Microstructure Flashcards
1
Q
Phase Transformations (solid - liquid)
A
- G = H -TS
- Liquid has a lower enthalpy as atoms can move around and rearrange
2
Q
Phase Transformations (solid state)
A
- Beta phase BCC has lower density and higher enthalpy as it can move around more
- Deriving free energy at undercooling means that at Teq, Delta G = 0 so
Delta S = Delta H / Teq since Delta G = Delta H - T Delta S
3
Q
2 Component systems
A
- At constant temp, a phase change can occur
- Phases = C - F + 1 (components - degrees of freedom + 1)
4
Q
Ideal solution
A
- no change in enthalpy upon mixing
- Atoms dont care who their neighbours are
5
Q
Diffusion
A
- Allows atoms to move through a solid. It is needed for chemical equilibrium as atoms will move to a higher potential phase
- Can be substitutional or interstitial diffusion
- Interstitial diffusion is easier as they are smaller and interstitial sites are rarely filled so they can just from site to site without blocking each other
6
Q
Ficks First Law
A
- Describes steady-state diffusion, where teh concentration gradient remains constant over time
- Explains how atoms or molecules move through a material due to concentration differences
- J (flux) = -D dC/dX
- Negative sign because diffusion occurs down the concentration gradient from high to low conc
7
Q
Interstitial Diffision
A
- Assumes chemical potential of all atoms are equal. Interstitials need to push atoms out of their equilibrium positions to jump positions which will raise the free energy of the crystal
- Raising the free energy of the crystal will cause the atoms to vibrate about their equilibrium positions
- Interstitials are able to jump of a large enough vibration disturbs the lattice
- Larger energy difference of disturbance = lower probability vibration will occur when interstitials wants to jump
8
Q
Substitutional Diffusion
A
- Needs a space to move into
- Requires a vacancy to move into and spaces are usually filled vacancies are known as defects/disturbance of normal lattice
- Delta H vac > 0 so energy is required to form a vacancy. Introducing this vacancy will increase entropy so it is energetically unfavourable for all vacancies to be eliminated
9
Q
Grain Boundaries
A
- Act as highways of diffusion so atoms move faster
- Grain boundaries have a more disturbed packing and free space so there is lower activation energy
- Grain boundaries are defined as regions of disorder in a material so atoms moving alone a grain boundary do not have to disturb the structure as much
- Lower activation energy and faster diffusion
10
Q
Ga Penetration into aluminium
A
- Shows that at low temperatures, grain boundary diffusion dominates. All transport is at grain boundaries at first and then as temperature increases, diffusion starts in the bulk
11
Q
Ficks Second Law
A
- Describes non-stead-state diffusion where concentration changes with time
- dc/dt = d/dx(Ddc/dx)
12
Q
Fixed Concentration Source at Surface
A
- A special solution at ficks second law
- Material with a homogrnous solute concentrated Co is exposed to a surface source of constant concentration Cs
13
Q
Boundaries
A
- In boundaries, atoms are in different environment due to broken bonds
- Number of broken bonds depends on how the crystal is cut as less broken bonds means more close packed planes
- Atoms form bonds to lower energy. At the surface, the atoms may lack neighbour as compared to the bulk due to broken bonds which leads to higher surface energy
- Each bond consists of two atoms sharing energy so when its broken, it affects two surfaces so each surface contributes 50% to the surface energy of that bond
14
Q
FCC model (boundaries)
A
- FCC place is the densest packed plane
- Slicing of the crystal makes it lose the 3 bonds to the next layer so energy per atom at the surface = 3E/2 (E = bond energy per atomic pair)
15
Q
Wulff Plot
A
- Calculates equilibrium shape based on surface energy
- Free energy prefers spherical shapes due to low surface area to volume ratio
- Predicts the lowest energy crystal shape
16
Q
Interfaces
A
- Region between 2 grains. A defect that describes when atomic arrangement is disrupted due to misalignment of adjacent grains
- At higher angles, the bonds are more disrupted and have lower energy than surface
- At low angles, change in orientation is very small so boundary is still ordered. Has much lower energy than other random boundaries
- For angles Theta < 15, boundary energy increases lineraly with the angle
- For angles Theta > 15m energy levels off as dislocations interact to form a more random boundary. This discolations overlap so it plateous.
- grain boundaries = 1/3 energy of free surface
17
Q
Tilt Boundary
A
- Theta is accomodated by a series of edge dislocations
- As theta increases, D decreases so dislocations become more densely packed
18
Q
Special Boundaries
A
- There are cases where even large misorientation angles can form a regular & low energy interface
- These boundaries will have a structured atomic arrangement which reduces boundary eneergy
19
Q
Twin Boundaries
A
- Mirror image so there is no difficulty in bridging the change
- Common in FCC metals with a 70.5 degree misorientation angle
- Energy drops at special boundaries so they are likely to form as they are energetically favourable
20
Q
Interphase Interfaces
A
- When 2 materials are joined, atomic alignment determine interface properties
- Coherent Interfaces means they align well and have low interfacial energy
- Semi-coherent interfaces leads to periodic dislocations that reduce strain
- Incoherent interfaces means there is a large misfit strain which leads to highly disordered, high energy interfaces
21
Q
Lattice Mismatch
A
- Occurs cause 2 joining materials have different lattice spacings
- For low mismatch (less than 5%), it will create a strained but coherent interface. These are known as misfit dislocations which allows the material to still bond even with the mixmatch
- High mismatch leads to dislocations. The strain builds up and to relieve the strain, edge dislocations form at regular intervals along the interface
- Smaller misfit leads to larger spacing as there is less dislocations. Larger misfit requires more dislocations to accommodate strain
- At straight of more than 25%, dislocations will overlap and the interface will be incoherent
22
Q
Mobility of interfaces
A
- Diffusion will be easier along the interface than the bulk due to increased atomic disorder: Qgb = 0.6Qbulk
- Strain is slower so impurities tend to accumulate in the boundaries
23
Q
Heterophase Boundary MOtion
A
- Phase boundaries between ordered and solid solution phases required atomic hopping and solute redistribution
- Atomic hopping: atoms can switch positions between phases
- Solute Redistribution: when a boundary moves so solute atoms (impurities or alloys) must redistribute to maintain equilibrium
- Atomic hopping is the slower process so it limits the process
24
Q
Glissile Interfaces
A
- Dislocation is fast in glissile interfaces
- It is a type of mobile interface that can move without diffusion as it is governed by dislocation glide and dependent on the direction of the Burger vector
25
Nucleation of Precipitates
1. For new phases, system starts in a homogenous alpha phase and the atoms start to diffuse randomly. Beta phase has a lower energy than alpha which means that the energy difference favors nucleation
2. After diffusion, local fluctuations where a cluster of beta phase atoms form. The cluster is unstable as its formation requires energy to create an interface. Energy barrier prevents immediate transformation into full beta phase so system is more likely still majority alpha
3. Cluster overcomes Delta Gn so it becames stable and lowers free energy. This leads to full phase transformation which is more stable
26
Volume misfit
1. Opposes transformation
2. When a new phase (beta) forms, there is often a mismatch in volume between them
3. Volume difference leads to elastic strain in surrounding matrix. This increases the total energy so system resists transformation so no new elastic energy is introduced
4. Beta forms inside alpha without breaking continuity which will cause alpha to deform elastically.
27
Critical radius of nucleaus
1. critical radius (take the derivative with respect to r and set it to 0)
2. If r < r* : nucleus shrinks
3. If r > r* : nucleus grows
28
Energy barrier for nucleation
1. Delta G*. It is the maximum energy required to form a stable nucleus
2. Nucleation is affected by surface energy (higher surface energy, harder nucleation) and temperature (higher temp means smaller driving force which increases energy barrier for nucleation)
29
Heterogenous precipitation
1. Precipitation is Usually heterogenous which means it occurs at specific sites rather than in bulk
2. Usually at grain boundaries as interfaces lower energy barriers
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grain boundary nucleation
1. More energetic grain boundaries favor nucleation by reducing shape factor
2. Heterogenous nucleation has lower energy barrier than homogenous nucleation
31
growth of precipitates
1. Small nuclei mean that the surrounding matrix is rich in solute. As the precipitates grow, they deplete solute from the matrix until equilibrium is reached
2. Growth starts with diffusion of atoms to the precipitate which is controlled by solute mobility. After that, rearrangement of atoms into the precipitates crystal structure
3. Followed by rearrangement of atoms into the precipitates crystal structure
4. Governed by the movement of the boundary. Far from the precipitate there is no change in concentration however nearer, there is a gradual concentration change
32
Diffusion control
1. Rejection of A atoms
2. When a new phase (Beta) is formed, it will have a different solute concentration than the surrounding alpha matrix
3. To maintain equilibrium, excess A atoms are rejected from growing precipitates and the concentration gradient around the precipitate increases over time
4. Rejected atoms diffuse away from the precipitate over distance L. Larger concentration difference means shorter L
5. Initially growth is fast but it slows down as B atoms need to be dragged from increasingly long distances away from the precipitate
33
Nucleation rate at high undercooling
1. Many small/fine precipitates.
2. High surface energy (high curvature, high gibbs free energy)
3. Over time, they will dissolve and reprecipitate (coarsen) into fewer larger precipitates
4. More effective than larger precipiates at strengthening but it has an energy penalty and is less thermodynamically favourable
34
Nucleation rate at low undercooling
1. Single Large precipiates
2. More stable
35
Coarsening
1. Larger precipitates will grow at the expense of smaller ones due to chemical potential
2. Solute concentration is higher near small precipitates than large ones which leads to diffusion
36
Heat treatment for Al ions
1. Rapid cooling (quench) locks Cu atoms in a super saturated solution
2. Aged meaning it is reheated at lower temperature to allow for controlled precipitation of Al2Cu to strengthen the material
37
Precipitation growth with Al ions
1. Guiner Preston Zones (not a real phase) : regions where Cu atoms cluster within the Al matrix but maintains full coherency wiht the Al Structure
2. Theta'' Phase : remained fully coherent with the Al matrix still but has a defined crystal structure and a new shape defined by the mismatch strain
3. Theta' Phase : precipitate grows so coherency is lost in the z direction (partially coherent.
4. Theta phase : fully incorporated and incoherent with the Al matrix. Has no preferencial alignment with the Al matrix
5. An equiaxed shape (sphere) forms due to lack of preferential orientation
38
Spinodal composition
1. Like nucleation except there is no critical nucleus. It happens everywhere.
2. Certain mixtures will spontaneously seperate into different phases when mixing enthalpy is position (unfavourable) so the mixture prefers to seperate
2. Occurs if convex hull criterian is met
3. Atoms move upwill against the concentration gradient due to differences in chemical potential
4. Leads to regions that become richer in one component and poorer in another. Eventually will lead to phase seperation without nucleation
5. Can only occur at low temps when Delta H > 0
39
Driving force for transofrmation
1. Driven by how gibbs energy varies with composition
2. If d2G/dX2 is negative, it is unstable and will lead to spinodal decomposition and if its position it will be a stable system
40
Opposing spinodal decomposition
1. gradient energy (same as interfacial energy) as it opposes raid composition changes as it means high interfacial energy so the system wants to reduce it by making transitions mor gradual
2. Size mismatch: atomic size differences create strain energy
41
Coherent Spinodal
1. region where spinal will actually occur rather than just be chemically favourable
42
Deformation
1. Essential in manufacturing for strengthening/shaping
2. Cold work will increase strength but also introduce dislocations.
3. Leads to higher surface area which means more grain boundaries and higher stored energy
43
Dislocation density
1. total length of dislocations per unit volume
44
Kink formation
1. Regions of localised deformation that accomodate strain efficiently
2. When a material experiences stress, deformation may localize in specific bands (common in materials wiht low crystal structure like HCP)
3. Usually forms at grain corners
45
partial dislocations
1. occurs at low straings and normally in FCC materials
2. When a dislocation splits into 2 which introduces a stacking fault where teh atomic stacking sequence is disrupted which reduces total energy
46
Recover
1. Helps to relieve stored energy while preserving overall grain structure
2. Starts will cell formation where dislocatons begin to rearrange and form tangled networks which act as low-energy configurations which reduce the amterials stored energy
3. Next, the dislocations will futher rearrange into well-defined subgrains (small low-angle boundaries). This reduces starin energy while keeping the material in a deformed shape
4. Lastly, the subgrains merge and grow larger lower energy structures which improves ductility and reduces hardness. This is driven by the reduction of total grain boundary energy
47
Nucleation on shear bands
1. Shear bands are localized zones of plastic deformation that form during high strain rate deformation. They are narrow zones of high dislocation density
2. Makes it easier to slip
3. Known as the runaway effect where deformation concentrates in shear bands and enhances recrystallisation in tose areas
47
recrystallisation
1. Strain-free grains nucleate, causing a drop in strength as microstructure rests
2. Driving force is the decrease in free energy
3. New grains will nucleate in dislocation free regions and grow into the surrounding matrix
48
Grain Growth
1. Usually undesiratble as it reduces material strength
2. Driving force for grain growth is the reduction of total boundarye nergy
49
Von Neuman Criterion
1. Determines whether a grain in a polycrystalline material will grow or shrink dependent on the number of edges/junctions
2. n > 6 : grow
n < 6 : shrink
n = 6 : stable
50
Boundary Speed
1. Migration is governed by atomic movement across the boundary
2. Higher curvature means more stored energy meaning faster diffusion
3. Grain growth is time-dependent but slows over time as grains grow large which drives force down
51
Zener Pinning
1. Occurs when small particles within a material inhibit the movement of grain boundaries
2. When a grain boundary encounters a particle, it bends around it, effectively pinning the boundary in place which leads to stabilisation of grwoth structures and prevents excessive grain growth
52
Steel
1. 2-phase micorstructure : pearlite (strong and hard) and ferrite (soft and ductile)
53
White Cast Iron
1. Made up of fine pearlite formed from liquid and cementite which makes the material hard and brittle
2. Starting from liquid, austenite forms until eutectic temp is reaches and the remaining liquid solidifies into a mixture of cementite and austenite. Once all the liquid has solidified, volume fraction of cementite increases as carbon solubility in austenite decreased
3. Once the eutectoid composition is reached, austenite decomposese into ferrite and cementite
54
Grey Cast Iron
1. Fe3C decomposes into graphite so it is less brittle
55
56
Pearlite
1. Consists of alternating ferrite and cementite which forms a fine layered structure ( alpha + Fe3C)
2. Pearlite is formed when austenite cools below the eutectic temp
3. There is no driving force for the formation of pearlite at the eutectoid temperature as the free energy of austenite ferrite and cementite are equal
4. Cementite will nuclete first in carbon-rich areas which causes iron rejection and enriches nearby areas in Fe
57
Hypoeutectic Steel
1. When ferrites form above the eutectoid temperatures.
2. Ferrites reject carbon into the remaining austenite which will enrich it
3. when it reaches the eutectoid temperature, the remaining carbon-rich austenite transforms into pearlite
58
pearlite spacing
1. Determined by cooling rate
2. Faster cooling leads in finer pearlite which slower cooling rate leads to coarse pearlite
59
Martensite
1. Forms when austenite is rapidly quenched (cooled quickly)
2. Known for its strength and toughness
3. A diffusionless transformation from FCC to BCT
4. When cooled so quickly, carbon atoms do not have time to diffuse so the transformation locks atoms in place in th BCT structure that leads to internal stress. More carbon means greater lattice deformation (lattie parameter) as the strain will elongate the structure
5. requires high undercooling to overcome strain and prevent diffusion
60
Martensite formation
1. Without diffusion, transformation occurs through glissile dislocation motion
2. Diffusion is too slow so dislocations move to the interface and a coherent interface allows the movement
3. Dislocations must lie on a single glide plane to prevent interference from locking the dislocations in place
61
Bain Strain
1. Caused by the formation from FCC to BCT
2.Combo of shear deformation and volume expansion
62
Martensite tempering
1. heat treated so carbon can redistribute
2. At less than 100 degrees, carbon atoms migrate to dislocations and defects.
3. At 100 -200, fine E carbide (Fe2.4C) precipitates form inside the martensite. Small fine particles of cementite will also start to form.
4. At 200 - 300 degrees, retained austenite decomposes into ferrite and cementite which form on the grain boundaries
5. At 400 -700 degrees, E carbide transforms into stable cementite spherozdizes and the martensite fully relaxes into ferrite
63
Bainite
1. Bainite laths (thin plates of ferrite) do not grow through the grain but rather stop at boundaries
2. Forms at temps between pearlite and martensite and is a mix of ferrite and cementite butdiffers from pearlite in morphology
3. Occurs without atomic diffusion
64
Upper Bainite
1. 400-550 degrees
2. Carbon is diffused fast allowing carbon to escape growing alpha plates
3. Cementite forms outside the baintic ferrite planes
65
Lower bainite
1. 250 - 400 degrees
2. Slower carbon diffusion means carbon canot escape and precipitates within the ferrite
3. Cementite forms inside the planes
4. Growth is diffusion controlled
