PhaseTransformations

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TOPIC 10: PHASE TRANSFORMATIONS: 

• Transforming one phase into another takes time. • How does the rate of transformation depend on time and T? • How can we control the transformation so that we can engineering non-equilibrium structures? • How different are the mechanical properties of non-equilibrium structures? TOPIC 10: PHASE TRANSFORMATIONS

IMPORTANCE OF COOLING TIME: 

IMPORTANCE OF COOLING TIME Cu-Ni alloy Slow cooling Equilibrium phases Fast cooling Non-equilibrium phases

COOLING AUSTENITE: 

COOLING AUSTENITE Mainly interested in eutectoid cooling: g  a + Fe3C (pearlite), 0.77 wt% C Cooling rate can result in a wide variety of phases and microstructures Equilibrium phases: pearlite, bainite Non-equilibrium phases: martensite

MECHANICAL PROPERTIES: 

MECHANICAL PROPERTIES Martensite Tempered martensite Bainite Fine pearlite Coarse pearlite Strength Ductility • Can control the formation of specific phases and microstructure so that desired properties result

FRACTION OF TRANSFORMATION: 

• Fraction transformed depends on time, at constant temperature (e.g., g  pearlite) • Transformation rate , r = 1/t0.5 FRACTION OF TRANSFORMATION Avrami equation (k, n are constants)

EUTECTOID TRANSFORMATION RATE ~ DT: 

• Growth of pearlite from austenite: • Reaction rate increases with DT. EUTECTOID TRANSFORMATION RATE ~ DT

TIME-TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS: 

• Fe-C system, Eutectoid composition (Co = 0.77wt%C) • Transformation at T = 675C. TIME-TEMPERATURE TRANSFORMATION (TTT) DIAGRAMS Also called isothermal transformation diagram

EX: COOLING HISTORY Fe-C SYSTEM: 

• Eutectoid composition, Co = 0.77wt%C • Begin at T > 727C • Rapidly cool to 625C and hold isothermally. • Cooling to lower temperatures results in finer microstructures EX: COOLING HISTORY Fe-C SYSTEM

PEARLITE MORPHOLOGY: 

• Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser. Two cases: • Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer. PEARLITE MORPHOLOGY

OTHER TRANSFORMATION PRODUCTS: 

• Bainite: --a strips with long, fine rods of Fe3C • Isothermal Transf. Diagram (Adapted from Fig. 10.8, Callister, 6e. (Fig. 10.8 from Metals Handbook, 8th ed., Vol. 8, Metallography, Structures, and Phase Diagrams, American Society for Metals, Materials Park, OH, 1973.) OTHER TRANSFORMATION PRODUCTS Note: reaction rate increases with decreasing temperature first, and then decreases

NUCLEATION AND GROWTH: 

• Reaction rate is a result of nucleation and growth of crystals. • Examples: NUCLEATION AND GROWTH Nucleation rate increases with DT Growth rate increases with T

OTHER PRODUCTS: MARTENSITE: 

• Martensite: --rapid cooling from above eutectoid temperature to room T --g(FCC) to Martensite (Body Centered Tetragonal) --involves collective motion of a lot of atoms • Isothermal Transf. Diagram • g to M transformation.. -- is rapid! At speed of sound -- % transf. depends on T only. OTHER PRODUCTS: MARTENSITE

OTHER PRODUCTS: Fe-C SYSTEM (2): 

11 • Martensite: --g(FCC) to Martensite (BCT) Adapted from Fig. 10.13, Callister 6e. (Adapted from Fig. 10.12, Callister, 6e. (Fig. 10.12 courtesy United States Steel Corporation.) • Isothermal Transf. Diagram • g to M transformation.. -- is rapid! -- % transf. depends on T only. (Adapted from Fig. 10.11, Callister, 6e. OTHER PRODUCTS: Fe-C SYSTEM (2)

PRODUCTS OF COOLING AUSTENITE: 

PRODUCTS OF COOLING AUSTENITE Slow cooling  pearlite Cool rapidly to upto 550 C, and hold  pearlite Cool rapidly to 550-225 C and hold  bainite Cool rapidly to below 225 C  martensite

COOLING EX: Fe-C SYSTEM (1): 

COOLING EX: Fe-C SYSTEM (1) 100% Austenite 100% Bainite 100% Bainite Rapidly cool to 350 C Hold for 10000 seconds Rapidly cool to room T

COOLING EX: Fe-C SYSTEM (2): 

COOLING EX: Fe-C SYSTEM (2) 100% Austenite 100% Austenite Mostly Martensite + traces of Austenite Rapidly cool to 250 C Hold for 100 seconds Rapidly cool to room T

COOLING EX: Fe-C SYSTEM (3): 

COOLING EX: Fe-C SYSTEM (3) 100% Austenite 50% Austenite, 50% Pearlite 50% Austenite, 50% Pearlite 50% Bainite, 50% Pearlite 50% Bainite, 50% Pearlite Rapidly cool to 650 C Hold for 20 seconds Rapidly cool to 400 C Hold for 1000 seconds Rapidly cool to room T

OTHER PRODUCTS: Fe-C SYSTEM (1): 

10 • Spheroidite: --a crystals with spherical Fe3C --diffusion dependent. --heat bainite or pearlite for long times --reduces interfacial area (driving force) • Isothermal Transf. Diagram Adapted from Fig. 10.9,Callister 6e. (Fig. 10.9 adapted from H. Boyer (Ed.) Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for Metals, 1997, p. 28.) (Adapted from Fig. 10.10, Callister, 6e. (Fig. 10.10 copyright United States Steel Corporation, 1971.) OTHER PRODUCTS: Fe-C SYSTEM (1)

TEMPERING MARTENSITE: 

18 • reduces brittleness of martensite, • reduces internal stress caused by quenching. Adapted from Fig. 10.24, Callister 6e. (Fig. 10.24 copyright by United States Steel Corporation, 1971.) Adapted from Fig. 10.25, Callister 6e. (Fig. 10.25 adapted from Fig. furnished courtesy of Republic Steel Corporation.) TEMPERING MARTENSITE

MECHANICAL PROPERTIES: 

MECHANICAL PROPERTIES Martensite Tempered martensite Bainite Fine pearlite Coarse pearlite Spheroidite Strength Ductility • Can control the formation of specific phases and microstructure through a cooling schedule so that desired properties result

HYPOEUTECTOID & HYPEREUTECTOID: 

HYPOEUTECTOID & HYPEREUTECTOID Eutectoid (0.77 wt% C)  pearlite (ferrite & cementite layers) Hypoeutectoid (< 0.77 wt% C)  pearlite & ferrite Hypereutectoid (> 0.77 wt% C)  pearlite & cementite Ferrite is soft and cementite is hard Thus, hardness and strength increase with carbon content Austenite Pearlite

HYPEReutectiod Steel TTT Curve: 

HYPEReutectiod Steel TTT Curve

Alloy Steel TTT Curve: 

Alloy Steel TTT Curve

Continuous Cooling Transformation (CCT): 

Continuous Cooling Transformation (CCT)

Continuous Cooling Transformation (CCT): 

Continuous Cooling Transformation (CCT)

Continuous Cooling Transformation (CCT): 

Continuous Cooling Transformation (CCT)

MECHANICAL PROP: Fe-C SYSTEM (1): 

MECHANICAL PROP: Fe-C SYSTEM (1)

MECHANICAL PROP: Fe-C SYSTEM (2): 

• Fine Pearlite vs Martensite: • Hardness: fine pearlite << martensite. Tempering martensite (holding at high temperature) reduces brittleness and residual stresses MECHANICAL PROP: Fe-C SYSTEM (2)

Slide29: 

19 Adapted from Fig. 10.27, Callister 6e. SUMMARY: PROCESSING OPTIONS

Slide30: 

Bainite coarse fine Austenite Martensite Moderate cooling (AS) Isothermal treatment (PCS) Tempered Martensite Pearlite AS: Alloy Steel PCS: Plain-carbon Steel Slow Cooling Rapid Quench Spheroidite Re-heat Re-heat

PRECIPITATION HARDENING: 

16 • Particles impede dislocations. • Ex: Al-Cu system • Procedure: --Pt A: solution heat treat (get a solid solution) --Pt B: quench to room temp. --Pt C: reheat to nucleate small q crystals within a crystals. • Other precipitation systems: • Cu-Be • Cu-Sn • Mg-Al Adapted from Fig. 11.22, Callister 6e. (Fig. 11.22 adapted from J.L. Murray, International Metals Review 30, p.5, 1985.) Adapted from Fig. 11.20, Callister 6e. PRECIPITATION HARDENING

Slide32: 

PRECIPITATION HARDENING • Two stage heat treatment. Procedure: --T0: solution heat treatment (get single phase solid solution) --Quench to T1. --T2: reheat to nucleate precipitates T0 T2

Slide33: 

PRECIPITATION HARDENING

Slide34: 

PRECIPITATION HARDENING

PRECIPITATE EFFECT ON TS, %EL: 

17 • 2014 Al Alloy: • TS peaks with precipitation time. • Increasing T accelerates process. • %EL reaches minimum with precipitation time. Adapted from Fig. 11.25 (a) and (b), Callister 6e. (Fig. 11.25 adapted from Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H. Baker (Managing Ed.), American Society for Metals, 1979. p. 41.) PRECIPITATE EFFECT ON TS, %EL