Solidification Flashcards
What is homogenous nucleation?
Nuclei form uniformly throughout the parent phase.
Nuclei form in the super-cooled liquid.
What is free energy?
Surface free energy + volume free energy
Function of internal energy and measurement of randomness.
What is surface free energy?
Results from the formation of the solid-liquid phase boundary during solidification. It helps with maintaining grain boundaries and it’s POSITIVE.
What is volume-free energy?
The driving force for the solidification transformation.
What is the activation free energy?
Free energy required for the formation of a stable nucleus.
What is the critical radius?
The minimum radius that needs to be surpassed for the nuclei to form.
If the radius is too small - the nuclei dissolve
If the radius is too big - is energetically more favorable for the nuclei to grow to reduce energy
The radius decreases if supercooling increases.
What is super-cooling?
The difference between the actual solidification temperature and the melting point (see figure).
Why is super-cooling necessary?
It is required for solidification, it promotes grain refinement - higher under-cooling, smaller grains, and stronger material.
What is heterogeneous nucleation?
Nuclei form preferentially at structural defects such as impurities, grain boundaries and dislocations.
How do grains form in a cast?
Next to the wall - high cooling rate, small grains are formed (many nuclei)
Farther away from the wall - columnar grains, grains are favorably oriented in the direction of the heat gradient
Middle of the cast - the grains don’t have any preferential directions and are bigger because all the heat of crystallization has already been used
What are dendrites?
When diffusion is not fast enough for the cooling speed, dendrites are formed.
They consist of a horizontal core with side arms perpendicular to it.
They form during solidification.
What is microporosity?
A larger area between the liquidus and solidus lines can lead to microporosity.
This is because there is a larger area of viscous material.
It is more difficult for viscous material to flow between the already-formed solidified parts, therefore this leads to gaps in the material which determine the micro-porosity.
This issue can be solved by adding additional material.
What is crystal segregation?
At high cooling rates, the rate of diffusion is affected. This leads to a coring effect in the grains as the center might have a higher concentration than the outer shell. A consequence is the difference in mechanical properties within the material, this can create difficulties when machining the part.
The concentration of low-melting elements increases towards the boundary.
What are low-solubility impurities?
During solidification impurities cannot dissolve in the solid phase and remain in the liquid, they are forced towards the grain boundaries.
This creates a boundary composition that differs from the grain itself.
This can lead to issues such as grain boundary corrosion (intercrystalline corrosion).
How can a finer grain size be achieved?
Adding different alloys during the solidification process to influence heterogeneous solidification (impurities act as nuclei) in order to achieve finer grain size (seeding).
