Chapter 10 Phase Transformations ISSUES TO ADDRESS Transforming

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Chapter 10: Phase Transformations ISSUES TO ADDRESS. . . • Transforming one phase into

Chapter 10: Phase Transformations ISSUES TO ADDRESS. . . • Transforming one phase into another takes time. Fe (Austenite) C FCC Fe C 3 Eutectoid transformation (cementite) + (ferrite) (BCC) • How does the rate of transformation depend on time and T ? • How can we slow down the transformation so that we can engineer non-equilibrium structures? • Are the mechanical properties of non-equilibrium structures better? Chapter 10 - 1

Phase Transformations Nucleation – nuclei (seeds) act as template to grow crystals – for

Phase Transformations Nucleation – nuclei (seeds) act as template to grow crystals – for nucleus to form rate of addition of atoms to nucleus must be faster than rate of loss – once nucleated, grow until reach equilibrium Driving force to nucleate increases as we increase T – supercooling (eutectic, eutectoid) Small supercooling few nuclei - large crystals Large supercooling rapid nucleation - many nuclei, small crystals Chapter 10 - 2

Solidification: Nucleation Processes • Homogeneous nucleation – nuclei form in the bulk of liquid

Solidification: Nucleation Processes • Homogeneous nucleation – nuclei form in the bulk of liquid metal – requires supercooling (typically 80 -300°C max) • Heterogeneous nucleation – much easier since stable “nucleus” is already present • Could be wall of mold or impurities in the liquid phase – allows solidification with only 0. 1 -10ºC supercooling Chapter 10 - 3

Homogeneous Nucleation & Energy Effects Surface Free Energy- destabilizes the nuclei (it takes energy

Homogeneous Nucleation & Energy Effects Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface) = surface tension GT = Total Free Energy = GS + GV Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy) r* = critical nucleus: nuclei < r* shrink; nuclei>r* grow (to reduce energy) Adapted from Fig. 10. 2(b), Callister 7 e. Chapter 10 - 4

Solidification r* = critical radius = surface free energy Tm = melting temperature HS

Solidification r* = critical radius = surface free energy Tm = melting temperature HS = latent heat of solidification T = Tm - T = supercooling Note: HS = strong function of T = weak function of T r* decreases as T increases For typical T r* ca. 100Å Chapter 10 - 5

Rate of Phase Transformations Kinetics - measure approach to equilibrium vs. time • Hold

Rate of Phase Transformations Kinetics - measure approach to equilibrium vs. time • Hold temperature constant & measure conversion vs. time How is conversion measured? X-ray diffraction – have to do many samples electrical conductivity – follow one sample sound waves – one sample Chapter 10 - 6

Fraction transformed, y Rate of Phase Transformation All out of material - done Fixed

Fraction transformed, y Rate of Phase Transformation All out of material - done Fixed T 0. 5 maximum rate reached – now amount unconverted decreases so rate slows rate increases as surface area increases t 0. 5 & nuclei grow log t Avrami rate equation => y = 1 - exp (-ktn) fraction transformed Adapted from Fig. 10, Callister 7 e. time – k & n fit for specific sample By convention r = 1 / t 0. 5 Chapter 10 - 7

Rate of Phase Transformations 135 C 119 C 1 10 113 C 102 C

Rate of Phase Transformations 135 C 119 C 1 10 113 C 102 C 88 C 102 43 C Adapted from Fig. 10. 11, Callister 7 e. (Fig. 10. 11 adapted from B. F. Decker and D. Harker, "Recrystallization in Rolled Copper", Trans AIME, 188, 1950, p. 888. ) 104 • In general, rate increases as T r = 1/t 0. 5 = A e -Q/RT – – R = gas constant T = temperature (K) A = preexponential factor Q = activation energy Arrhenius expression • r often small: equilibrium not possible! Chapter 10 - 8

Eutectoid Transformation Rate • Growth of pearlite from austenite: Adapted from Fig. 9. 15,

Eutectoid Transformation Rate • Growth of pearlite from austenite: Adapted from Fig. 9. 15, Callister 7 e. • Recrystallization rate increases with T. cementite (Fe 3 C) Ferrite ( ) pearlite growth direction 100 y (% pearlite) Austenite ( ) grain boundary Diffusive flow of C needed 600°C ( T larger) 50 650°C 675°C ( T smaller) Adapted from Fig. 10. 12, Callister 7 e. 0 Course pearlite formed at higher T - softer Fine pearlite formed at low T - harder Chapter 10 - 9

Nucleation and Growth • Reaction rate is a result of nucleation and growth of

Nucleation and Growth • Reaction rate is a result of nucleation and growth of crystals. 100 % Pearlite Nucleation rate increases with T Growth regime 50 Nucleation regime 0 t 0. 5 Growth rate increases with T log (time) Adapted from Fig. 10, Callister 7 e. • Examples: pearlite colony T just below TE Nucleation rate low Growth rate high T moderately below TE Nucleation rate med. Growth rate med. T way below TE Nucleation rate high Growth rate low Chapter 10 - 10

Transformations & Undercooling Þ + Fe 3 C • Eutectoid transf. (Fe-C System): •

Transformations & Undercooling Þ + Fe 3 C • Eutectoid transf. (Fe-C System): • Can make it occur at: 0. 76 wt% C 6. 7 wt% C 0. 022 wt% C . . . 727ºC (cool it slowly). . . below 727ºC (“undercool” it!) T(°C) 1600 d +L 1200 (austenite) 1000 +Fe 3 C Eutectoid: Equil. Cooling: Ttransf. = 727ºC 800 T 400 0 (Fe) 727°C +Fe 3 C Undercooling by Ttransf. < 727 C 0. 76 600 0. 022 ferrite L+Fe 3 C 1148°C 1 2 3 4 5 6 Fe 3 C (cementite) L 1400 Adapted from Fig. 9. 24, Callister 7 e. (Fig. 9. 24 adapted from Binary Alloy Phase Diagrams, 2 nd ed. , Vol. 1, T. B. Massalski (Ed. -in -Chief), ASM International, Materials Park, OH, 1990. ) 6. 7 Co , wt%C Chapter 10 - 11

Isothermal Transformation Diagrams y, % transformed • Fe-C system, Co = 0. 76 wt%

Isothermal Transformation Diagrams y, % transformed • Fe-C system, Co = 0. 76 wt% C • Transformation at T = 675°C. 100 T = 675°C 50 0 10 2 1 T(°C) Austenite (stable) 10 4 time (s) TE (727 C) 600 Pearlite isothermal transformation at 675°C 500 400 % 100 te 50%pearli 0% 700 Austenite (unstable) 1 10 10 2 10 3 10 4 10 5 Adapted from Fig. 10. 13, Callister 7 e. (Fig. 10. 13 adapted from H. Boyer (Ed. ) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1977, p. 369. ) time (s) Chapter 10 - 12

Effect of Cooling History in Fe-C System • Eutectoid composition, Co = 0. 76

Effect of Cooling History in Fe-C System • Eutectoid composition, Co = 0. 76 wt% C • Begin at T > 727°C • Rapidly cool to 625°C and hold isothermally. T(°C) Austenite (stable) 700 600 TE (727 C) Austenite (unstable) % 100 pea 50% 500 Adapted from Fig. 10. 14, Callister 7 e. (Fig. 10. 14 adapted from H. Boyer (Ed. ) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28. ) Pearlite 400 1 10 10 2 10 3 10 4 10 5 time (s) Chapter 10 - 13

Non-Equilibrium Transformation Products: Fe-C • Bainite: -- lathes (strips) with long rods of Fe

Non-Equilibrium Transformation Products: Fe-C • Bainite: -- lathes (strips) with long rods of Fe 3 C --diffusion controlled. • Isothermal Transf. Diagram 800 Austenite (stable) T(°C) A 100% pearlite/bainite boundary 100% bainite 400 a (ferrite) TE P 600 Fe 3 C (cementite) B A 10 103 (Adapted from Fig. 10. 17, Callister, 7 e. (Fig. 10. 17 from Metals Handbook, 8 th ed. , Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973. ) % 100 10 -1 50% 0% 200 5 m 105 time (s) Adapted from Fig. 10. 18, Callister 7 e. (Fig. 10. 18 adapted from H. Boyer (Ed. ) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28. ) Chapter 10 - 14

Spheroidite: Fe-C System • Spheroidite: (ferrite) -- grains with spherical Fe 3 C --diffusion

Spheroidite: Fe-C System • Spheroidite: (ferrite) -- grains with spherical Fe 3 C --diffusion dependent. --heat bainite or pearlite for long times Fe 3 C --reduces interfacial area (driving force) (cementite) 60 m (Adapted from Fig. 10. 19, Callister, 7 e. (Fig. 10. 19 copyright United States Steel Corporation, 1971. ) Chapter 10 - 15

Martensite: Fe-C System • Martensite: -- (FCC) to Martensite (BCT) Fe atom sites x

Martensite: Fe-C System • Martensite: -- (FCC) to Martensite (BCT) Fe atom sites x x x 60 m (involves single atom jumps) potential C atom sites x x x (Adapted from Fig. 10. 20, Callister, 7 e. • Isothermal Transf. Diagram 800 Austenite (stable) T(°C) A P 600 Adapted from Fig. 10. 22, Callister 7 e. 400 A 200 10 -1 B 5 0% 0% 10 10 TE 0% 50% 90% 103 (Adapted from Fig. 10. 21, Callister, 7 e. (Fig. 10. 21 courtesy United States Steel Corporation. ) • to M transformation. . 0% M+A M+A Martensite needles Austenite 105 -- is rapid! -- % transf. depends on T only. time (s) Chapter 10 - 16

Martensite Formation (FCC) slow cooling (BCC) + Fe 3 C quench M (BCT) tempering

Martensite Formation (FCC) slow cooling (BCC) + Fe 3 C quench M (BCT) tempering M = martensite is body centered tetragonal (BCT) Diffusionless transformation BCT few slip planes BCT if C > 0. 15 wt% hard, brittle Chapter 10 - 17

Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni,

Phase Transformations of Alloys Effect of adding other elements Change transition temp. Cr, Ni, Mo, Si, Mn retard + Fe 3 C transformation Adapted from Fig. 10. 23, Callister 7 e. Chapter 10 - 18

Cooling Curve plot temp vs. time Adapted from Fig. 10. 25, Callister 7 e.

Cooling Curve plot temp vs. time Adapted from Fig. 10. 25, Callister 7 e. Chapter 10 - 19

Dynamic Phase Transformations On the isothermal transformation diagram for 0. 45 wt% C Fe-C

Dynamic Phase Transformations On the isothermal transformation diagram for 0. 45 wt% C Fe-C alloy, sketch and label the time-temperature paths to produce the following microstructures: a) 50% fine pearlite and 50% bainite b) 100% martensite c) 50% martensite and 50% austenite Chapter 10 - 20

Example Problem for Co = 0. 45 wt% a) 50% fine pearlite and 50%

Example Problem for Co = 0. 45 wt% a) 50% fine pearlite and 50% bainite 800 first make pearlite T (°C) then bainite A P B 600 fine pearlite lower T A+a A+B A 400 A+P 50% M (start) M (50%) M (90%) 200 Adapted from Fig. 10. 29, Callister 5 e. 0 0. 1 10 103 time (s) 105 Chapter 10 - 21

Example Problem for Co = 0. 45 wt% b) 100 % martensite – quench

Example Problem for Co = 0. 45 wt% b) 100 % martensite – quench = rapid cool 1. c) 50 % 800 martensite A+a A T (°C) and 50 % A+P austenite P 600 B A+B A 400 50% M (start) M (50%) M (90%) d) 200 Adapted from Fig. 10. 29, Callister 5 e. c) 0 0. 1 10 103 time (s) 105 Chapter 10 - 22

Mechanical Prop: Fe-C System (1) • Effect of wt% C Adapted from Fig. 9.

Mechanical Prop: Fe-C System (1) • Effect of wt% C Adapted from Fig. 9. 30, Callister 7 e. (Fig. 9. 30 courtesy Republic Steel Corporation. ) TS(MPa) 1100 YS(MPa) Co < 0. 76 wt% C Hypoeutectoid Hypo Hyper Co > 0. 76 wt% C Adapted from Fig. 9. 33, Callister 7 e. 9. 33 copyright 1971 by United Hypereutectoid (Fig. States Steel Corporation. ) %EL Hypo Hyper 80 100 900 hardness 40 700 50 500 0 0. 5 1 0 Adapted from Fig. 10. 29, Callister 7 e. (Fig. 10. 29 based on data from Metals Handbook: Heat Treating, Vol. 4, 9 th ed. , V. Masseria (Managing Ed. ), American Society for Metals, 1981, p. 9. ) 0. 76 0 0. 76 300 Impact energy (Izod, ft-lb) Pearlite (med) ferrite (soft) Pearlite (med) Cementite (hard) 1 0. 5 0 wt% C • More wt% C: TS and YS increase, %EL decreases. Chapter 10 - 23

Mechanical Prop: Fe-C System (2) • Fine vs coarse pearlite vs spheroidite Hypo Hyper

Mechanical Prop: Fe-C System (2) • Fine vs coarse pearlite vs spheroidite Hypo Hyper 90 Hypo Hyper fine pearlite 240 160 coarse pearlite spheroidite 80 0 • Hardness: • %RA: 0. 5 1 wt%C Ductility (%AR) Brinell hardness 320 spheroidite 60 coarse pearlite fine pearlite 30 0 0 fine > coarse > spheroidite fine < coarse < spheroidite 0. 5 1 wt%C Adapted from Fig. 10. 30, Callister 7 e. (Fig. 10. 30 based on data from Metals Handbook: Heat Treating, Vol. 4, 9 th ed. , V. Masseria (Managing Ed. ), American Society for Metals, 1981, pp. 9 and 17. ) Chapter 10 - 24

Mechanical Prop: Fe-C System (3) • Fine Pearlite vs Martensite: Brinell hardness Hypo 600

Mechanical Prop: Fe-C System (3) • Fine Pearlite vs Martensite: Brinell hardness Hypo 600 Hyper martensite Adapted from Fig. 10. 32, Callister 7 e. (Fig. 10. 32 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. 8 A, p. 1776. ) 400 200 0 fine pearlite 0 0. 5 1 wt% C • Hardness: fine pearlite << martensite. Chapter 10 - 25

Tempering Martensite • reduces brittleness of martensite, • reduces internal stress caused by quenching.

Tempering Martensite • reduces brittleness of martensite, • reduces internal stress caused by quenching. TS(MPa) YS(MPa) 1800 Adapted from Fig. 10. 34, Callister 1400 7 e. (Fig. 10. 34 adapted from Fig. 1200 furnished courtesy of Republic Steel 1000 Corporation. ) TS YS 60 50 %RA 40 30 %RA 800 200 400 9 m 1600 Adapted from Fig. 10. 33, Callister 7 e. (Fig. 10. 33 copyright by United States Steel Corporation, 1971. ) 600 Tempering T (°C) • produces extremely small Fe 3 C particles surrounded by . • decreases TS, YS but increases %RA Chapter 10 - 26

Summary: Processing Options Austenite ( ) slow cool moderate cool Adapted from Fig. 10.

Summary: Processing Options Austenite ( ) slow cool moderate cool Adapted from Fig. 10. 36, Callister 7 e. rapid quench Bainite Martensite ( + Fe 3 C layers + a proeutectoid phase) ( + Fe 3 C plates/needles) (BCT phase diffusionless transformation) Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends reheat Ductility Strength Pearlite Tempered Martensite ( + very fine Fe 3 C particles) Chapter 10 - 27

ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 10 - 28

ANNOUNCEMENTS Reading: Core Problems: Self-help Problems: Chapter 10 - 28