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Heat Treatment (time and temperature) ⇒ ⇒ Microstructure ⇒ Mechanical Properties
Chapter Outline: Phase Transformations in Metals
¾ Kinetics of phase transformations
¾ Multiphase Transformations
¾ Phase transformations in Fe-C alloys
¾ Isothermal Transformation Diagrams
¾ Mechanical Behavior
¾ Tempered Martensite
Not tested: 10.6 Continuous Cooling Transformation Diagrams
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
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Phase transformations (change of the microstructure) can be divided into three categories:
Phase transformations. Kinetics.
¾ Diffusion-dependent with no change in phase composition or number of phases present (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.)
¾ Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations)
¾ Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure (e.g. martensitic transformation discussed in later in this chapter)
Phase transformations do not occur instantaneously. Diffusion-dependent 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.
University of Tennessee, Dept. of Materials Science and Engineering (^) 3
Most phase transformations involve change in composition ⇒ redistribution of atoms via diffusion is required.
The process of phase transformation involves:
Kinetics of phase transformations
¾ Nucleation of of the new phase - formation of stable small particles (nuclei) of the new phase. Nuclei are often formed at grain boundaries and other defects.
¾ Growth of new phase at the expense of the original
phase.
S-shape curve: percent of material transformed vs. the logarithm of time.
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
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A nucleus is only stable if further growth reduces the
energy of the system. For r > r c the nucleus is stable.
Nucleation
University of Tennessee, Dept. of Materials Science and Engineering (^) 7
Let us consider eutectoid reaction as an example
eutectoid reaction:
γ (0.76 wt% C)
α (0.022 wt% C)
Fe 3 C
The S-shaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation (nucleation rate increases with supercooling) and not by diffusion (which occurs faster at higher T).
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 8
Isothermal Transformation (or TTT) Diagrams
(Temperature, Time, and % Transformation)
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TTT Diagrams
The thickness of the ferrite and cementite layers in pearlite is ~ 8:1. The absolute layer thickness depends on the temperature of the transformation. The higher the temperature, the thicker the layers.
Fine pearlite
Austenite → pearlite transformation
α ferrite Coarse pearlite
Fe 3 C
Austenite (stable)
Denotes that a transformation is occurring
Eutectoid temperature
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
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TTT Diagrams
¾ The family of S-shaped curves at different T are used to construct the TTT diagrams.
¾ The TTT diagrams are for the isothermal (constant T) transformations (material is cooled quickly to a given temperature before the transformation occurs, and then keep it at that temperature).
¾ At low temperatures, the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.
¾ Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite ( fine pearlite ).
¾ At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite ( coarse pearlite ).
¾ At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite. Additional curves for proeutectoid transformation must be included on TTT diagrams.
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Spheroidite
- Annealing of pearlitic or bainitic microstructures at elevated temperatures just below eutectoid (e.g. 24 h at 700 C) leads to the formation of new microstructure – spheroidite - spheres of cementite in a ferrite matrix.
- Composition or relative amounts of ferrite and cementite are not changing in this transformation, only shape of the cementite inclusions is changing.
- Transformation proceeds by C diffusion – needs high T.
- Driving force for the transformation - reduction in total ferrite - cementite boundary area
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
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Martensite (I)
- Martensite forms when austenite is rapidly cooled (quenched) to room T.
- It forms nearly instantaneously when the required low temperature is reached. The austenite-martensite does not involve diffusion → no thermal activation is needed, this is called an athermal transformation.
- Each atom displaces a small (sub-atomic) distance to
transform FCC γ-Fe (austenite) to martensite which has
a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit cell axis is longer than the other two).
- Martensite is metastable - can persist indefinitely at room temperature, but will transform to equilibrium phases on annealing at an elevated temperature.
- Martensite can coexist with other phases and/or microstructures in Fe-C system
- Since martensite is metastable non-equilibrium phase, it does not appear in phase Fe-C phase diagram
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The martensitic transformation involves the sudden
reorientation of C and Fe atoms from the FCC solid
solution of γ-Fe (austenite) to a body-centered
tetragonal (BCT) solid solution (martensite).
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 16
TTT Diagram including Martensite
Austenite-to-martensite is diffusionless and very fast. The amount of martensite formed depends on temperature only.
A: Austenite P: Pearlite B: Bainite M: Martensite
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Mechanical Behavior of Fe-C Alloys (II)
The strength and hardness of the different microstructures is inversely related to the size of the microstructures (fine
structures have more phase boundaries inhibiting
dislocation motion).
Mechanical properties of bainite, pearlite, spheroidite
Considering microstructure we can predict that
¾ Spheroidite is the softest
¾ Fine pearlite is harder and stronger than coarse pearlite
¾ Bainite is harder and stronger than pearlite
Mechanical properties of martensite
Of the various microstructures in steel alloys
¾ Martensite is the hardest, strongest and the most brittle
The strength of martensite is not related to microstructure. Rather, it is related to the interstitial C atoms hindering
dislocation motion (solid solution hardening, Chapter 7)
and to the small number of slip systems.
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 20
Mechanical Behavior of Fe-C Alloys (III)
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Tempered Martensite (I)
Martensite is so brittle that it needs to be modified for
practical applications. This is done by heating it to
250-650 oC for some time (tempering) which produces
tempered martensite , an extremely fine-grained and
well dispersed cementite grains in a ferrite matrix.
¾ Tempered martensite is less hard/strong as
compared to regular martensite but has enhanced
ductility (ferrite phase is ductile).
¾ Mechanical properties depend upon cementite
particle size: fewer, larger particles means less
boundary area and softer, more ductile material -
eventual limit is spheroidite.
¾ Particle size increases with higher tempering
temperature and/or longer time (more C diffusion)
- therefore softer, more ductile material.
Introduction to Materials Science, Chapter 10, Phase Transformations in Metals
University of Tennessee, Dept. of Materials Science and Engineering (^) 22
Tempered Martensite (II)
Electron micrograph of tempered martensite
Higher temperature & time: spheroidite (soft)
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Reading for next class:
Chapter 11: Thermal Processing of Metal Alloys
¾ Process Annealing, Stress Relief
¾ Heat Treatment of Steels
¾ Precipitation Hardening
