to 100 ~15 Cu-Al-Ni 14/14.5 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 ~10 Cu-Zn-X (X=Si,Sn,Al) few wt % X -180 to 200 ~10 In- Ti 18/23 at % Ti 60 to 100 ~4 Ni-Al 36/38 at % Al -180 to 100 ~10 Ni- Ti ~49/51 at % Ni -50 to 110 ~30 Fe-Pt ~25 at % Pt ~-130 ~4 Mn -Cu 5/35 wn % Cu -250 to 180 ~25 Fe- Mn -Si 32 wt % Mn -200 to 150 ~100

56. 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

57. Applications of Shape Memory Alloys 57

58. 58 Existing and potential SMA applications in the biomedical domain

59. SMAs in Bio-medical Devices

60. Bone Anchors Robotic arms Medical Stents

61. 61 Existing and potential SMA applications in the automotive domain

62. 62 Existing and potential SMA applications in the aerospace domain

63. Temperature Dependency of Linear Expansion Coefficient 63 Substances that expand at the same rate in every direction are called isotropic

64. 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.

65. 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:

66. 66 The linear expansion coefficient α vs. temperature for ceramic AlN samples

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68. 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 .

69. 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

70. Effects on strain 70