WK16 - Kinetics & Phase Diagrams Flashcards
What is diffusion? What’s the difference between inter-diffusion and self-diffusion?
Diffusion is the transport of material via atomic motion (energy is required to overcome barriers for atomic diffusion)
Inter - diffusion of impurities between two regions in response to a concentration gradient
Self - diffusion of atoms in a one-element material, where atoms of the same type exchange
What is vacancy and interstitial diffusion?
Vacancy - atomic bonds around the vacancy break allowing a new atom to fill the vacancy (atom migration moves in the opposite direction to the vacancy)
Interstitial - atoms can diffuse fairly quickly as their bonding is weaker and there are more interstitial sites to jump to (atoms need to be small)
• tends to be quicker than vacancy (smaller atoms, less distortion to lattice)
What is steady state diffusion? What is the concentration gradient also known as?
Flux does not change with time
Concentration profile is a function of position
Concentration gradient also known as the driving force of diffusion (minus sign indicates diffusion occurs down the concentration gradient)
What is Non-steady state diffusion? (Flick’s Second Law)
Both concentration profile and concentration gradient change with time
Most of the cases in reality
What is the activation energy? How is it described?
For atoms to jump to a vacancy or interstitial to jump to another site, it needs enough thermal energy to break its bond and move
Probability of a jump is described by the Arrhenius equation*
* Arrhenius dependence is just stating a parameter has an EXP(x) dependence on temperature
What are the factors that make diffusion faster? (6 total)
- Open crystal structures/smaller diffusing atoms
- Lower density materials
- Materials with low melting temperatures
- Materials with secondary bonding
- Cations (in ionic materials)
- Polycrystalline materials
What is creep? What are the three main phases of creep? What is it dependent on?
Tendency of a solid material to slowly deform under long-term mechanical stresses (slow deformation)
- Primary (creep rate decreases with time), 2. Secondary creep (steady state creep with a constant slope), 3. Tertiary Creep (creep increases with time, accelerating to rupture)
Time | temperature | load
Diffusional Creep and Dislocstion Creep are the two main mechanisms of Creep, what are they?
Diffusional Creep - Occurs when material is transported across the specimen by diffusion through several possible pathways
• Nabarro-Herring Creep (through bulk crystal lattice
• Coble Creep (Diffusion along grain boundaries)
Dislocation (Power-Law) Creep - due to dislocations
• dislocations can climb from one slip plane to another making it easier to move past obstacles (at elevated temps).
What are the main material factors that can affect creep?
- Stress-assisted vacancy diffusion
- Grain boundary sliding
- Grain boundary diffusion
- Dislocation motion
Increasing grain size can help remove some of these mechanisms
How can creep be minimised? Main temp at which creep will occur for a material?
- High melting temps
- High elastic modules
- Large grain sizes (inhibits grain boundary sliding)
Creep occurs at > 0.4Tm (where Tm is melting temp)
What does it mean if a microstructure has texture?
Texture - distribution of crystallographic orientations of a polycrystalline sample in a specific direction
What is annealing?
Hearing a material to certain temperatures allows the atoms within to move around through diffusion
Diffusions processes can remove the effect of cold-working by annihilating dislocations and vacancies, then new grains forms
Cold worked grains - many small grains
Annealed grins - less larger grains
• ductility increases, tensile strength decreased
Describe the process that drives recrystallisation
Cold worked material have many small grains resulting in lots of internal strain energy
Strain energy gives a driving force to recrystallise new grains with less strain
Driving force = difference between the internal energy in the strained and unstrained grains
Define recrystallisation temp. How is affected by cold working?
“The temperature required to completely recrystallise the material in an hour”
0.3 to 0.7 of Tm (typically)
Temperature decreases as %CW increases
What is Quenching?
Rapid cooling, by immersion in water or oil, of a metal from the high temperature at which it has been shaped
• cooling the metal slowly will cause the microstructure to change as it cools
What is the solubility limit? What happens above this level?
“The maximum amount of a component that can be dissolved into a phase”
• Above this level the component will precipitate
C has limit of 6.67wt.% in Fe
What affects the formation of another phase (in reference to the atoms involved)
- Crystal structure of the main phase
- Atomic radii of the elements
- Electronegativity of the different elements
- Valency of each element
How do you convert from weight percentage to atomic percentage? Derive the expression for this
C’1 = C1A2/(C1A2 + C2A1) * 100
Describe a phase and multi phase material, how does it differ to a grain boundary?
Phase - a region that has uniform physical and chemical characteristics
Multi phase - A heterogenous system has two or more distinct phases separated by phase boundaries
GB - different orientations meet within the same phase
What is the difference between equilibrium and metastable?
Equilibrium - a system is in this state if it does not change with time at a constant temperature, pressure and composition
Metastable - slow kinetics may result in a material behaving like it’s in equilibrium, despite not being in its lowest energy states
• local minimum of free energy
• Diamond metastable of Carbon
What is isomorphous system?
Simplest binary system where components of A can completely accommodate the components of B (100% soluble with each other)
How does a phase diagram look in non-equilibrium conditions?
This is when the atoms aren’t given infinite time to move back to their equilibrium points
Layers at each tie line remain at that composition after solidification, forming layered grains with a Ni (example) content in the core.
This means grain boundaries will melt first as they have lowest Ni (point of weakness)
What is binary Eutectic system?
Next simplest diagram, where a binary alloys components aren’t 100% insoluble with each other.
Hence u have 3 phases: Liquid (L + a, L + b), alpha, beta
What is the Eutectic (invariant) point? What is the Eutectic Isotherm?
A single point where the liquid phase (L) goes straight to the solid phase of the two components (alpha + beta)
Eutectic Isotherm - the temperature at which this occurs
What is a Eutectic reaction (ER)? Why is it unique?
L —> alpha + beta
Due to completely different compositions of alpha and beta, ER drives but redistributions of Pb and Sn by atomic diffusion
A layered (or lamellar) structure forms called eutectic structure
Draw 4 phase bubbles which transforms from: 1. Liquid, alpha + Liquid, alpha + beta
What is important to consider?
How do the names of these phases differ?
Must calculate alpha percentage in alpha + Liquid
Then calculate alpha in alpha + beta
alpha + L : primary alpha (denoted via ‘)
Alpha + beta : Eutectic alpha
What is an intermetallic? How does it appear on a phase diagram?
Is a phase of a different crystal structure to the main phase and at a very specific composition
Show as vertical straight lines - only be 100% of the phases at one specific place on the composition graph
Tend to be brittle and problematic
What is the Eutectoid reaction?
Reaction occurs from one solid phase to two NEW solid phases
Alpha —> beta + gamma
What is a Peritectic reaction?
Solid + liquid form a new solid phase after cooling
Alpha + liquid —> beta
Reaction is slow, and becomes harder with time as beta is separating the Alpha and liquid
What is a congruent transformation?
Involves no change in COMPOSITION
L —> gamma in Ti
Alpha —> gamma in Fe (most significant)
