Chapter 10: Phase Transformations

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1 Chapter 10: Phase Transformations ISSUES TO ADDRESS... Transforming one phase into another takes time. Fe (Austenite) Eutectoid transformation Fe 3 C (cementite) + C FCC (ferrite) (BCC) How does the rate of transformation depend on time and temperature? Is it possible to slow down transformations so that non-equilibrium structures are formed? Are the mechanical properties of non-equilibrium structures more desirable than equilibrium ones? Chapter 10-1

2 Nucleation Phase Transformations nuclei (seeds) act as templates on which crystals grow for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss once nucleated, growth proceeds until equilibrium is attained Driving force to nucleate increases as we increase T supercooling (eutectic, eutectoid) superheating (peritectic) Small supercooling slow nucleation rate - few nuclei - large crystals Large supercooling rapid nucleation rate - many nuclei - small crystals Chapter 10-2

3 Solidification: Nucleation Types Homogeneous nucleation nuclei form in the bulk of liquid metal requires considerable supercooling (typically ºC) Heterogeneous nucleation much easier since stable nucleating surface is already present e.g., mold wall, impurities in liquid phase only very slight supercooling (0.1-10ºC) Chapter 10-3

4 Homogeneous Nucleation & Energy Effects Surface Free Energy - destabilizes the nuclei (it takes energy to make an interface) 2 G S 4 r = surface tension r* = critical nucleus: for r < r* nuclei shrink; for r >r* nuclei grow (to reduce energy) Adapted from Fig.10.2(b), Callister & Rethwisch 8e. G T = Total Free Energy = G S + G V Volume (Bulk) Free Energy stabilizes the nuclei (releases energy) 4 3 G V r G G 3 volume free energy unit volume Chapter 10-4

5 r * 2 H f Tm T Solidification r* = critical radius = surface free energy T m = melting temperature H f = latent heat of solidification T = T m - T = supercooling Note: H f and are weakly dependent on T r* decreases as T increases For typical T r* ~ 10 nm Chapter 10-5

6 Rate of Phase Transformations Kinetics - study of reaction rates of phase transformations To determine reaction rate measure degree of transformation as function of time (while holding temp constant) How is degree of transformation measured? X-ray diffraction many specimens required electrical conductivity measurements on single specimen measure propagation of sound waves on single specimen Chapter 10-6

7 Fraction transformed, y Rate of Phase Transformation Fixed T transformation complete 0.5 t 0.5 log t Avrami equation => y = 1- exp (-kt n ) fraction transformed maximum rate reached now amount unconverted decreases so rate slows rate increases as surface area increases & nuclei grow time Adapted from Fig , Callister & Rethwisch 8e. k & n are transformation specific parameters By convention rate = 1 / t 0.5 Chapter 10-7

8 Temperature Dependence of Transformation Rate 135 C 119 C 113 C 102 C 88 C 43 C Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from B.F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888.) For the recrystallization of Cu, since rate = 1/t 0.5 rate increases with increasing temperature Rate often so slow that attainment of equilibrium state not possible! Chapter 10-8

9 Fe 3 C (cementite) Transformations & Undercooling Eutectoid transf. (Fe-Fe 3 C system): For transf. to occur, must cool to below 727ºC (i.e., must undercool ) ferrite T(ºC) +L (austenite) Eutectoid: 1148ºC L + Fe 3 C 0.76 wt% C 6.7 wt% C wt% C +Fe 3 C Equil. Cooling: T transf. = 727ºC T +Fe 3 C Undercooling by T transf. < 727 C L+Fe 3 C 727ºC Adapted from Fig. 9.24,Callister & Rethwisch 8e. (Fig adapted from Binary Alloy Phase Diagrams, 2nd ed., Vol. 1, T.B. Massalski (Ed.-in- Chief), ASM International, Materials Park, OH, 1990.) (Fe) C, wt%c Chapter 10-9

10 y (% pearlite) The Fe-Fe 3 C Eutectoid Transformation Transformation of austenite to pearlite: Austenite ( ) grain boundary Adapted from Fig. 9.15, Callister & Rethwisch 8e. cementite (Fe 3 C) Ferrite ( ) pearlite growth direction For this transformation, 100 rate increases with 600ºC ( T larger) [T eutectoid T ] (i.e., T). 650ºC Adapted from ºC ( T smaller) Diffusion of C during transformation Carbon diffusion Fig , Callister & Rethwisch 8e. Coarse pearlite formed at higher temperatures relatively soft Fine pearlite formed at lower temperatures relatively hard Chapter 10-10

11 y, % transformed Generation of Isothermal Transformation Diagrams Consider: The Fe-Fe 3 C system, for C 0 = 0.76 wt% C A transformation temperature of 675ºC T = 675ºC T(ºC) Austenite (unstable) Austenite (stable) Pearlite isothermal transformation at 675ºC time (s) T E (727ºC) time (s) Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369.) Chapter 10-11

12 Austenite-to-Pearlite Isothermal Transformation Eutectoid composition, C 0 = 0.76 wt% C Begin at T > 727ºC Rapidly cool to 625ºC Hold T (625ºC) constant (isothermal treatment) T(ºC) 700 Austenite (unstable) Austenite (stable) T E (727ºC) Pearlite Adapted from Fig ,Callister & Rethwisch 8e. (Fig adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) time (s) Chapter 10-12

13 Fe 3 C (cementite) Transformations Involving Noneutectoid Compositions Consider C 0 = 1.13 wt% C 900 T(ºC) 1600 T(ºC) A A + C A A + P P T E (727ºC) (austenite) T +L L +Fe 3 C +Fe 3 C L+Fe 3 C 727ºC Adapted from Fig , Callister & Rethwisch 8e. time (s) (Fe) Adapted from Fig. 9.24, Callister & Rethwisch 8e. C, wt%c Hypereutectoid composition proeutectoid cementite Chapter 10-13

14 800 T(ºC) A Austenite (stable) A Bainite: Another Fe-Fe 3 C Transformation Product Bainite: -- elongated Fe 3 C particles in -ferrite matrix -- diffusion controlled Isothermal Transf. Diagram, C 0 = 0.76 wt% C P 100% pearlite 100% bainite B T E Fe 3 C (cementite) (ferrite) 5 m Adapted from Fig , Callister & Rethwisch 8e. (Fig from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.) Adapted from Fig , Callister & Rethwisch 8e. time (s) Chapter 10-14

15 Spheroidite: Another Microstructure for the Fe-Fe 3 C System Spheroidite: -- Fe 3 C particles within an -ferrite matrix -- formation requires diffusion -- heat bainite or pearlite at temperature just below eutectoid for long times -- driving force reduction of -ferrite/fe 3 C interfacial area (ferrite) Fe 3 C (cementite) 60 m Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright United States Steel Corporation, 1971.) Chapter 10-15

16 60 m Martensite: A Nonequilibrium Transformation Product Martensite: -- (FCC) to Martensite (BCT) Fe atom sites x x x x x x potential C atom sites Adapted from Fig , Callister & Rethwisch 8e. Isothermal Transf. Diagram Adapted from Fig , Callister & Rethwisch 8e. 800 T(ºC) A Austenite (stable) A M + A M + A M + A P B T E 0% 50% 90% time (s) Martensite needles Austenite Adapted from Fig , Callister & Rethwisch 8e. (Fig courtesy United States Steel Corporation.) to martensite (M) transformation.. -- is rapid! (diffusionless) -- % transf. depends only on T to which rapidly cooled Chapter 10-16

17 Martensite Formation (FCC) slow cooling (BCC) + Fe 3 C quench M (BCT) tempering Martensite (M) single phase Diffusionless transformation BCT has body centered tetragonal (BCT) crystal structure BCT if C 0 > 0.15 wt% C few slip planes hard, brittle Chapter 10-17

18 Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni, Mo, Si, Mn retard + Fe 3 C reaction (and formation of pearlite, bainite) Adapted from Fig , Callister & Rethwisch 8e. Chapter 10-18

19 Conversion of isothermal transformation diagram to continuous cooling transformation diagram Continuous Cooling Transformation Diagrams Adapted from Fig , Callister & Rethwisch 8e. Cooling curve Chapter 10-19

20 Isothermal Heat Treatment Example Problems On the isothermal transformation diagram for a 0.45 wt% C, Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures: a) 42% proeutectoid ferrite and 58% coarse pearlite b) 50% fine pearlite and 50% bainite c) 100% martensite d) 50% martensite and 50% austenite Chapter 10-20

21 Solution to Part (a) of Example Problem a) 42% proeutectoid ferrite and 58% coarse pearlite Isothermally treat at ~ 680ºC -- all austenite transforms to proeutectoid and coarse pearlite. W pearlite C = = T (ºC) A A M (start) M (50%) M (90%) Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C A + P B A + P A + B 50% W = = 0.42 Adapted from Fig , Callister 5e time (s) Chapter 10-21

22 Solution to Part (b) of Example Problem b) 50% fine pearlite and 50% bainite Isothermally treat at ~ 590ºC T (ºC) 50% of austenite transforms to fine pearlite. Then isothermally treat at ~ 470ºC all remaining austenite transforms to bainite A A M (start) M (50%) M (90%) Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C A + P B A + P A + B 50% Adapted from Fig , Callister 5e time (s) Chapter 10-22

23 Solutions to Parts (c) & (d) of Example Problem c) 100% martensite rapidly quench to room temperature d) 50% martensite & 50% austenite -- rapidly quench to ~ 290ºC, hold at this temperature 800 T (ºC) A A M (start) M (50%) M (90%) Fe-Fe 3 C phase diagram, for C 0 = 0.45 wt% C A + P B A + P A + B 50% d) Adapted from Fig , Callister 5e. c) time (s) Chapter 10-23

24 Impact energy (Izod, ft-lb) Mechanical Props: Influence of C Content Pearlite (med) ferrite (soft) Adapted from Fig. 9.30, Callister & Rethwisch 8e. C 0 < 0.76 wt% C Hypoeutectoid C 0 > 0.76 wt% C Hypereutectoid Pearlite (med) C ementite (hard) Adapted from Fig. 9.33, Callister & Rethwisch 8e. TS(MPa) 1100 YS(MPa) Hypo Hyper hardness wt% C %EL wt% C Increase C content: TS and YS increase, %EL decreases 50 Hypo Hyper Adapted from Fig , Callister & Rethwisch 8e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, p. 9.) Chapter 10-24

25 Brinell hardness Ductility (%RA) Mechanical Props: Fine Pearlite vs. Coarse Pearlite vs. Spheroidite Hardness: %RA: Hypo Hyper fine pearlite coarse pearlite spheroidite wt%c fine > coarse > spheroidite fine < coarse < spheroidite Hypo Hyper spheroidite 30 coarse pearlite fine pearlite wt%c Adapted from Fig , Callister & Rethwisch 8e. (Fig based on data from Metals Handbook: Heat Treating, Vol. 4, 9th ed., V. Masseria (Managing Ed.), American Society for Metals, 1981, pp. 9 and 17.) Chapter 10-25

26 Brinell hardness Mechanical Props: Fine Pearlite vs. Martensite Hypo Hyper martensite fine pearlite wt% C Hardness: fine pearlite << martensite. Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from Edgar C. Bain, Functions of the Alloying Elements in Steel, American Society for Metals, 1939, p. 36; and R.A. Grange, C.R. Hribal, and L.F. Porter, Metall. Trans. A, Vol. 8A, p ) Chapter 10-26

27 9 m Tempered Martensite Heat treat martensite to form tempered martensite tempered martensite less brittle than martensite tempering reduces internal stresses caused by quenching TS(MPa) YS(MPa) 1800 Adapted from Fig , Callister & Rethwisch 8e. (Fig adapted from Fig. furnished courtesy of Republic Steel Corporation.) YS %RA TS %RA Tempering T (ºC) Adapted from Fig , Callister & Rethwisch 8e. (Fig copyright by United States Steel Corporation, 1971.) tempering produces extremely small Fe 3 C particles surrounded by tempering decreases TS, YS but increases %RA Chapter 10-27

28 Strength Ductility Summary of Possible Transformations slow cool Austenite ( ) moderate cool rapid quench Adapted from Fig , Callister & Rethwisch 8e. Pearlite ( + Fe 3 C layers + a proeutectoid phase) Bainite ( + elong. Fe 3 C particles) Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Martensite (BCT phase diffusionless transformation) reheat Tempered Martensite ( + very fine Fe 3 C particles) Chapter 10-28

29 ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 10-29