Chapter 11 Phase Transformations

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

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. 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

4. 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).

5. 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

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7. Nucleation and Growth • Rate is a result of nucleation and growth of crystals. • Examples:

8. Fe3C Fe g Eutectoid (cementite) transformation + (Austenite) a C (BCC) FCC (ferrite) Nucleation of a spherical solid particle in a liquid c11f01 • The change in free energy DG (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 a transformation: 1) the difference between the solid and liquid phases (volume free energy, DGV); and 2) the solid-liquid phase boundary (surface free energy, DGS). • Transforming one phase into another takes time. Liquid DG =DGS + DGV

9. Surface Free Energy-destabilizes the nuclei (it takes energy to make an interface) g = surface tension DGT = Total Free Energy = DGS + DGV Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy) Homogeneous Nucleation & Energy Effects r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy)

10. r* = critical radius g = surface free energy Tm = melting temperature Hf= latent heat of solidification (fusion) DT = Tm - T = supercooling  r* decreases asT increases For typicalTr* ~ 10nm Solidification • Note:Hf and  are weakly dependent on T

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

12. Rate of Phase Transformation Avrami equation => y = 1- exp (-ktn) transformation complete maximum rate reached – now amount unconverted decreases so rate slows rate increases as surface area increases & nuclei grow By convention rate = 1 / t0.5 Fixed T Fraction transformed, y 0.5 t0.5 Fraction transformed depends on time log t fraction transformed time 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. 12

13. 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. 135C 119C 113C 102C 88C 43C 1 10 102 104 Temperature Dependence of Transformation Rate 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.) Arrhenius expression

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

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

16. Isothermal Transformation Diagrams c11f13 • 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.

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

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

19. Transformations Involving Noneutectoid Compositions T(°C) T(°C) 900 1600 d A L 1400 800 TE (727°C) A g +L + g 1200 L+Fe3C A C (austenite) 700 P 1000 + Fe3C (cementite) g +Fe3C a P A 600 800 DT a +Fe3C 500 600 0.022 0.76 1 10 102 103 104 1.13 400 727°C time (s) 0 1 2 3 4 5 6 6.7 (Fe) C, wt%C Adapted from Fig. 10.28, Callister & Rethwisch 3e. Consider C0 = 1.13 wt% C Adapted from Fig. 11.16, Callister & Rethwisch 3e. Hypereutectoid composition – proeutectoid cementite 19

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

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

23. 800 Austenite (stable) TE T(°C) A P 100% pearlite 600 100% bainite B 400 A 200 0% 100% 50% 10 -1 3 5 10 10 10 time (s) Bainite: Non-Equil Transformation Products • elongated Fe3C particles in a-ferrite matrix • diffusion controlled • a lathes (strips) with long rods of Fe3C Martensite Cementite Ferrite

24. 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).

25. 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

26. c11f20 Pearlitic Steel partially transformed to Spheroidite

27. 800 Austenite (stable) TE T(°C) A P 600 B 400 A 100% 50% 0% 0% 200 M + A 50% M + A 90% M + A time (s) -1 10 3 5 10 10 10 Martensite Formation • Isothermal Transformation Diagram Martensite needles Austenite • 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

28. 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.

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

30. Effect of Adding Other Elements c11f24 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.

31. c11f23 • 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 austeniticstructure. • Treatment (a) • Rapidly cool to 350˚C • Hold for 104 seconds • Quench to room temperature Bainite, 100%

32. c11f23 • 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 austeniticstructure. • Treatment (b) • Rapidly cool to 250˚C • Hold for 100 seconds • Quench to room temperature Austenite, 100% Martensite, 100%

33. c11f23 • 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 austeniticstructure. • 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 Bainite, 50% Final: 50% Bainite, 50% Pearlite

34. Continuous Cooling Transformation Diagrams c11f26 • 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.

35. 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.

36. 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

37. 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.

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

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

40. Mechanical Properties: Fe-C System c11f31

41. Tempered Martensite c11f34 • 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

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

43. Rockwell C and Brinell Hardness c11f36 Hardness versus tempering time for a water-quenched eutectoid plain carbon steel (1080) that has been rapidly quenched to form martensite.

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45. 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

46. Phase Diagram for Precipitation Hardened Alloy c11f40 • 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).

47. Precipitation Heat Treatment c11f43 • 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.

48. 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 precipitatesthat 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.

49. 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 agingprocess occurs at room temperature Artificial agingIf solution heat treated, requires heating to speed up the precipitation

50. 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.