Phase Transformation Flashcards
Categories of phase transformation
Diffusion dependent with no change in composition/number of phase, diffusion dependent with change in composition/number of phase, diffusion-independent
Examples of diffusion dependent phase transformation with no change in composition/number of phases
Melting, solidification, recrystallization, congruent phase transformations
Examples of diffusion dependent phase transformation with changes in composition or number of phases
eutectic, peritectic, eutectoid, peritectoid, precipitation
Example of diffusion independent phase transformation
martensitic
Eutectic
Liquid transforms into two solids
Peritectic
Liquid plus solid transforms into a different solid phase
Eutectoid
Solid transforms into two solids
Peritectoid
Two solids transform into one different solid
Factors that affect phase transformation
Composition at interface, kinetics of reaction, diffusion vs. nucleation driven, heat flow
What is the controlling parameter of the phase transformation
Slowest of affecting factors
Why is slowest factor controlling
Uses more of available driving force, leaving less for other processes
Rate of phase transformation
1/(t_0.5)
Percent of material transformed vs. log time
S-shaped curve
If there is a positive relation between temperature and rate
The phase transformation is diffusion driven
If there is a negative relation between temperature and rate
Phase transformation is nucleation driven
Avrami equation
y = 1 - exp(-kt^n)
What are k and n in Avrami
time independent constants
What is y in Avrami
Sigmoidal time-dependent transformation fraction
What is n in Avrami specifically
Order of phase transformation
Why do rates not have constant relation with temperature IRL
Mechanism can switch from nucleation to diffusion dominated
Martensitic transformation
Crystal lattice displacement decreases free energy
Martensitic transformation
Crystal lattice displacement decreases free energy
Bainitic transformation
Both diffusional and martensitic transformations occur together
What controls kinetics of martensitic transformation
Elastic strain energy
Elastic strain energy
0.5v(\sigma^2)/E
v in elastic strain energy
Volume
Coherent conjugation
Aligned planes
Why martensitic transformations only occur under high driving force conditions
High kinetic barrier
Result of martensitic reformation of atoms/lattices
Doesn’t complete at starting temp; must decrease temp as transformation progresses
Martensitic hysteresis
Reverse transformation finishes at a higher temperature than Ms
Speed of martensitic transformation after start
Very fast
Speed of martensitic transformation in general
Speed of sound in material
