Nucleation And Growth And Microstucture Development Flashcards
What are the two aspects phase transformations can be understood as
Thermodynamic aspect - will it it happen
- is it energetically advantageous for the process to go forward
-note that latent heat is released if it does happen
Kinetic aspect - how will it happen
-mostly dependent on speed of diffusion
-the actual process of phase transformation is nucleation and growth
What is latent heat
Energy lost by a material due to the phase change, usually released into the environment as heat
What is kinetics
The process of how something happens
This includes how fast and how long something will occur for
How do we know what form a structure will take based on a energy versus tempreture graph
The lower energy state will always be advantageous at any tempreture
(T/F) The phase transformation at a temperature at a temp require the grain or loss of the energy difference between phases
T
In terms of grain growth, what is usually advantageous
Making only one grain grow since it decreases GB and thus diffusion and thus creep
Also more uniform properties
How can we ensure only one grain is formed
By controlling nucleation and growth of grains
What are the main phase transformations
Recrystalization and grain growth
Solidification
Solid-solid phase transformations
What is nucleation
The process of creating nucleui which are the first spec of a new phase
What is growth in the context of phase transformation
Process of increasing nucleus size
What does a phase diagram depict about nucleation and growth
It shows the tempretures which phases begin to nucleate and where they grow, replacing other phases
What is isothermic solidification
Solidification at one temperature- phase changes an growth happens with no temperature change
What is recalesance
Energy release as latent heat, increasing the temperature to the proper Tm
Undercooling cools the temperature to colder then the Tm
How much undercooking occurs
Varies base on impurities in the substance
What is delt T
Undercooking
How do impurities effect delta T
Increase
What process follows undercooling
Recalesence
(T/F) Undercooling can be thought of as activation energy for solidification/phase change
T
(T/F) Undercooling has more energy than recalesence
F
They are equal in energy magnitude and opposite directions
This is because undercooling is the energy passed the freezing temp (or other phase change temp) and recalesence is the energy released from the undercooled substance to get it to the phase change temp
How can undercooling be controlled
By controlling how much (impurities) and when it’s offered (Tm)
What is Solidification
Transforming from liquid to solid
Nucleation of a soldi phase
Does Solidification start at Tm
No, it does not start until undercooling occurs, this stores enough extra energy to pass the activation hump by releasing it via recalescence, the solidification then starts
So not at the first time the substance is at the Tm but rather the second time
Does a liquid or a solid have more energy
A liquid has more energy then a solid
This means to make a solid, energy is relased
What happens to energy when a solid phase is nucleated
Local energy decreases as solids have less energy then liquids
What is the energy lost due to solid formation
V * delta G (l-s)
What is delta G (l-s)
Energy of formation of the solid per unit volume
What is the volume of a sphere
(4/3)PI(r^3)
What is a interface, what is an interphase interphase
Interface : A defect which occurs that contracts what’s occurring
Interphase interface : a defect with a defect energy based on area
Describes the interface which occurs during solidification
The interface is a interphase interface
An interface is created between solid and liquid
This defect require energy input
Essentially some of the phase is going from solid to liquid which uses energy
This is contradictory to the rest of the phase which is going from liquid to solid, releasing energy
What is the amount of energy increase due to interference in soldification
A* y(s-l)
What is y(s-l)
Surface energy between the L and s phase per unit area
What is the area of a circle
4pi(r^2)
So what is the total change in energy
Decrease energy going from liquid to solid but then add energy for the defects which go from solid to liquid
(T/F) All defects increase energy
F
All defects increase energy except for vaccancies
What are the two types of nucleation
Homogenous
Heterogenous
Why does nucleation from spheres
It has the smallest surface to volume ratio (causes surface tension) and thus this shape forms as it is the path of least resistance
A small surface area is desirable
In terms of sign, what do we want from the total energy after solidifcation
Lower then 0 since that means the system lowered its energy which means it has soldified
We need this to be less then 0 since we need it to eventually balance the latent heat energy that will be released, this needs to balance to a net 0 - the Tm
What is the critical free energy change
The activation energy for nucleation
In order for solidification to occur (a solid phase to nucleate), a minimum energy must be reached for activation energy to be surpassed
The critical value is the minimum energy value where the process will continue, over this value it will also continue
Found by finding the local max of the free energy by radius
Is there a critical radius for nucleation
Yes, atoms can clump together, clumped atoms have a higher chance of nucleation since more energy is available
The critical radius is the average radius of a clump of atoms which have enough energy to pass the activation energy and begin to nucleate
What is the relationship between radius of a group of atoms and nucleation chance
Increased radius = increased nucleation chance as more energy is available to pass the activate energy gap
Expand on the statement, nucleation is random
Random number of atoms collide and thus clump, this forms random radius clumps. Thus, the probability of a clump forming with a high enough energy to pass the activation energy is random
Nucleus don’t form one atom at a time, they form in groups of atoms
What is homogenous nucleation
Nucleation that occurs naturally, without the help of foreign bodies
Undercooling is a must for solidification to begin as it is needed to pass the activation energy
The bigger the undercooling, the greater value of delta G (l-s) per volume and this the lower barrier to nucleation (delta G *)
Relationship between undercooling and barrier to nucleation
Larger undercooling = greater delta G (l-s) per volume = lower barrier to nucleation (delta G *)
Why does greater undercooling = greater delta G (l-s) per unit volume
Greater undercooling means that the substance is even colder the the Tm, this means there is more energy stored internally in the liquid structure that needs to be released to become a solid
Since there is a greater energy difference between the liquid and solids at colder tempretures, the delta G (l-s) would be higher and thus more energy is relased to get to the right tempreture
How can we decrease uncertainty in nucleation
1) Cool rapidly
- causes substance to reach a larger delta T due to overshoot and thus a larger delta G (L-S) and a lower delta G* and thus is easier to nucleate
2) Change activation energy by decreasing y(s-l)
-this is what heterogenous nucleation does
What is delta G *
The critical energy
Activation energy
The amount of energy needed to overcome the defects in the substance
What forms can a spherical shape take in terms of nucleation
1) a perfect circle shape
2) a wetting shape - a dome shape
Both forms will have the same volume
What is the difference between a wetting vs non-wetting shape
In a wetting shape, there is a greater interface between the primary object and the secondary object than in the non-wetting case - a section of the interface of the primary object and a tertiary object is replaced
Therefore, depending on the surface energy (Y) of the phases, the wetting shape would likley lower the surface energy as the tertiary object tends to have lower surface energy (we are typically talking about solid, solid, liquid)
Lowering surface energy is beneficial as it lowers delta G* and thus makes it easier to nucleate
What type if nucleation takes into account wetting
Heterogenous nucleation
How is wetting typical defined in terms of nucleation and how does this describe the difference between homogenous and heterogenous nucleation
Wetting is typically defined by a contact angle
The angle defines the drop properties of the shape, mainly surface tension
The delta G het = delta G homo * f(angle) where f(angle) is a function of the contact angle
So essentially, the activation energy is the same as homogenous but with the decrease in energy due to surface tension in the shape of the solid to liquid defects formed which decrease the surface energy
Contact angle determines how much of an effect wetting has
Is there every a case where heterogenous nucleation is more difficult then homogenous nucleation
Not usually, unless there is an extreme radius that are thermodynamically unlikely
What is the difference between heterogenous and homogenous nucleation
Heterogenous has help from foreign objects while homogenous does not
Heterogeneous has a lower activation energy as it has lower interfacial energy as a result of wetting which typically lowers the surface energy term
The curve of the temp vs nucleation rate graph
The slope/curve gets shifted up (easier to nucleate and less undercooling required) in heterogenous vs homogenous
What is the relationship between diffusion and nucleation
When you have higher diffusion, molecules move around more
Thus
Higher diffusion = more collisions = higher chance of larger r (since there are more collisions) in a unit of time = higher change r > r* = higher nucleation rate
What is the contradiction of the effect of temperature on nucleation
Increase temperature = increase diffusion = more collisions = higher change of r > r*= more nucleation
But
Increase temperature = increased delat G * = less stable nuclei
These effects need to be summed to create a single curve
What factors effect nucleation rate
Temperature - higher temperature means less stable nuclei since higher temperature increases delta G*
Diffusion - higher diffusion (and thus higher temperature) = more collisions = higher chance r > r*
So diffusion increases the number of atom clumps that pass the activation energy and that effect changes with temperature
Temperature increases the activation energy
(T/F) More undercooling is required for het. Nucleation
F
Less undercooling needed, lower activation energy
Is diffusion effected by homogenous versus heterogenous nucleation
Diffusion is unaffected by whether it is homogenous or heterogeneous - just effecting collisons
Is the instability of nuclei more or less prominent with heterogenous or homogenous nucleation
Less prominent with heterogenous, this shifts the temp nucleation rate upwards
Describes the curve of the temp nucleation curve
Above the curve, limited by delta G * (stable nuclei) (increased by temperature)
Bellow the curve, limited by diffusivity
How does thermodynamics play a role in the growth stage
The solidified nucleus would still want to maintain a decrease in energy as it grows larger
Thus it wats to increase the volume of the solidified component but not the surface area between solid and liquid
(T/F) Increasing the surface area between solid and liquid decreases the overall energy
F
Increases
(T/F) increasing the volume of the solidified component decreases the over all energy
T
What are the tow main mechanisms for growth of nuclei in terms of solidification
Planar growth
Dendritic growth - most metal exhibit this
What is planar growth
Nuclei grow one layer at a time
Common in polymers
Uncommon in metals
What is dendritic growth
Tree like growth - a tree but organized
Common in metals
Tends to occur at a higher rate than planar
Has primary (trunk) and secondary (branched) dendritic arms, can have tertiary arms
How do dendrites relate/form a grain
Each grain can have multiple dendrites in it
Dendrites can preferentially grow in one crystallographic direction
The favourably orientated chill grains develop into columnar grains
What is SDAS
Secondary Dendritic Arm Spacing
The space between the arms or bumps of the secondary dendritic arms
Why do we want to control SDAS
Since a decrease in SDAS = increase in UTS
What is the relationship between SDAS and ts
Ts is the time to 100 % solidification
Increased ts = increased SDAS and vice versa
So the longer it takes to soldifiy, the greater the arm spacing which makes sense since it takes time to elongate the arms to the greater SDAS’s
What is the relationship between temperature and SDAS
Increase temp = increased SDAS
Higher temp = higher diffusion = higher growth as there is more kinetic energy
What are the key relationships that deal with temperature nucleation and growth
Lower temp = increase undercooling
= lower delta G *
= increases nucleation rate
= less ts
= less SDAS
= higher UTS
What is casting
The pouring of metal into a mould as a primary manufacturing technique
What is the result of casting in terms of grain structure (microstructure)
The outside has heterogenous nucleation - lots of nuclei, smaller grains
This is followed by a area of columnar zone consisting of grains grown from the heterogenous zone
Then, in the Center, there is a equiaxed grains - homogenous nucleation
How do heterogenous nuclei grow versus homogenous
Heterogenous tend to form lots of small nuclei/grains, this typically occurs near the outside of the part since it is the easiest (energetically) and the outside tends to cool very fast (large area of exposed surface). The small grains from heterogenous nucleation tend to grow into columnar grains, there grains are sectioned by grains with the same orientation. This has to do with how only favourably orientated grains will grow. The other grains just do not grow and stay small around the outer edge of the part.
Homogenous nucleation produces small, fine grains with uniform properties called equiaxed grains. The grains are uniform since this part of the structure gets to cool slowly, allowing homogenous nucleation to occur.
Which type of nucleation tends to be unwanted in the microstructure of a part
Heterogenous, although it is easier to nucleate, the grains often have un-uniform properties making it hard to work with
Why is the natural casting microstructure undesirable
Grains have uniform properties which means that the entire material will have vastly different properties in different spots
What can be done to avoid the natural un-uniform heterogenous grain growth structure
Inoculation: the addition of other things to act as heterogenous nucleation sites (increased nucleation) and and also act as grain refiners (decrease growth)
This creates a structure where only equiaxed grains grow - preventing unwanted un-unifrom grains
What is a single crystalline turbine blade in relation to nucleation and growth
An application in which a single grain crystalline blade is formed which decreases the stress concentrations along the grain boundaries due to creep, making the blades more efficient
Typically you have equiaxed grains
By chilling the plate, we can create a turbine blade where only the columnar grains survive and thus decreasing the amount of grain boundaries and directional un-uniformity, decreasing stress concentrations
We can create a single grain structure by chilling the nucleation site so only columnar grains survive and then also adding a pig tail, this only fits one grain in it so only one grain can continue to grow, creating a single grain turbine blade
Why is metal 3D printing not the highest strength
There is reduced strength due to the un-uniform structure
Products tends to either have extended trucks and underdeveloped dendrites or reduced trunks but developed dendrites
These defects decrease the strength of the part so they tend to be week
What is a solid-solid phase transformation
A transformation from a solid phase to a solid phase
Like solidification, requires undercooling
Heterogenous nucleation is more advantageous than homogenous nucleation
Possible heterogenous nucleation sites are GB, surface and dislocations
What is the difference between a solid to solid phase transformation and a solidification transformation in terms of nucleation
Solid-solid transformation is much slower since diffusion is much slower in solid in liquid
What is the major assumption made when analyzing microstructure, especially when using phase diagrams
Equilibrium cooling : Lowe cooling rate which is low enough to allow diffusion to occur such that the elemental composition is identical to those describe by equilibrium phase diagrams
What does equilibrium cooling mean in a a physical sense
Every time we drop a degrees, we wait a very long time for everything to return to its lowest possible energy level
This is continuous cooling at a very slow rate - makes it easy to find the tempretrue at a given time and thus the phase diagrams can be used to predict the microstructure
What doe phase diagrams represent in terms of microstructure
Equilibrium structure
(T/F) At different temps, as substance has different phase compositions for the same elemental compositions
T
The occurs since diffusion is going on, even if the composition of the substance has not changed
When do we have columnar grains
When one phase is present, the grains will naturally form columnar grains
Remember too add the grain boundaries
When do precipitates form
Precipitates form as phase cools and solubility temp forces solutes out of the structure
Ideally this happens at the grains but they are usually along the GB
We wants it to precipitate in grains since this is strong but it is hard to get. Precipitates only along the GB leads to a brittle, easy to get structure
(T/F) Phase diagrams are time based
F
If something occurs rapidly, this means it occurred at a fast cooling rate
When do lamella structures occur
When we pass through a eutectic or eutectoid point
Why do lamella structures form?
Since the system wants to get to a steady state as quickly as possible, it changes the way it transforms
To increase the speed of diffusion between two grains, the diffusion should be minimized for rapid diffusion to occur, the grains stick to each other - this is a for of heterogenous nucleation- this allows fast exchanges
The lamella structure takes this one step further and forms long grains that are stacked, one on top of another, with alternating grains types. This allows for maximum surface area contract which creates the fastest diffusion.
Thus lamella structures are stacked alternating stacked layers of grain
How can we break up a phase diagram with respect to a eutectic point
Hypoeuteutic - less the eutectic, to the left
-primary (pro-eutectic) phase formed
- before the eutectic transformation occurs
- formed phase stays in microstructure after eutectic tranformation
Eutectic point
- pre-formed primary phase exists
-remaining L undergoes eutectic transformation to transform into a lamella structure
Hypereutectic - post eutectic
-post transformation
-primary phase exists and lamella structure exists
What are primary or pro-eutectic phases
Phases formed prior to reaching the eutectic or eutectoid tempreture
Why do we typically not see precipitate in structures even though our math says there should be some
We calculate based on ideal assumptions in equilibrium cooling and that there are no interactions between sub systems
In actuality this is not the case
By nature cooling can’t be equllibrium since it is a change of state
The second condition of no interactions causes the biggest issue, the precipitate probably go absorbed into another part of the structure
What assumptions do we make when analyzing a microstructure using the phase diagram
Equilibrium cooling
There is no interaction between subsystems
What is interlamella spacing
The distance between the Center of two lamella layers of the same phase
How does inter-lamella spacing effect strength
There is an inverse proptionality between inter-lamella spacing and microhardness
Less spacing = higher strength
What happens if we deviate from ideal conditions by increasing the cooling rate
By increasing the cooling rate
-diffusion no longer goes to completion
-a time pressure for diffusion is created (not strong since by the time the time pressure became enough of a factor, diffusion would be heavily limited by cooling too fast)
- increased cooling rate = more undercooling = lower delta G * = more nucleation
-at increased cooling rate, a finer lamella structure forms out of nessessity
What is the relationship between cooling rate and inter-lamella spacing
Increased cooling rate = smaller interlaces spacing
Grains grow in a more vertical manner
What is the relationship between cooling rate and strength
Since a faster cooling rate = less inter-lamella spacing = greater microhardness
Faster cooling rate = greater strength
Why does increasing cooling rate infinitely not work to make a structure very strong
We begin to limit diffusion significantly
What changes in terms of planar growth when cooling rate is increased significantly
Not enough time for elemental composition to reach equilibrium values
We get cores which are regions of states within a grain due to uneven cooling
What happens when cooling rate is increase significantly in terms of dendritic growth
Not enough time for diffusion to occur completely- go to equilibrium values
The arms grow at a rapid pace, into each other, trapping liquid inside pockets within the structure
This process is called macrosegregation
What happens when to solutes when diffusion is limited
Not as much solute can be absorbed into the solvent when diffusion is limited, this causes solute rejection, excpecially when temp is lowered
Thus the amount of solute in the solvent phase is reduced regardless of temprature
The ejected solute forms a precipitate since it can’t diffuse into the phase
Microsegreation vs macrosegregation
Small precipitate trapped in liquid between SDA vs large precipitates
Why does the dendritic arms expand at increased cooling rate
At high cooling rate, diffusion is limited
By restricting diffusion, we block the structure from getting to the lowest energy level
The system wants to lower the energy level as fast as possible, even if it has to increase the Surface area
This causes it to rapidly expand, trapping pockets of liquid and precipitate between the secondary dendritic arms
Typically the L would reabsorb and distribute the precipitate/ solute atoms elsewhere, but since it is shut off, the precipitate stays, creating a secondary phase
Are precipitate pure solute
No, they just have a higher level of solute
When a precipitate forms, the surrounding area has a lower [] of solute
What is an example of the phase diagram being wrong from actuality
When you have increased cooling rate, precipitates may form due to limited diffusion. This creates a secondary phase where the phase diagram says there should only be one phase
Is the phase diagram equilibrium condition or non-equilibrium conditions and what does this mean
Equlibrium
Equilibrium conditions are we allow diffusion to go to complete ion and there is a low cooling rate
Non-eq conditions are we limit the extent of diffusion (on a spectrum) by increasing the cooling rate
What is quenching in terms of diffusionin
Increasing the cooling rate to the point where diffusion is almost completely limited
How do we fix segregation
The problem is cause by a lack of diffusion so we need to increase diffusion
We do this through a process called homogeniztion
What is homogenization
A type of heat treatment
Heat is provided to speed up diffusion - waiting for things to diffuse out with help from extra kinetic energy in the form of heat
We want to increase to a temp which will not create a phase change
Works since the system wants to go back to a state with the lowest energy possible which is always a single phase (found on the phase diagram)
What does the SDAS represent
Diffusion distance
Is homogenization temp dependent? Why or why not
It is
Because diffusion is temp dependent
Why is macro segregation hard to fix
The diffusion distance is to large, it would take a long time to rectify
It’s so big we can see it with our eyes
In theory, with infinite time, it could diffuse out
