Phase Transformations Flashcards
What is a phase transformation?
The conversion of a material from one phase (parent) to one or more new phases (products)
List the most important phase transformations
Solidification Eutectic Eutectoid Precipitation Polymorphic/allotropic
What is a glissile interface?
Parent and product phases have the same composition.
Rate limited by interface mobility
Temperature effects negligible
Military transformation
Slides easily (often by dislocation motion)
Give an example of a transformation where glissile interfaces form.
Martensite and twinning
List features of a non-glissile transformation where the parent and product phases have the same composition.
Rate is limited by interface mobility
Transport is thermally activated
Civilian
Example of a phase transformation where non-glissile interfaces formand the parent and products have the same compositions..
Grain growth
Solidification of pure metal
List features of a non-glissile transformation where the parent and product phases do not have the same composition.
Long range diffusion required
Interface controlled, diffusion controlled or mixed control depending on the balance of thermodynamics and kinetics
Transport is thermally activated thus strongly temperature dependent.
Example of a phase transformation where a non-glissile interface forms and the parent and products have different compositions.
Eutectic/eutectoid transformations
Where does the driving force for solidification come from?
Difference in free energies of the parent and product phases.
What are the 4 stages of a precipitation reaction?
Nucleation
Growth
Impingement
Coarsening
How are thermodynamics and kinetics balanced in solidification when considering microstructure size?
Kinetics is often determined by diffusion, we would expect the reaction to be faster for finer microstructure as shorter diffusion distances. However finer microstructure reduces the interface energy so the reduces the driving force of the reaction and the reaction is slowed.
Drive the term for the Gibbs-Thompson effect for the growth of a dendrite.
Check
Why do dendrites grow faster than a flat interface if there is a lower driving force?
Thermal diffusion of latent heat away from the curved end of a dendrite is faster than from a flat surface.
Derive an expression for growth velocity of a dendrite in terms of latent heat and undercooling.
Check
Does diffusion only occur in alloys?
No, you can have self-diffusion.
In an ideal solution, diffusion leads to what?
Mixing
In an alloy with a miscibility gap, diffusion leads to what?
Separation.
Diffusion is governed by differences in what?
Chemical potential!!!
Not composition.
Give the equation for Fick 1
Check
Give an expression relating distance travelled and diffusion coefficient for Fick 1
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Give an expression for jump frequency in Fick 1 using activation energy (interstitial).
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Give an expression for the diffusion coefficient in Fick 1 in terms of jump frequency.
Check
Give an expression for jump frequency in Fick 1 using activation energy (substitutional).
Check
Derive Fick 1 in terms of mobility and chemical potential.
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