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MSE 2090: Introduction to Materials Science
Chapter 10, Phase Transformations
Heat Treatment
(
time
and temperature)
⇒
Microstructure
Chapter Outline: Phase Transformations
¾
Kinetics of phase transformations
¾
Homogeneous and heterogeneous nucleation
¾
Growth, rate of the phase transformation
¾
Metastable and equilibrium states
¾
Phase transformations in FeC alloys
¾
Isothermal Transformation Diagrams
Not tested:
in 10.5 Bainite, Spheroidite, Martensite… (from p. 360)
10.6 Continuous Cooling Transformation Diagrams
10.710.9 Mechanical behavior of FeC alloys,
Tempered Martensite
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MSE 2090: Introduction to Materials Science
Chapter 10, Phase Transformations
Phase transformations
(change of the microstructure) can
be divided into three categories:
Phase transformations.
Kinetics.
¾
Diffusiondependent with no change in phase
composition or number of phases present
(e.g.
melting, solidification of pure metal, allotropic
transformations, recrystallization, etc.)
¾
Diffusiondependent
with
changes
in
phase
compositions and/or number of phases
(e.g. eutectic
or eutectoid transformations)
¾
Diffusionless phase transformation
 by cooperative
small displacements of all atoms in structure, e.g.
martensitic transformation (discussed in this chapter but
not tested)
Phase transformations do not occur instantaneously.
Diffusiondependent phase transformations can be rather
slow and the final structure often depend on the rate of
cooling/heating.
We need to consider the time dependence or
kinetics
of the phase transformations.
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MSE 2090: Introduction to Materials Science
Chapter 10, Phase Transformations
Phase transformations involve change in structure and (for
multiphase systems) composition
⇒
rearrangement and
redistribution of atoms via diffusion is required.
The process of phase transformation involves:
Kinetics of phase transformations
¾
Nucleation
of the new phase(s)  formation of stable
small particles (nuclei) of the new phase(s). Nuclei are
often formed at grain boundaries and other defects.
¾
Growth
of the new phase(s) at the expense of the
original phase(s).
Sshape curve
: percent of
material transformed vs.
the logarithm of time.
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MSE 2090: Introduction to Materials Science
Chapter 10, Phase Transformations
Nucleation
Nucleation can be
Heterogeneous
– the new phase appears on the walls of
the container, at impurity particles, etc.
Homogeneous
– solid nuclei spontaneously appear within
the undercooled phase.
Let’s consider solidification of a liquid phase undercooled
below the melting temperature as a simple example of a
phase transformation.
solid
solid
liquid
liquid
homogeneous
nucleation
heterogeneous
nucleation
supercooled
liquid
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MSE 2090: Introduction to Materials Science
Chapter 10, Phase Transformations
Gibbs free energy in analysis of phase transitions
It is convenient to analyze phase transformations occurring
under conditions of constant temperature (T) and pressure
(P) by using
Gibbs free energy (G)
.
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 Fall '10
 leoindzhiglei
 Thermodynamics, Phase transition, Supercooling

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