Material Engineering - Lecture - Phase Tranf, Lecture notes of Material Engineering

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Phase

Transformations

Fe 3 C (cementite) orthorhombic Martensite BCT Austenite FCC Ferrite BCC

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

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).

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

r * = critical nucleus: for r < r * nuclei shrink; for r > r * nuclei grow (to reduce energy) Homogeneous Nucleation & Energy Effects  GT = Total Free Energy =  GS +  GV Surface Free Energy- destabilizes the nuclei (it takes energy to make an interface)

2

GS 4 r

 = surface tension Volume (Bulk) Free Energy – stabilizes the nuclei (releases energy)   G   r  G V 3 3 4 unit volume volumefree energy  G  

Solidification

H T

T

r

f

m

Note:  H

f

and  are weakly dependent on  T

 r * decreases as  T increases

For typical  T r * ~ 10 nm

 Hf = latent heat of solidification (fusion) Tm = melting temperature = surface free energy  T = Tm - T = supercooling r* = critical radius

2

• Fraction transformed depends on time. fraction transformed time y ^1 ^ e ktn Avrami Eqn.
• Transformation rate depends on T. 1 10 102 104 0 50 100 y (%) log (t) min Ex: recrystallization of Cu r ^

t

1. 5  (^) Ae Q /RT activation energy

• r often small: equil not possible y log (t) Fixed T 0

t

FRACTION OF TRANSFORMATION

Generation of Isothermal Transformation Diagrams

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

Consider:

5

• Reaction rate is a result of nucleation and growth of crystals.
  • Examples: % Pearlite 0 50 100 Nucleation regime Growth regime t 50 log (time) Nucleation rate increases w/ T Growth rate increases w/ T Nucleation rate high T just below TE T moderately belowTE T way below TE Nucleation rate low Growth rate high  (^)  (^)  pearlite colony Nucleation rate med Growth rate med. Growth rate low

Nucleation and Growth

Isothermal Transformation Diagrams 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.

Transformations Involving Noneutectoid Compositions

Hypereutectoid composition – proeutectoid cementite

Consider C

0

= 1.13 wt% C

Fe 3 C (cementite) 1600 1400 1200 1000 800 600 400 (^0 1 2 3 4 5 6) 6. L  (austenite)  (^) + L  +Fe 3 C  (^) +Fe 3 C L +Fe 3 C

(Fe) C, wt%C

T (°C)

727°C  T 0.022^ 0. 1.

Strength^ Ductility Martensite T Martensite bainite fine pearlite coarse pearlite spheroidite General Trends Possible Transformations

10 103 105

time (s)

10

400 600 800

T (°C)

Austenite (stable) 200 P B

TE

0% 50%^ 100% A A

Bainite: Non-Equil Transformation Products

 elongated Fe 3 C particles in -ferrite matrix  (^) diffusion controlled  lathes (strips) with long rods of Fe 3 C  100% bainite 100% pearlite Martensite Cementite Ferrite

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).