Phase Transformation - Kasetsart...

Kasetsart University

Dr.Peerapong Triyacharoen Department of Materials Engineering

213211: Phase Transformation

Phase TransformationISSUES TO ADDRESS...• Transforming one phase into another takes time.

• How does the rate of transformation depend ontime and T?

• How can we slow down the transformation so thatwe can engineering non-equilibrium structures?

• Are the mechanical properties of non-equilibriumstructures better?

γ (Austenite)

Eutectoid transformation

Fe3C (cementite)

α (ferrite)

+(BCC)

Fraction of Transformation• Fraction transformed depends on time.

fraction transformed time

y = 1− e−ktn

Avrami Eqn.

• Transformation rate depends on T.

1 10 10 2 10 40

10 0 135°

43°Cy (%)

log (t) min

Ex: recrystallization of Cu

r = 1t

= Ae−Q /RT

activation energy

• r often small: equilibrium is not possible!

log (t)

Fixed T

Transformation & Undercooling• Can make it occur at:

...727ºC (cool it slowly) ...below 727ºC (“undercool” it!)

• Eutectoid transf. (Fe-C System):

α ferrite

4000 1 2 3 4 5 6 6

γ austenite

γ+Fe 3C

cementiteα+Fe 3C

L+Fe 3C

(Fe) Co, wt% C

Eutectoid:

727°C

T(°C)

γ ⇒ α + Fe 3 C0.77wt%C

0.022wt%C6.7 wt%C

Undercooling by ∆T: T transf. < 727ºC

Equil. cooling: T transf. = 727ºC

Eutectoid Transformation Rate ~ ∆T

γαααα

pearlite growth direction

Austenite ( γ) grain boundary

cementite (Fe 3C)

ferrite (α)

Diffusive flow of C needed

αγ γ

• Growth of pearlite from austenite:

• Reaction rateincreases with∆T. 675°C

(∆ T smaller)

1 10 10 2 10 3time (s)

600°C (∆ T larger)

650°C

Isothermal Transformation Diagram• Fe-C system, Co = 0.77wt%C Transformation at T = 675°C.

1 10 10 2 10 3 10 4 10 5

0%pearlite

Austenite (stable) T E (727°C)Austenite (unstable)

Pearlite

T(°C)

01 10 2 10 4

T=675°C

time (s)

isothermal transformation at 675°C

Ex: Cooling in Fe-C System• Eutectoid composition, Co = 0.77wt%C• Begin at T > 727°C• Rapidly cool to 625°C and hold isothermally.

Pearlite Morphology

- Smaller ∆T: colonies are larger

- Larger ∆T: colonies are smaller

• 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.

Non-Equil. Transformation Products: Fe-C

Bainite reaction rate:

rbainite = e−Q /RT

• Bainite:--α lathes (strips) with long

rods of Fe3C--diffusion controlled.

(cementite)

α (ferrite)

10 103 105

time (s)10-1

T(°C)Austenite (stable)

100% bainite

pearlite/bainite boundary100% pearlite

Other Products: Fe-C System (I)

60 µm

α (ferrite)

(cementite)

• Spheroidite:--α crystals with spherical Fe3C--diffusion dependent.--heat bainite or pearlite for long times--reduces interfacial area (driving force)

10 103 105time (s)10-1

Spheroidite100% spheroidite

100% spheroidite

Other Products: Fe-C System (II)• Martensite: γ (FCC) to Martensite (BCT)

xpotential C atom sites

Fe atom sites

(involves single atom jumps)

time (s)10 103 10510-1

100%50%

M + AM + A

0%50%90%

Martentite needlesAustenite

• γ to M transformation..-- is rapid!-- % transf. depends on T only.

• Effect of quenching medium:Medium

airoil

Severity of Quenchsmall

moderatelarge

Hardnesssmall

moderatelarge

• Effect of geometry:When surface-to-volume ratio increases:

--cooling rate increases--hardness increases

Positioncentersurface

Cooling ratesmalllarge

Hardnesssmalllarge

Quenching Medium & Geometry

Cooling Ex.(a) Rapidly cool to 350°C, hold

for 104s, quench to Troom

(b) Rapidly cool to 250°C, hold for 100s, quench to Troom

(c) Rapidly cool to 650°C, hold for 20s, rapidly cool to

400°C, hold for 103s, quench to Troom

(a)(b)

Mechanical Prop: Fe-C System (I)Hypoeutectoid Hypereutectoid

Pearlite (med)

Ferrite (soft) Pearlite (med)

Cementite (hard)

Mechanical Prop: Fe-C System (II)

Mechanical Prop: Fe-C System (III)

Summary: Processing OptionsAustenite ( γ)

Bainite (α + Fe 3C plates/needles)

Pearlite (α + Fe 3C layers + a

proeutectoid phase)

Martensite (BCT phase diffusionless

transformation)

Tempered Martensite (α + very fine

Fe 3C particles)

slow cool

moderate cool

rapid quench

reheat

Martensite T Martensite

bainite fine pearlite

coarse pearlite spheroidite

General Trends

Problem 10.16

Phase Transformation - Kasetsart...

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