Lecture 03/10 Flashcards
What is the transformation rate?
Dependence of reaction progress on time
What are the classifications of transformations?
- Simple diffusion dependent
- Diffusion- dependent
- Diffusionless
Do most transformations occur instantaneously?
No
How do phase transformations work?
Formation of numerous small particles of the new phase which increase in size until the transformation is complete
What are the two stages of phase transformation?
Nucleation and growth
What are the types of nucleation?
Homogenous and heterogenous
Describe how nuclei form during homogeneous nucleation
Nuclei of the new phase form uniformly throughout the parent phase
Describe how nuclei form during heterogeneous nucleation
Nuclei form at structural inhomogeneities such as grain boundaries
What are the contributions to the total free energy change that accompanies a solidification transformation during homogeneous nucleation?
- Free energy difference between the solid and liquid phases
- Energy from the formation of the solid-liquid phase boundary
Is the energy from the formation of the solid-liquid phase boundary in homogeneous nucleation positive or negative?
Positive
Describe the total free energy change curve
- As the solid particle begins to form, the free energy increases
- If the cluster reaches a size r*, the growth will continue with a decrease in free energy
- If the cluster is less than r*, it will shrink and redisolve
What is a subcritical particle called?
Embryo
What is a particle called if the radius is greater than r* ?
Nucleus
During homogeneous nucleation, the lowering of temperatures below the equilibrium solidification temperature means?
Solidification occurs more readily
During homogeneous nucleation, what happens for the number of stable nuclei when the temperature is lowered below Tm?
Magnitude of n* increases
When is it easier for nucleation to occur? What is this called?
At surfaces and interfaces, heterogeneous nucleation
During growth, the product phase particles depend on?
Transformation temperature
During growth, transformations that occur near Tm correspond to?
Low nucleation and high growth rates leading to few but large particles
During growth, if the material is supercooled to a low temperature where the transformation is very low what happens?
It’s possible that nonequilibrium phase structures are formed
During growth, the rate of transformation and the time required for the transformation to proceed are?
Inversely proportional
What is the kinetics of a transformation?
Time dependence of rate
Simple diffusion-dependent transformation have no change in?
Number or composition of phases
Diffusionless transformation occurs when?
A metastable phase is produced
Nucleation involves?
Appearance of nuclei
Growth involves?
Nuclei increasing in size, resulting in the disappearance of some of the parent phase
Transformation reaches completion if?
Growth of the nuclei is allowed to proceed until the equilibrium fraction is attained
If the temperature is below equilibrium solidification, is the volume free energy positive or negative?
Negative
What is a subcritical particle?
The cluster of atoms have a radius less than the critical value
As temperature decreases, what happens to the critical radius and activation free energy?
Both decrease
What is supercooling?
Solidification will begin after the temperature has been lowered to below the melting temperature
For solid transformations, a plot fraction transformation vs log time yields?
S shaped curve
The time dependence of transformation is represented by the?
Avrami equation
